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Title: Elements of Agricultural Chemistry

Author: Thomas Anderson

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ELEMENTS

OF

AGRICULTURAL CHEMISTRY

BY

THOMAS ANDERSON, M.D.

F.R.S.E., F.C.S.

PROFESSOR OF CHEMISTRY IN THE UNIVERSITY OF GLASGOW, AND CHEMIST TO THE
HIGHLAND AND AGRICULTURAL SOCIETY OF SCOTLAND.

EDINBURGH:

ADAM AND CHARLES BLACK.

1860.


ERRATUM.

Page 190, line 11, for "gallon" read "ton."

PRINTED BY R. AND R. CLARK, EDINBURGH.


Transcriber's note: Many of the tables needed to be split to fit space
constraints. Minor typos have been corrected and footnotes moved to the
end of the chapters. A word surrounded by underscores like _this_
signifies the word is italics in the text. For numbers and equations,
underscores before bracketed numbers in equations denote a subscript.



PREFACE.


The object of the present work is to offer to the farmer a concise
outline of the general principles of Agricultural Chemistry. It has no
pretensions to be considered a complete treatise on the subject. On the
contrary, its aim is strictly elementary, and with this view I have
endeavoured, as far as possible, to avoid unnecessary technicalities so
as to make it intelligible to those who are unacquainted with the
details of chemical science, although I have not hesitated to discuss
such points as appeared essential to the proper understanding of any
particular subject.

The rapid progress of agricultural chemistry, and the numerous
researches prosecuted under the auspices of agricultural societies and
private experimenters in this and other countries, render it by no means
an easy task to make a proper selection from the mass of facts which is
being daily accumulated. In doing this, however, I have been guided by a
pretty intimate knowledge of the wants of the farmer, which has induced
me to enlarge on those departments of the subject which bear more
immediately on the every-day practice of agriculture; and for this
reason the composition and properties of soils, the nature of manures,
and the principles by which their application ought to be governed, have
been somewhat minutely treated.

In all cases numerical details have been given as fully as is consistent
with the limits of the work; and it may be right to state that a
considerable number of the analyses contained in it have been made in my
own laboratory, and that even when I have preferred to quote the results
of other chemists, they have not unfrequently been confirmed by my own
experiments.

     UNIVERSITY OF GLASGOW,
     _1st November 1860._




CONTENTS.


                                                                       Page

INTRODUCTION                                                              1


CHAPTER I.

THE ORGANIC CONSTITUENTS OF PLANTS.

Carbon ... Carbonic Acid ... Hydrogen ... Nitrogen ... Nitric Acid ...
Ammonia ... Oxygen ... Sources whence obtained ... The Atmosphere ...
The Soil ... Source of the Inorganic Constituents of Plants ... Manner
in which the Constituents of Plants are absorbed                          8


CHAPTER II.

THE PROXIMATE CONSTITUENTS OF PLANTS.

The Saccharine and Amylaceous Constituents ... Cellulose ... Incrusting
Matter ... Starch ... Lichen Starch ... Inuline ... Gum ... Dextrine ...
Sugar ... Mucilage ... Pectine and Pectic Acid ... Oily or Fatty Matters
... Margaric, Stearic, and Oleic Acids ... Wax ... Nitrogenous or
Albuminous Constituents of Plants and Animals ... Albumen ... Fibrine
... Casein ... Diastase                                                  40


CHAPTER III.

THE CHANGES WHICH TAKE PLACE IN THE FOOD OF
PLANTS DURING THEIR GROWTH.

Changes occurring during Germination ... Changes during the After-Growth
of the Plant ... Decomposition of Carbonic Acid ... Decomposition of
Water ... Decomposition of Ammonia ... Decomposition of Nitric Acid      54


CHAPTER IV.

THE INORGANIC CONSTITUENTS OF PLANTS.

The Amount of Inorganic Matters in Different Plants ... The Relative
Proportions of Ash in the Different Parts of Plants ... Influence of the
Nature of the Soil on the Proportion of Mineral Matters in the Plant ...
The Composition of the Ashes of Plants ... Classification of Different
Plants                                                                   63


CHAPTER V.

THE SOIL--ITS CHEMICAL AND PHYSICAL CHARACTERS.

The Origin of Soils ... Composition of Crystalline and Sedimentary Rocks
... their Disintegration ... Chemical Composition of the Soil ...
Fertile and Barren Soils ... Mechanical Texture of Soils ... Absorbent
Action of Soils ... their Physical Characters ... Relation to Heat and
Moisture ... The Subsoil ... Classification of Soils                     83


CHAPTER VI.

THE IMPROVEMENT OF THE SOIL BY MECHANICAL
PROCESSES.

Draining ... Its Advantageous Effects ... Subsoil and Deep Ploughing
... Improving the Soil by Paring and Burning ... Warping
... Mixing of Soils ... Chalking                                        137


CHAPTER VII.

THE GENERAL PRINCIPLES OF MANURING.

Fundamental Principles upon which Manures are applied ... _Special_ and
_General_ Manures ... Importance of this distinction ... Views regarding
the Theory of Manures ... Remarks on Special Manures ... Action of
Manures on the Chemical and Physical Properties of a Soil ... Remarks on
the Application of Manures                                              152


CHAPTER VIII.

THE COMPOSITION AND PROPERTIES OF FARM-YARD
AND LIQUID MANURES.

Farm-yard Manure ... Urine ... Composition of ... Dung ... Composition
of ... Farm-yard Manure ... Composition of ... Management of Dung-Heaps
... Box-feeding ... Fermentation and application of Manure ... Liquid
Manure ... Composition and application of ... Sewage Manure ... Its
composition and application                                             166


CHAPTER IX.

THE COMPOSITION AND PROPERTIES OF VEGETABLE
MANURES.

Rape-Dust, Mustard, Cotton and Castor Cake ... Composition of various
Oil-Cakes ... Malt-Dust, Bran, Chaff, etc. ... Straw and Saw-dust ...
Manuring with Fresh Vegetable Matter ... Green Manuring ... Sea-Weed ...
Composition of various Sea-Weeds ... Leaves ... Peat                    195


CHAPTER X.

THE COMPOSITION AND PROPERTIES OF ANIMAL
MANURES.

Guano, different varieties of ... Average composition of ... Division
into Ammoniacal and Phosphatic ... Characters of ... Adulteration of ...
Application of ... Pigeons' Dung ... Urate and Sulphated Urine ...
Night-Soil and Poudrette ... Hair, Skin, Horn, Wool, etc. ... Blood ...
Fish ... "Fish-Guano"--Bones                                            204


CHAPTER XI.

THE COMPOSITION AND PROPERTIES OF MINERAL
MANURES.

Mineral Manures ... Sulphate and Muriate of Ammonia ... Sulphomuriate
of Ammonia ... Ammoniacal Liquor ... Nitrates of Potash and Soda ...
Muriate and Sulphate of Potash ... Chloride of Sodium, or Common Salt
... Carbonates of Potash and Soda ... Silicates of Potash and Soda ...
Sulphate of Magnesia ... Phosphate of Lime ... Bone-ash ... Coprolites
... Apatite ... Sombrero Guano ... Superphosphates and Dissolved Bones
... Biphosphate of Lime or Soluble Phosphates ... Phospho-Peruvian Guano
... Lime ... Chalk ... Marl ... Application and Action of Lime on Soils
... Sulphate of Lime or Gypsum                                          226


CHAPTER XII.

THE VALUATION OF MANURES.

The Principle on which Manures are valued ... Its application to
different simple and complex Manures ... Method of Calculation ...
General Remarks                                                         255


CHAPTER XIII.

THE ROTATION OF CROPS.

Its necessity explained ... Quantity of Mineral Matters in the produce
of an Acre of Different Crops ... The Theory of Rotation                266


CHAPTER XIV.

THE FEEDING OF FARM STOCK.

The Principles of Feeding ... The Composition of different Animals in
different stages of Fattening ... The Composition of the Food of Animals
... Milk ... The Principal Varieties of Cattle Food ... General
Observations on Feeding                                                 276




AGRICULTURAL CHEMISTRY.




INTRODUCTION.


That the phenomena of vegetation are dependent on certain chemical
changes occurring in the plant, by which the various elements of its
food are elaborated and converted into vegetable matter, was very early
recognised by chemists; and long before the correct principles of that
science were established, Van Helmont maintained that plants derived
their nourishment from water, while Sir Kenelm Digby, Hook, Bradley, and
others, attributed an equally exclusive influence to air, and enlarged
on the practical importance of the conclusions to be deduced from their
views. These opinions, which were little better than hypotheses, and
founded on very imperfect chemical data, are mentioned by Jethro Tull,
the father of modern agriculture, only to deny their accuracy; and he
contended that the plants absorb and digest the finer particles of the
earth, and attributed the success of the particular system of husbandry
he advocated to the comminution of the soil, by which a larger number
of its particles are rendered sufficiently small to permit their ready
absorption by the roots. Popular opinion at that time was in favour of
the mechanical rather than the chemical explanation of agricultural
facts, and Tull's work had the effect of confirming this opinion, and
turning attention away from the application of chemistry to agriculture.
Indeed, no good results could have followed its study at that time, for
chemistry, especially in those departments bearing more immediately on
agriculture, was much too imperfect, and it was only towards the close
of the last century, when Lavoisier established its true principles,
that it became possible to pursue it with any prospect of success.

Very soon after Lavoisier's system was made known, Lord Dundonald
published his "Treatise on the Intimate Connexion between Chemistry and
Agriculture," in which the important bearings of the recent chemical
discoveries on the practice of agriculture were brought prominently
under the notice of the farmer, and almost at the same time De Saussure
commenced those remarkable researches, which extended over a long series
of years, and laid the foundation of almost all our accurate knowledge
of the chemistry of vegetation. Saussure traced with singular care and
accuracy the whole phenomena of the life of plants, and indicated the
mode in which the facts he established might be taken advantage of in
improving the cultivation of the soil. But neither his researches, nor
Lord Dundonald's more direct appeal to the farmer, excited the attention
they deserved, or produced any immediate effect on the progress of
agriculture. It was not till the year 1812 that the interest of
practical men was fairly awakened by a course of lectures given by Sir
Humphrey Davy, at the instance of Sir John Sinclair, who was at that
time president of the Board of Agriculture. In these lectures, written
with all the clearness and precision which characterised their author's
style, the results of De Saussure's experiments were for the first time
presented to the farmer in a form in which they could be easily
understood by him, the conclusions to which they led were distinctly
indicated, and a number of useful practical suggestions made, many of
which have been adopted into every-day practice, and become so
thoroughly incorporated with it, that their scientific origin has been
altogether forgotten. A lively interest was excited by the publication
of Davy's work, but it soon died out, and the subject lay in almost
complete abeyance for a considerable number of years. Nor could any
other result be well expected, for at that time agriculture was not ripe
for chemistry, nor chemistry ripe for agriculture. The necessities of a
rapidly increasing population had not yet begun to compel the farmer to
use every means adapted to increase the amount of production to its
utmost limit; and though the fundamental principles of chemistry had
been established, its details, especially in that department which
treats of the constituents of plants and animals, were very imperfectly
known. It is not surprising, therefore, that matters should have
remained almost unchanged for the comparatively long period of nearly
thirty years. Indeed, with the exception of the investigation of soils
by Schübler, and some other inquiries of minor importance, and which, in
this country at least, excited no attention on the part of the
agriculturist, nothing was done until the year 1840, when Liebig
published his treatise on _Chemistry, in its application to Agriculture
and Physiology_.

Saussure's researches formed the main groundwork of Liebig's treatise,
as they had before done for Davy's; but the progress of science had
supplied many new facts which confirmed the opinions of the older
chemists in most respects, and enabled Liebig to generalise with greater
confidence, and illustrate more fully the principles upon which
chemistry ought to be applied to agriculture. Few works have ever
produced a more profound impression. Written in a clear and forcible
style, dealing with scientific truths in a bold and original manner, and
producing a strong impression, as well by its earnestness as by the
importance of its conclusions, it was received by the agricultural
public with the full conviction that the application of its principles
was to be immediately followed by the production of immensely increased
crops, and by a rapid advance in every branch of practical agriculture.
The disappointment of these extravagant expectations, which _chemists_
themselves foresaw, and for which they vainly attempted to prepare the
agriculturist, was followed by an equally rapid reaction; and those who
had embraced Liebig's views, and lauded them as the commencement of a
new era, but who had absurdly expected an instantaneous effect, changed
their opinion, and contemned, as strongly as they had before supported,
the application of chemistry to agriculture.

That this effect should have been produced is not unnatural; for
practical men, having at that time little or no knowledge of chemistry,
were necessarily unable to estimate its true position in relation to
agriculture, and forgetting that this department of science was still in
its early youth, and burthened with all the faults and errors of youth,
they treated it as if it were already perfect in all its parts. Neither
could they distinguish between the fully demonstrated scientific truths,
and the uncertain, though probable conclusions deduced from them; and
when the latter, as occasionally happened, proved to be at variance with
practice, it is not surprising: that this should have produced a feeling
of distrust on the part of persons incapable, from an imperfect, and
still oftener from no knowledge of science, of drawing the line of
demarcation, which Liebig frequently omitted to do, between the positive
fact and the hypothetical inference, which, however probable, is, after
all, merely a suggestion requiring to be substantiated by experiment.
This omission, which the scientific reader can supply for himself,
becomes a source of serious misapprehension in a work addressed to
persons unacquainted with science, who adopt indiscriminately both the
facts and the hypotheses of the author. And this is no doubt the cause
of the vary different estimation in which the work of the Giessen
Professor was held by scientific and practical men.

Liebig's treatise was followed, in the year 1844, by the publication of
Boussingault's _Economic Rurale_, a work winch excited at the time
infinitely less interest than Liebig's, although it is really quite as
important a contribution to scientific agriculture. It is distinguished
by entering more fully into the special details of the application of
chemistry to agriculture, and contains the results of the author's
numerous searches both in the laboratory and the field. Boussingault
possesses the qualification, at present somewhat rare, of combining a
thorough knowledge of practical agriculture with extended scientific
attainments; and his investigations, which have been made with direct
reference to practice, and their results tested in the field, are the
largest and most valuable contribution to the exact data of scientific
agriculture which has yet been made public.

The year 1844 was also distinguished by the foundation of the
Agricultural Chemistry Association of Scotland, an event of no small
importance in the history of scientific agriculture. That association
was instituted through the exertions of a small number of practical
farmers, for the purpose of pursuing investigations in agricultural
chemistry, and affording to its members assistance in all matters
connected with the cultivation of the soil, and has formed the model of
similar establishments in London, Dublin, and Belfast, as well as in
Germany; and it is peculiarly creditable to the intelligence and energy
of the practical farmers of Scotland, that with them commenced a
movement, which has already found imitators in so many quarters, and
conferred such great benefits on agriculture. Within the last ten or
twelve years, and mainly owing to the establishment of agricultural
laboratories, great progress has been made in accumulating facts on
which to found an accurate knowledge of the principles of agricultural
chemistry, and the number of chemists who have devoted themselves to
this subject has considerably increased, though still greatly less than
its exigencies require.

Notwithstanding all that has recently been done, it must not be
forgotten that we have scarcely advanced beyond the threshold, and that
it is only by numerous and frequently repeated experiments that it is
possible to arrive at satisfactory results. Agricultural inquiries are
liable to peculiar fallacies due to the perturbing influence of climate,
season, and many other causes, the individual effects of which can only
be eliminated with difficulty, and much error has been introduced, by
hastily generalising from single experiments, in place of awaiting the
results of repeated trials. Hence it is that the progress of scientific
agriculture must necessarily be slow and gradual, and is not likely to
be marked by any great or startling discoveries. Now that the relations
of science to practice are better understood, the extravagant
expectations at one time entertained have been abandoned, and, as a
necessary consequence, the interest in agricultural chemistry has again
increased, and the conviction daily gains ground that no one who wishes
to farm with success, can afford to be without some knowledge of the
scientific principles of his art.




CHAPTER I.

THE ORGANIC CONSTITUENTS OF PLANTS.


When the water naturally existing in plants is expelled by exposure to
the air or a gentle heat, the residual dry matter is found to be
composed of a considerable number of different substances, which have
been divided into two great classes, called the organic and the
inorganic, or mineral constituents of plants. The former are readily
combustible, and on the application of heat, catch fire, and are
entirely consumed, leaving the inorganic matters in the form of a white
residuum or ash. All plants contain both classes of substances; and
though their relative proportions vary within very wide limits, the
former always greatly exceed the latter, which in many cases form only a
very minute proportion of the whole weight of the plant. Owing to the
great preponderance of the organic or combustible matters, it was at one
time believed that the inorganic substances formed no part of the true
structure of plants, and consisted only of a small portion of the
mineral matters of the soil, which had been absorbed along with their
organic food; but this opinion, which probably was never universally
entertained, is now entirely abandoned, and it is no longer doubted that
both classes of substances are equally essential to their existence.

Although they form so large a proportion of the plant, its organic
constituents are composed of no more than four elements, viz.:--

     Carbon.
     Hydrogen.
     Nitrogen.
     Oxygen.

The inorganic constituents are much more numerous, not less than
thirteen substances, which appear to be essential, having been observed.
These are--

     Potash.
     Soda.
     Lime.
     Magnesia.
     Peroxide of Iron.
     Silicic Acid.
     Phosphoric Acid.
     Sulphuric Acid.
     Chlorine.

And more rarely

     Manganese.
     Iodine.
     Bromine.
     Fluorine.

Several other substances, among which may be mentioned alumina and
copper, have also been enumerated; but there is every reason to believe
that they are not essential, and the cases in which they have been found
are quite exceptional.

It is to be especially noticed that none of these substances occur in
plants in the free or uncombined state, but always in the form of
compounds of greater or less complexity, and extremely varied both in
their properties and composition.

It would be out of place, in a work like the present, to enter into
complete details of the properties of the elements of which plants are
composed, which belongs strictly to pure chemistry, but it is necessary
to premise a few observations regarding the organic elements, and their
more important compounds.

_Carbon._--When a piece of wood is heated in a close vessel, it is
charred, and converted into charcoal. This charcoal is the most familiar
form of carbon, but it is not absolutely pure, as it necessarily
contains the ash of the wood from which it was made. In its purest form
it occurs in the diamond, which is believed to be produced by the
decomposition of vegetable matters, and it is there crystallized and
remarkably transparent; but when produced by artificial processes,
carbon is always black, more or less porous, and soils the fingers. It
is insoluble in water, burns readily, and is converted into carbonic
acid. Carbon is the largest constituent of plants, and forms, in round
numbers, about 50 per cent of their weight when dry.

_Carbonic Acid._--This, the most important compound of carbon and
oxygen, is best obtained by pouring a strong acid upon chalk or
limestone, when it escapes with effervescence. It is a colourless gas,
extinguishing flame, incapable of supporting respiration, much heavier
than atmospheric air, and slightly soluble in water, which takes up its
own volume of the gas. It is produced abundantly when vegetable matters
are burnt, as also during respiration, fermentation, and many other
processes. It is likewise formed daring the decay of animal and
vegetable matters, and is consequently evolved from dung and compost
heaps.

_Hydrogen_ occurs in nature only in combination. Its principal compound
is water, from which it is separated by the simultaneous action of an
acid, such as sulphuric acid and a metal, in the form of a transparent
gas, lighter than any other substance. It is very combustible, burns
with a pale blue flame, and is converted into water. It is found in all
plants, although in comparatively small quantity, for, when dry, they
rarely contain more than four or five per cent. Its most important
compound is water, of which it forms one-ninth, the other eight-ninths
consisting of oxygen.

_Nitrogen_ exists abundantly in the atmosphere, of which it forms nearly
four-fifths, or, more exactly, 79 per cent. It is there mixed, but not
combined with oxygen; and when the latter gas is removed, by introducing
into a bottle of air some substance for which the former has an
affinity, the nitrogen is left in a state of purity. It is a transparent
gas, which is incombustible and extinguishes flame. It is a singularly
inert substance, and is incapable of directly entering into union with
any other element except oxygen, and with that it combines with the
greatest difficulty, and only by the action of the electric spark--a
peculiarity which has very important bearings on many points we shall
afterwards have to discuss. Nitrogen is found in plants to the extent of
from 1 to 4 per cent.

_Nitric Acid._--This, the most important compound of nitrogen and
oxygen, can be produced by sending a current of electric sparks through
a mixture of its constituents, but in this way it can be obtained only
in extremely small quantity. It is much more abundantly produced when
organic matters are decomposed with free access of air, in which case
the greater proportion of their nitrogen combines with the atmospheric
oxygen. This process, which is known by the name of nitrification, is
greatly promoted by the presence of lime or some other substance, with
which the nitric acid may combine in proportion as it is formed. It
takes place, to a great extent, in the soil in India and other hot
climates; and our chief supplies of saltpetre, or nitrate of potash, are
derived from the soil in these countries, where it has been formed in
this manner. The same change occurs, though to a much smaller extent, in
the soil in temperate climates.

_Ammonia_ is a compound of nitrogen and hydrogen, but it cannot be
formed by the direct union of these gases. It is a product of the
decomposition of organic substances containing nitrogen, and is produced
when they are distilled at a high temperature, or allowed to putrefy out
of contact of the air. In its pure state it is a transparent and
colourless gas, having a peculiar pungent smell, and highly soluble in
water. It is an alkali resembling potash and soda, and, like these
substances, unites with the acids and forms salts, of which the sulphate
and muriate are the most familiar. In these salts it is fixed, and does
not escape from them unless they be mixed with lime, or some other
substance possessing a more powerful affinity for the acid with which it
is united.

_Oxygen_ is one of the most widely distributed of all the elements, and,
owing to its powerful affinities, is the most important agent in almost
all natural changes. It is found in the air, of which it forms 21 per
cent, and in combination with hydrogen, and almost all the other
chemical elements. In the pure state it possesses very remarkable
properties. All substances burn in it with greater brilliancy than they
do in atmospheric air, and its affinity for most of the elements is
extremely powerful. When diluted with nitrogen, it supports the
respiration of animals; but in the pure state it proves fatal after the
lapse of an hour or two. It is found in plants, in quantities varying
from 30 to 36 per cent.

It is worthy of observation, that of the four organic elements, carbon
only is fixed, and the other three are gases; and likewise, when any two
of them unite, their compound is either a gaseous or a volatile
substance. The charring of organic substances, which is one of their
most characteristic properties, and constantly made use of by chemists
as a distinctive reaction, is due to this peculiarity; for when they are
heated, a simpler arrangement of their particles takes place, the
hydrogen, nitrogen, and oxygen unite among themselves, and carry off a
small quantity of carbon, while the remainder is left behind in the form
of charcoal, and is only consumed when access of the external air is
permitted.

Now, in order that a plant may grow, its four organic constituents must
be absorbed by it, and that this absorption may take place, it is
essential that they be presented to it in suitable forms. A seed may be
planted in pure carbon, and supplied with unlimited quantities of
hydrogen, nitrogen, oxygen, and inorganic substances, and it will not
germinate; and a plant, when placed in similar circumstances, shows no
disposition to increase, but rapidly languishes and dies. The obvious
inference from these facts is, that these substances cannot be absorbed
when in the _elementary_ state, but that it is only after they have
entered into certain forms of combination that they acquire the property
of being readily taken up, and assimilated by the organs of the plant.

It was at one time believed that many different compounds of these
elements might be absorbed and elaborated, but later and more accurate
experiments have reduced the number to four--namely, carbonic acid,
water, ammonia, and nitric acid. The first supplies carbon, the second
hydrogen, the two last nitrogen, while all of them, with the exception
of ammonia, may supply the plant with oxygen as well as with that
element of which it is the particular source.

There are only two sources from which these substances can be obtained
by the plant, viz. the atmosphere and the soil, and it is necessary that
we should here consider the mode in which they may be obtained from
each.

_The Atmosphere as a source of the Organic Constituents of
Plants._--Atmospheric air consists of a mixture of nitrogen and oxygen
gases, watery vapour, carbonic acid, ammonia, and nitric acid. The two
first are the largest constituents, and the others, though equally
essential, are present in small, and some of them in extremely minute
quantity. When deprived of moisture and its minor constituents, 100
volumes of air are found to contain 21 of oxygen and 79 of nitrogen.
Although these gases are not chemically combined in the air, but only
mechanically mixed, their proportion is exceedingly uniform, for
analyses completely corresponding with these numbers have been made by
Humboldt, Gay-Lussac, and Dumas at Paris, by Saussure at Geneva, and by
Lewy at Copenhagen; and similar results have also been obtained from air
collected by Gay-Lussac during his ascent in a balloon at the height of
21,430 feet, and by Humboldt on the mountain of Antisano in South
America at a height of 16,640 feet. In short, under all circumstances,
and in all places, the relation subsisting between the oxygen and
nitrogen is constant; and though, no doubt, many local circumstances
exist which may tend to modify their proportions, these are so slow and
partial in their operations, and so counterbalanced by others acting in
an opposite direction, as to retain a uniform proportion between the
main constituents of the atmosphere, and to prevent the undue
accumulation of one or other of them at any one point.

No such uniformity exists in the proportion of the minor constituents.
The variation in the quantity of watery vapour is a familiar fact, the
difference between a dry and moist atmosphere being known to the most
careless observer, and the proportions of the other constituents are
also liable to considerable variations.

_Carbonic Acid._--The proportion of carbonic acid in the air has been
investigated by Saussure. From his experiments, made at the village of
Chambeisy, near Geneva, it appears that the quantity is not constant,
but varies from 3·15 to 5·75 volumes in 10,000; the mean being 4·15.
These variations are dependent on different circumstances. It was found
that the carbonic acid was always more abundant during the night than
during the day--the mean quantity in the former case being 4·32, in the
latter 3·38. The largest quantity found during the night was 5·74,
during the day 5·4. Heavy and continued rain diminishes the quantity of
carbonic acid, by dissolving and carrying it down into the soil.
Saussure found that in the month of July 1827, during the time when nine
millimetres of rain fell, the average quantity of carbonic acid amounted
to 5·18 volumes in 10,000; while in September 1829, when 254 millimetres
fell, it was only 3·57. A moist state of the soil, which is favourable
to the absorption of carbonic acid, also diminishes the quantity
contained in the air, while, on the other hand, continued frosts, by
retaining the atmosphere and soil in a dry state, have an opposite
effect. High winds increase the carbonic acid to a small extent. It was
also found to be greater over the cultivated lands than over the lake of
Geneva; at the tops of mountains than at the level of the sea; in towns
than in the country. The differences observed in all these cases, though
small, are quite distinct, and have been confirmed by subsequent
experimenters.

_Ammonia._--The presence of ammonia in the atmosphere appears to have
been first observed by Saussure, who found that when the sulphate of
alumina is exposed to the air, it is gradually converted into the double
sulphate of alumina and ammonia. Liebig more recently showed that
ammonia can always be detected in rain and snow water, and it could not
be doubted that it had been absorbed from the atmosphere. Experiments
have since been made by different observers with the view of determining
the quantity of atmospheric ammonia, and their results are contained in
the subjoined table, which gives the quantity found in a million parts
of air.

Kemp                                         3·6800

        { 12 feet above the surface          3·5000
Pierre  { 25 feet  do.  do.                  0·5000

Graeger                                      0·3230

          { By day                           0·0980
Fresenius { By night                         0·1690

      {          { Maximum                   0·0317
      { In Paris { Minimum                   0·0177
      {          { Mean                      0·0237
Ville {
      {          { Maximum                   0·0276
      { Environs { Minimum                   0·0165
      { of Paris { Mean                      0·0210

Of these results, the earlier ones of Kemp, Pierre, and Graeger are
undoubtedly erroneous, as they were made without those precautions which
subsequent experience has shown to be necessary. Even those of the other
observers must be taken as giving only a very general idea of the
quantity of ammonia in the air, for a proportion so minute as one
fifty-millionth cannot be accurately determined even by the most
delicate experiments. For this reason, more recent experimenters have
endeavoured to arrive at conclusions bearing more immediately upon
agricultural questions, by determining the quantity of ammonia brought
down by the rain. The first observations on this subject were made by
Barral in 1851, and they have been repeated during the years 1855 and
1856 by Mr. Way. In 1853, Boussingault also made numerous experiments
on the quantity of ammonia in the rain falling at different places, as
well as in dew and the moisture of fogs. He found in the imperial
gallon--

                                                            Grs.
Rain         { Paris                                        0·2100
             { Liebfrauenberg                               0·0350


Dew,          Liebfrauenberg           { Maximum            0·4340
                                       { Minimum            0·0714

            { Liebfrauenberg                                0·1790
Fog         { Paris                                         9·6000

It thus appears that in Paris the quantity of ammonia in rain-water is
just six times as great as it is in the country, a result, no doubt, due
to the ammonia evolved during the combustion of fuel, and to animal
exhalations, and to the same cause, the large quantity contained in the
moisture of fogs in Paris may also be attributed. Barral and Way have
made determinations of the quantity of ammonia carried down by the rain
in each month of the year, the former using for this purpose the water
collected in the rain-gauges of the Paris Observatory, and representing,
therefore, a town atmosphere; the latter, that from a large rain-gauge
at Rothamsted, at a distance from any town. According to Barral the
ammonia annually deposited on an acre of land amounts to 12·28 lbs., a
quantity considerably exceeding that obtained by Way, whose experiments
being made at a distance from towns, must be considered as representing
more accurately the normal condition of the air. His results for the
years 1855 and 1856 are given below, along with the quantities of nitric
acid found at the same time.

_Nitric Acid._--The presence of nitric acid in the air appears to have
been first observed by Priestley at the end of the last century, but
Liebig, in 1825, showed that it was always to be found after
thunder-storms, although he failed to detect it at other times. In 1851
Barral proved that it is invariably present in rain-water, and stated
the quantity annually carried down to an acre of land at no less than
41·29 lbs. But at the time his experiments were made, the methods of
determining very minute quantities of nitric acid were exceedingly
defective, and Way, by the adoption of an improved process, has shown
that the quantity is very much smaller than Barral supposed, and really
falls short of three pounds. His results for ammonia, as well as nitric
acid, are given in the subjoined table.

---------------------+----------------+----------------+-----------------+
|                    | Nitric Acid in | Ammonia in     | Total Nitrogen  |
|                    | Grains.        | Grains.        | in Grains.      |
|                    +-------+--------+-------+--------+--------+--------+
|                    | 1855. | 1856.  | 1855. |  1856. |  1855. |  1856. |
+--------------------+-------+--------+-------+--------+--------+--------+
|January             |  230  | 1564   | 1244  |  5,005 |  1084  |  4,526 |
|February            |  944  |  544   | 2337  |  4,175 |  2169  |  3,579 |
|March               | 1102  |  866   | 4513  |  2,108 |  3995  |  1,945 |
|April               |  325  | 1063   | 1141  |  8,614 |  1024  |  7,369 |
|May                 | 1840  | 3024   | 4206  | 18,313 |  3939  | 15,863 |
|June                | 3303  | 2046   | 5574  |  4,870 |  5447  |  4,540 |
|July                | 2680  | 1191   | 9620  |  2,869 |  8615  |  2,670 |
|August              | 3577  | 2125   | 4769  |  4,214 |  4870  |  4,021 |
|September           |  732  | 1756   | 3313  |  5,972 |  2917  |  5,373 |
|October             | 4480  | 2075   | 7592  |  3,921 |  7414  |  3,767 |
|November            | 1007  | 1371   | 3021  |  2,591 |  2749  |  2,489 |
|December            |  664  | 2035   | 2438  |  4,070 |  2180  |  3,352 |
+--------------------+-------+--------+-------+--------+--------+--------+
|Total in pounds for}|       |        |       |        |        |        |
|the whole year     }|  2·98 |   ·280 |  7·11 |   9·53 |   6·63 |   8·31 |
+--------------------+-------+--------+----------------+--------+--------+

No attempts have been made to determine the proportion of nitric acid in
air, but its quantity is undoubtedly excessively minute, and materially
smaller than that of ammonia. At least this conclusion seems to be a
fair inference from Way's researches, as well as the recent experiments
of Boussingault on the proportion of nitric acid contained in rain, dew,
and fog, made in a manner exactly similar to those on the ammonia,
already quoted. According to his experiments an imperial gallon
contains--

                         Grs.
Rain. {Paris             0·0708
      {Liebfrauenberg    0·0140

Dew.  {Maximum           0·0785
      {Minimum           0·0030

Fog.  {Paris             0·7092
      {Liebfrauenberg    0·0718

Although it thus appears that Barral's results have been only partially
confirmed, enough has been ascertained to show that the quantity of
ammonia and nitric acid in the air is sufficient to produce a material
influence in the growth of plants. The large amount of these substances
contained in the dew is also particularly worthy of notice, and may
serve to some extent to explain its remarkably invigorating effect on
vegetation.

_Carburetted Hydrogen._--Gay-Lussac, Humboldt, and Boussingault have
shown, that when the whole of the moisture and carbonic acid have been
removed from the air, it still contains a small quantity of carbon and
hydrogen; and Saussure has rendered it probable that they exist in a
state of combination as carburetted hydrogen gas. No definite proof of
this position has, however, as yet been adduced, and the function of the
compound is entirely unknown. It is possible that the presence of carbon
and hydrogen may be due to a small quantity of organic matter; but,
whatever be its source, its amount is certainly extremely small.

_Sulphuretted Hydrogen and Phosphuretted Hydrogen._--The proportion of
these substances is almost infinitesimal; but they are pretty general
constituents of the atmosphere, and are apparently derived from the
decomposition of animal and vegetable matters.

The preceding statements lead to the important conclusion, that the
atmosphere is capable of affording an abundant supply of all the organic
elements of plants, because it not only contains nitrogen and oxygen in
the free state, but also in those forms of combination in which they are
most readily absorbed, as well as a large quantity of carbonic acid,
from which their carbon may be derived. At first sight it may indeed
appear that the quantity of the latter compound, and still more that of
ammonia, is so trifling as to be of little practical importance. But a
very simple calculation serves to show that, though relatively small,
they are absolutely large, for the carbonic acid contained in the whole
atmosphere amounts in round numbers to

     2,400,000,000,000 tons,

and the ammonia, assuming it not to exceed one part in fifty millions,
must weigh

     74,000,000 tons,

quantities amply sufficient to afford an abundant supply of these
elements to the whole vegetation of our globe.

_The Soil as a Source of the Organic Constituents of Plants._--When a
portion of soil is subjected to heat, it is found that it, like the
plant, consists of a combustible and an incombustible part; but while in
the plant the incombustible part or ash is small, and the combustible
large, these proportions are reversed in the soil, which consists
chiefly of inorganic or mineral matters, mixed with a quantity of
combustible or organic substances, rarely exceeding 8 or 10 per cent,
and often falling considerably short of this quantity.

The organic matter exists in the form of a substance called humus, which
must be considered here as a source of the organic constituents of
plants, independently of the general composition of the soil, which will
be afterwards discussed.

The term _humus_ is generic, and applied by chemists to a rather
numerous group of substances, very closely allied in their properties,
several of which are generally present in all fertile soils. They have
been submitted to examination by various chemists, but by none more
accurately than by Mulder and Herman, to whom, indeed, we owe almost all
the precise information we possess on the subject. The organic matters
of the soil may be divided into three great classes; the first
containing those substances which are soluble in water; the second,
those extracted by means of caustic potash; and the third, those
insoluble in all menstrua. When a soil is boiled with a solution of
caustic potash, a deep brown fluid is obtained, from which acids
precipitate a dark brown flocculent substance, consisting of a mixture
of at least three different acids, to which the names of humic, ulmic,
and geic acids have been applied. The fluid from which they have been
precipitated contains two substances, crenic and apocrenic acids, while
the soil still retains what has been called insoluble humus.

The acids above named do not differ greatly in chemical characters, but
they have been subdivided into the humic, geic, and crenic groups, which
present some differences in properties and composition. They are
compounds of carbon, hydrogen, and oxygen, and are characterised by so
powerful an affinity for ammonia that they are with difficulty obtained
free from that substance, and generally exist in the soil in combination
with it. They are all products of the decomposition of vegetable
matters in the soil, and are formed during their decay by a succession
of changes, which may be easily traced by observing the course of events
when a piece of wood or any other vegetable substance is exposed for a
length of time to air and moisture. It is then found gradually to
disintegrate with the evolution of carbonic acid, acquiring first a
brown and finally a black colour. At one particular stage of the process
it is converted into one or other of two substances, called humin and
ulmin, both insoluble in alkalies, and apparently identical with the
insoluble humus of the soil; but when the decomposition is more advanced
the products become soluble in alkalies, and then contain humic, ulmic,
and geic acids, and finally, by a still further progress, crenic and
apocrenic acids are formed as the result of an oxidation occurring at
certain periods of the decay.

The roots and other vegetable debris remaining in the soil undergo a
similar series of changes, and form the humus, which is found only in
the surface soil, that is to say, in the portion which is now or has at
some previous period been occupied by plants, and the quantity of humus
contained in any soil is mainly dependent on the activity of vegetation
on it. Numerous analyses of humus compounds extracted from the soil have
been made, and have served to establish a number of minor differences in
the composition even of those to which the same name has been applied,
due manifestly to the fact that their production is the result of a
gradual decomposition, which renders it impossible to extract from the
soil one pure substance, but only a variable mixture of several, so
similar to one another in properties, that their separation is very
difficult, if not impossible. For this reason great discrepancies exist
in the statements made regarding them by different observers, but this
is a matter of comparatively small importance, as their exact
composition has no very direct bearing on agricultural questions, and it
will suffice to give the names and chemical formulæ of those which have
been analysed and described,--

Ulmic acid from long Frisian turf    C_{40}  H_{18}  O_{16}
Humic acid from hard turf            C_{40}  H_{15}  O_{15}
Humic acid from arable soil          C_{40}  H_{16}  O_{16}
Humic acid from a pasture field      C_{40}  H_{14}  O_{14}
Geic acid                            C_{40}  H_{15}  O_{17}
Apocrenic acid                       C_{48}  H_{12}  O_{24}
Crenic acid                          C_{24}  H_{12}  O_{16}

It is only necessary to observe further, that these formulæ indicate a
close connection with woody fibre, and the continuous diminution of the
hydrogen and increase of oxygen shows that they must have been produced
by a gradually advancing decay.

The earlier chemists and vegetable physiologists attributed to the humus
of the soil a much more important function than it is now believed to
possess.

It was formerly considered to be the exclusive, or at least the chief
source of the organic constituents of plants, and by absorption through
the roots to yield to them the greater part of their nutriment. But
though this view has still some supporters, among whom Mulder is the
most distinguished, it is now generally admitted that humus is not a
_direct_ source of the organic constituents of plants, and is not
absorbed as such by their roots, although it is so _indirectly_, in as
far as the decomposition which it is constantly undergoing in the soil
yields carbonic acid, which can be absorbed. The older opinion is
refuted by many well-ascertained facts. As regards the exclusive origin
of the carbon of plants from humus, it is easy to see that this at least
cannot be true, for humus, as already stated, is itself derived solely
from the decomposition of vegetable and animal matters; and if the
plants on the earth's surface were to be supported by it alone, the
whole of their substance would have to return to the soil in the same
form, in order to supply the generation which succeeds them. But this is
very far from being the case, for the respiration of animals, the
combustion of fuel, and many other processes, are annually converting a
large quantity of these matters into carbonic acid; and if there were no
other source of carbon but the humus of the soil, the amount of
vegetable life would gradually diminish, and at length become entirely
extinct. Schleiden, who has discussed this subject very fully, has made
an approximative calculation of the total quantity of humus on the
earth's surface, and of the carbon annually converted into carbonic acid
by the respiration of man and animals, the combustion of wood for fuel,
and other minor processes; and he draws the conclusion that, if there
were no other source of carbon except humus, the quantity of that
substance existing in the soil would only support vegetation for a
period of sixty years.

The particular phenomena of vegetation also afford abundant evidence
that humus cannot be the only source of carbon. Thus Boussingault has
shown that on the average of years, the crops cultivated on an acre of
land remove from it about one ton more organic matter than they receive
in the manure applied to them, although there is no corresponding
diminution in the quantity of humus contained in the soil. An instance
which leads still more unequivocally to the same conclusion is given by
Humboldt. He states that an acre of land, planted with bananas, yields
annually about 152,000 pounds weight of fruit, containing about 32,000
pounds, or almost exactly 14 tons of carbon; and as this production goes
on during a period of twenty years, there must be withdrawn in that time
no less than 280 tons of carbon. But the soil on an acre of land weighs,
in round numbers, 1000 tons, and supposing it to contain 4 per cent of
humus, the total weight of carbon in it would amount to little more than
20 tons.

It is obvious from these and many other analogous facts that humus
cannot be the only or even a considerable source of the carbon of
plants, although it is still contended by some chemists that it may be
absorbed to a small extent. But even this is at variance with many
well-known facts. For if humus were absorbed, it might be expected that
vegetation would be most luxuriant on soils containing abundance of that
substance, especially if it existed in a soluble and readily absorbable
form; but so far from this being the case, nothing is more certain than
that peat, in which these conditions are fulfilled, is positively
injurious to most plants. On the other hand, our daily experience
affords innumerable examples of plants growing luxuriantly in soils and
places where no humus exists. The sands of the sea-shore, and the most
barren rocks, have their vegetation, and the red-hot ashes which are
thrown out by active volcanoes are no sooner cool than a crop of plants
springs up on them.

The conclusions to be drawn from these considerations have been further
confirmed by the direct experiments of different observers. Boussingault
sowed peas, weighing 15·60 grains, in a soil composed of a mixture of
sand and clay, which had been heated red-hot, and consequently contained
no humus, and after 99 days' growth, during which they had been watered
with distilled water, he found the crop to weigh 68·72 grains, so that
there had been a fourfold increase. Similar experiments have been made
by Prince Salm Horstmar, on oats and rape sown in a soil deprived of
organic matter by ignition, in which they grew readily, and arrived at
complete maturity. One oat straw attained a height of three feet, and
bore 78 grains; another bore 47; and a third 28--in all 153. These when
dried at 212° weighed 46·302 grains, and the straw 45·6 grains. The most
satisfactory experiments, however, are those of Weigman and Polstorf,
these observers having found that it was possible to obtain a
two-hundred-fold produce of barley in an entirely artificial soil,
provided care was taken to give it the _physical_ characters of a
fertile soil. They prepared a mixture of six parts of sand, two of
chalk, one of white bole, and one of wood charcoal; to which was added a
small quantity of felspar, previously fused with marble and some soluble
salts, so as to imitate as closely as possible the inorganic parts of a
soil, and in it they planted twelve barley pickles. The plants grew
luxuriantly, reaching a height of three feet, and each bearing nine
ears, containing 22 pickles. The grain of the twelve plants weighed 2040
grains.

These experiments show that plants can grow and produce seed when the
most scrupulous care is taken to deprive them of every trace of humus.
But Saussure has gone further, and shown that even when present, humus
is not absorbed. He allowed plants of the common bean and the Polygonum
Persicaria to grow in solutions of humate of potash, and found a very
trifling diminution in the quantity of humic acid present; but the value
of his experiments is invalidated by his having omitted to ascertain
whether the diminution of humic acid which he observed was really due
to absorption by the plant. This omission has been supplied by Weigman
and Polstorf. They grew plants of mint (Mentha undulata) and of
Polygonum Persicaria in solutions of humate of potash, and placed beside
the glass containing the plant, another perfectly similar, and
containing only the solution of humate of potash. The solution, which
contained in every 100 grains, 0·148 grains of solid matter, consisting
of humate of potash, etc. was found to become gradually paler, and at
the end of a month, during which time the plants had increased by 6-1/2
inches, the quantity of solid matter in 100 grains had diminished to
0·132. But the solution contained in the other glass, and in which no
plant had grown, had diminished to 0·136, so that the absorption could
not have amounted to more than 0·004 grains for every 100 grains of
solution employed. This quantity is so small as to be within the limits
of error of experiment, and we are consequently entitled to draw the
conclusion that humus, even under the most favourable circumstances, is
not absorbed by plants.

But though not directly capable of affording nutriment to plants, it
must not, on that account, be supposed that humus is altogether devoid
of importance, for it is constantly undergoing decomposition in the
soil, and thus becomes a source of carbonic acid which can be absorbed,
and, as we shall afterwards more particularly see, it exercises very
important functions in bringing the other constituents of the soil into
readily available forms of combination.

It has been already observed that carbon, hydrogen, nitrogen, and
oxygen, cannot be absorbed by plants when uncombined, but only in the
forms of water, carbonic acid, ammonia, and nitric acid. It is scarcely
necessary to detail the grounds on which this conclusion has been
arrived at in regard to carbon and hydrogen, for practically it is of
little importance whether they can be absorbed or not, as the former is
rarely, the latter never, found uncombined in nature. Neither can there
be any doubt that water and carbonic acid are the only substances from
which these elements can be obtained. Every-day experience convinces us
that water is essential to vegetation; and Saussure, and other
observers, have shown that plants will not grow if they are deprived of
carbonic acid, and that they actually absorb that substance abundantly
from the atmosphere. The evidence for the non-absorption of oxygen lies
chiefly in the fact that plants obtain, in the form of water and
carbonic acid, a larger quantity of that element than they require, and
in place of absorbing, are constantly exhaling it. The form in which
nitrogen may be absorbed has given rise to much difference of opinion.
In the year 1779, Priestley commenced the examination of this subject,
and drew from his experiments the conclusion, that plants absorb the
nitrogen of the air. Saussure shortly afterwards examined the same
subject, and having found, that when grown in a confined space of air,
and watered with pure water, the nitrogen of the plants underwent no
increase, he inferred that they derived their entire supplies of that
element from ammonia, or the soluble nitrogenous constituents of the
soil or manure. Boussingault has since re-examined this question, and by
a most elaborate series of experiments, in which the utmost care was
taken to avoid every source of fallacy, he was led to the conclusion,
that when haricots, oats, lupins, and cresses were grown in calcined
pumice-stone, mixed with the ash of plants, and supplied with air
deprived of ammonia and nitric acid, their nitrogen underwent no
increase. It has been objected to these experiments, that the plants
being confined in a limited bulk of air, were placed in an unnatural
condition, and Ville has recently repeated them with a current of air
passing through the apparatus, and found a slight increase in the
nitrogen, due, as he thinks, to direct absorption. It is much more
probable, however, that it depends on small quantities of ammonia or
nitric acid which had not been completely removed from the air by the
means employed for that purpose, for nothing is more difficult than the
complete abstraction of these substances, and as the gain of nitrogen
was only 0·8 grains, while 60,000 gallons of air, and 13 of water, were
employed in the experiment, which lasted for a considerable time, it is
reasonable to suppose that a sufficient quantity may have remained to
produce this trifling increase.

While these experiments show that plants maintain only a languid
existence when grown in air deprived of ammonia and nitric acid, and
hence, that the direct absorption of nitrogen, if it occur at all, must
do so to a very small extent, the addition of a very minute quantity of
the former substance immediately produces an active vegetation and rapid
increase in size of the plants. Among the most striking proofs of this
are the experiments of Wolff, made by growing barley and vetches in a
soil calcined so as to destroy organic matters, and then mixed with
small quantities of different compounds of ammonia. He found that when
the produce from the calcined soil was represented by 100, that from the
different ammoniacal salts was--

                        Barley.     Vetches.

Muriate of Ammonia       257·2      176·4
Carbonate of Ammonia     123·6      173·8
Sulphate of Ammonia      203·6      125·2

These experiments not only prove that ammonia can be absorbed, but they
also indirectly confirm the statement already made, that humus is not
necessary; for in some instances the produce was higher than that
obtained from the uncalcined soil with the same manures, although it
contained four per cent of humus.

On such experiments Liebig rests his opinion that ammonia is the
exclusive source of the nitrogen of plants, and although he has recently
admitted that it may be replaced by nitric acid, it is obvious that he
considers this a rare and exceptional occurrence. The evidence, however,
for the absorption of nitric acid appears to rest on as good grounds as
that of ammonia, for experience has shown that nitrate of soda acts
powerfully as a manure, and its effect must be due to the nitric acid,
and not to the soda, for the other compounds of that alkali have no such
effect. Wolff has illustrated this point by a series of experiments on
the sunflower, of which we shall quote one. He took two seeds of that
plant, and sowed them on the 10th May, in a soil composed of calcined
sand, mixed with a small quantity of the ash of plants, and added at
intervals during the progress of the experiment, a quantity of nitrate
of potash, amounting in all to 17·13 grains. The plants were watered
with distilled water, containing carbonic acid in solution, and the pot
in which they grew was protected from rain and dew by a glass cover. On
the 19th August one of the plants had attained a height of above 28
inches, and had nine fine leaves and a flower-bud; the other was about
20 inches high, and had ten leaves. On the 22d August, one of the plants
having been accidentally injured, the experiment was terminated. The
plants, which contained 103·16 grains of dry matter, were then carefully
analysed, and the quantity of nitrogen contained in the soil after the
experiment and in the seed was determined.

                                   Grains.
Nitrogen in the dry plants         1·737   }
    "    remaining in the soil     0·697   }  2·434

    "    in the nitrate of potash  2·370   }
    "    in the seeds              0·029   }  2·399
                                             ------
                            Difference        0·035

Hence, the nitrogen contained in the plants must, in this instance, have
been obtained entirely from the nitrate of potash, for the quantity
contained in it and in the seeds is exactly equal to that in the plants
and the soil, the difference of 0·03 grains being so small that it may
be safely attributed to the errors inseparable from such experiments.
For the sake of comparison, an exactly similar experiment was made on
two seeds grown without nitrate of potash, and in this instance, after
an equally long period of growth, the largest plant had only attained a
height of 7·5 inches, and had three small pale and imperfectly developed
leaves. They contained only 0·033 grains of nitrogen, while the seeds
contained 0·032--indicating that, under these circumstances, there was
no increase in the quantity of that element.

But, independently of these experimental results, it may be inferred
from general considerations, that nitric acid must be one of the sources
from which plants derive their nitrogen. It has been already stated,
that the humus contained in the soil consists of the remains of decayed
plants, and there is every reason to suppose that the primeval soil
contained no organic matters, and that the first generation of plants
must have derived the whole of their nitrogen from, the atmosphere. If,
therefore, it be assumed that ammonia is the only source of the
nitrogen of plants, it would follow, that as that substance cannot be
produced by the direct union of its elements, the quantity of ammonia in
the air could only remain undiminished in the event of the whole of the
nitrogen of decaying plants returning into that form. But this is
certainly not the case, for every time a vegetable substance is burned,
part of its nitrogen is liberated in the free state, and in certain
conditions of putrefaction, nitric acid is produced. Now, if ammonia be
the only form in which nitrogen is absorbed, there must be a gradual
diminution of the quantity contained in the air; and further, there must
either be some continuous source of supply by which its quantity is
maintained, or there must be some other substance capable of affording
nitrogen in a form fitted for the maintenance of plant life. As regards
the first alternative, it must be stated that we know of no source other
than the decomposition of plants from which ammonia can be derived, and
we are therefore compelled to adopt the second alternative, and to admit
that there must be some other source of nitrogen, and it cannot be
doubted, from what has been already stated, that it is from nitric acid
only that it can be obtained.

It must be admitted, then, that carbonic acid, ammonia, nitric acid, and
water, are the great organic foods of plants. But while they have
afforded to them an inexhaustible supply of the last, the quantity of
the other three available for food are limited, and insufficient to
sustain their life for a prolonged period. It has been shown by
Chevandrier, that an acre of land under beech wood accumulates annually
about 1650 lb. of carbon. Now, the column of air resting upon an acre of
land contains only about 15,500 lb. of carbon, and the soil may be
estimated to contain 1 per cent., or 22,400 lb. per acre, and the whole
of this carbon would therefore be removed, both from the air and the
soil, in the course of little more than 23 years. But it is a familiar
fact, that plants continue to grow with undiminished luxuriance year
after year in the same soil, and they do so because neither their carbon
nor their nitrogen are permanently absorbed; they are there only for a
period, and when the plant has finished its functions, and dies, they
sooner or later return into their original state. Either the plant
decays, in which case its carbon and nitrogen pass more or less rapidly
into their original state, or it becomes the food of animals, and by the
processes of respiration and secretion, the same change is indirectly
effected. In this way a sort of balance is sustained; the carbon, which
at one moment is absorbed by the plant, passes in the next into the
tissues of the animal, only to be again expired in that state in which
it is fitted to commence again its round of changes.

But while there is thus a continuous circulation of these constituents
through both plants and animals, there are various changes which tend to
liberate in the free state a certain quantity both of the carbon and
nitrogen of plants, and these being thus removed from the sphere of
organic life, there would be a gradual diminution in the amount of
vegetation at the earth's surface, unless this loss were counterbalanced
by some corresponding source of gain. In regard to carbonic acid the
most important source is volcanic action, but the loss of nitrogen,
which is far more important and considerable, is restored by the direct
combination of its elements. The formation of nitric acid during thunder
storms has been long familiar; but it would appear from the recent
experiments of Clöez, which, should they be confirmed by farther
enquiry, will be of much importance, that this compound is also
produced without electrical action when air is passed over certain
porous substances, saturated with alkaline and earthy compounds.
Fragments of calcined brick and pumice stone were saturated with
solution of carbonate of potash, with carbonates of lime and magnesia
and other mixtures, and a current of air freed from nitric acid and
ammonia passed over them for a long period, at the end of which notable
quantities of nitric acid were detected.

_Source of the Inorganic Constituents of Plants._--The inorganic
constituents of plants being all fixed substances, it is sufficiently
obvious that they can only be obtained from the soil, which, as we shall
afterwards see, contains all of them in greater or less abundance, and
has always been admitted to be the only substance capable of supplying
them. The older chemists and physiologists, however, attributed no
importance to these substances, and from the small quantities in which
they are found in plants, imagined that they were there merely
accidental impurities absorbed from the soil along with the humus, which
was at that time considered to be their organic food. This opinion,
sufficiently disproved by the constant occurrence of the same substances
in nearly the same proportions, in the ash of each individual plant, has
been further refuted by the experiments of Prince Salm Horstmar, who has
established their importance to vegetation, by experiments upon oats
grown on artificial soils, in each of which one inorganic constituent
was omitted. He found that, without silica, the grain vegetated, but
remained small, pale in colour, and so weak as to be incapable of
supporting itself; without lime, it died when it had produced its second
leaf; without potash and soda, it grew only to the height of three
inches; without magnesia, it was weak and incapable of supporting
itself; without phosphoric acid, weak but upright; and without sulphuric
acid, though normal in form, the plant was feeble, and produced no
fruit.

_Manner in which the Constituents of Plants are absorbed._--Having
treated of the sources of the elements of plants, it is necessary to
direct attention to the mode in which they enter their system.

_Water._--The absorption of water by plants takes place in great
abundance, and is connected with many of the most important phenomena of
vegetation. It is principally absorbed by the roots, and passes into the
tissues of the plant, where a part of it is decomposed, and goes to the
formation of certain of its organic compounds; while by far the larger
quantity, in place of remaining in it, is again exhaled by the leaves.
The extent to which this takes place is very large. Hales found that a
sunflower exhaled in twelve hours about 1 lb. 5 oz. of water, but this
quantity was liable to considerable variation, being greater in dry, and
less in wet weather, and much diminished during the night. Saussure made
similar experiments, and observed that the quantity of water exhaled by
a sunflower amounted to about 220 lb. in four months. The exhalation of
plants has recently been examined with great accuracy by Lawes. His
experiments were made by planting single plants of wheat, barley, beans,
peas, and clover, in large glass jars capable of holding about 42 lb. of
soil, and covered with glass plates, furnished with a hole in the centre
for the passage of the stem of the plant. Water was supplied to the soil
at certain intervals, and the jars were carefully weighed. The result of
the experiments, continued during a period of 172 days, is given in the
following table, which shows the total quantity of water exhaled in
grains:--

Wheat                    113,527
Barley                   120,025
Beans                    112,231
Peas                     109,082
Clover, cut 28th June     55,093

It further appears, that the exhalation is not uniform, but increases
during the active growth of the plant, and diminishes again when that
period is passed. These variations are shown by the subjoined tables, of
which the first gives the total exhalation, and the second the average
daily loss of water during certain periods.


TABLE I.--_Showing the Number of Grains of Water given off by the Plants
during stated divisional Periods of their Growth._

+-----------+--------+--------+--------+--------+--------+--------+---------+
|Description|9 Days. |31 Days.|27 Days.|34 Days.|30 Days.|14 Days.| 27 Days.|
|           +--------+--------+--------+--------+--------+--------+---------+
|of Plant.  | From   |From    |From    |From    |From    |From    | From    |
|           | Mar. 19|Mar. 28 |Apr. 28 |May 25  |June 28 |July 28 | Aug. 11 |
|           | to     |to      |to      |to      |to      |to      | to      |
|           |Mar. 28.|Apr. 28.|May 25. |June 28.|July 28.|Aug. 11.| Sept. 7.|
+-----------+--------+--------+--------+--------+--------+--------+---------+
|Wheat      | 129    | 1268   |  4,385 | 40,030 | 46,060 | 15,420 |   6235  |
|Barley     | 129    | 1867   | 12,029 | 37,480 | 45,060 | 17,046 |   6414  |
|Beans      |  88    | 1854   |  4,846 | 30,110 | 58,950 | 12,626 |   3657  |
|Pease      | 101    | 1332   |  2,873 | 36,715 | 62,780 |  5,281 |    ...  |
|Clover     | 400    | 1645   |  2,948 | 50,100 |   ...  |    ... |    ...  |
+-----------+--------+--------+--------+--------+--------+--------+---------+


TABLE II.--_Showing the average daily Loss of Water (in Grains) by the
Plants, within several stated divisional Periods of their Growth._

+------------+--------+--------+--------+--------+--------+--------+--------+
| Description|9 Days. |31 Days.|27 Days.|34 Days.|30 Days.|14 Days.|27 Days.|
|            +--------+--------+--------+--------+--------+--------+--------+
| of Plant.  | From   | From   | From   | From   | From   | From   | From   |
|            |Mar. 19 |Mar. 28 |Apr. 28 |May 25  |June 28 |July 28 |Aug. 11 |
|            | to     | to     | to     | to     | to     | to     | to     |
|            |Mar. 28.|Apr. 28.|May 25. |June 28.|July 28.|Aug. 11.|Sept. 7.|
+------------+--------+--------+--------+--------+--------+--------+--------+
| Wheat      | 14·3   | 40·9   | 162·4  | 1177·4 | 1535·3 | 1101·4 | 230·9  |
| Barley     | 14·3   | 60·2   | 445·5  | 1102·3 | 1502·0 | 1217·6 | 237·5  |
| Beans      |  9·7   | 59·8   | 179·5  |  885·6 | 1965·0 |  901·8 | 135·4  |
| Peas       | 11·2   | 42·9   | 106·4  | 1079·8 | 2092·7 |  377·2 |  ...   |
| Clover     | 44·4   | 53·0   | 109·2  | 1473·5 |   ...  |   ...  |  ...   |
+------------+--------+--------+--------+--------+--------+--------+--------+

Similar experiments were made with the same plants in soils to which
certain manures had been added, and with results generally similar.
Calculating from these experiments, we are led to the apparently
anomalous conclusion that the quantity of water exhaled by the plants
growing on an acre of land greatly exceeds the annual fall of rain;
although it is obvious that of all the rain which falls, only a small
proportion can be absorbed by the plants growing on the soil, for a
large quantity is carried off by the rivers, and never reaches their
roots. It has been calculated, for instance, that the Thames carries off
in this way at least one-third of the annual rain that falls in the
district watered by it, and the Rhine nearly four-fifths. Of course this
large exhalation must depend on the repeated absorption of the same
quantity of water, which, after being exhaled, is again deposited on the
soil in the form of dew, and passes repeatedly through the plant. This
constant percolation of water is of immense importance to the plant, as
it forms the channel through which some of its other constituents are
carried to it.

_Carbonic Acid._--While the larger part of the water which a plant
requires is absorbed by its roots, the reverse is the case with carbonic
acid. A certain proportion no doubt is carried up through the roots by
the water, which always contains a quantity of that gas in solution, but
by far the larger proportion is directly absorbed from the air by the
leaves. A simple experiment of Boussingault's illustrates this
absorption very strikingly. He took a large glass globe having three
apertures, through one of which he introduced the branch of a vine, with
twenty leaves on it. With one of the side apertures a tube was
connected, by means of which the air could be drawn slowly through the
globe, and into an apparatus in which its carbonic acid was accurately
determined. He found, in this way, that while the air which entered the
globe contained 0·0004 of carbonic acid, that which escaped contained
only 0·0001, so that three-fourths of the carbonic acid had been
absorbed.

_Ammonia and Nitric Acid._--Little is known regarding the mode in which
these substances enter the plant. It is usually supposed that they are
entirely absorbed by the roots, and no doubt the greater proportion is
taken up in this way, but it is very probable that they may also be
absorbed by the leaves, at least the addition of ammonia to the air in
which plants are grown, materially accelerates vegetation. It is
probable, however, that the rain carries down the ammonia to the roots,
and there is no doubt that that derived from the decomposition of the
nitrogenous matters in the soil is so absorbed.

_Inorganic Constituents._--The inorganic constituents of course are
entirely absorbed by the roots; and it is as a solvent for them that the
large quantity of water continually passing through the plants is so
important. They exist in the soil in particular states of combination,
in which they are scarcely soluble in water. But their solubility is
increased by the presence of carbonic acid contained in the water, and
which causes it to dissolve, to some extent, substances otherwise
insoluble. It is in this way that lime, which occurs in the soil
principally as the insoluble carbonate, is dissolved and absorbed. And
phosphate of lime is also taken up by water containing carbonic acid, or
even common salt in solution. The amount of solubility produced by these
substances is extremely small; but it is sufficient for the purpose of
supplying to the plant as much of its mineral constituents as are
required, for the quantity of water which, as we have already seen,
passes through a plant is very large when compared with the amount of
inorganic matters absorbed. It has been shown by Lawes and Gilbert, that
about 2000 grains of water pass through a plant for every grain of
mineral matter fixed in it, so that there is no difficulty in
understanding how the absorption takes place.

It is worthy of notice, however, that the absorption of the elements of
plants takes place even though they may not be in solution in the soil,
the roots apparently possessing the power of directly acting on and
dissolving insoluble matters; but a distinction must be drawn between
this and the view entertained by Jethro Tull, who supposed that they
might be absorbed in the solid state, provided they were reduced to a
state of sufficient comminution. It is now no longer doubted that,
whatever action the roots may exert, the constituents of the plant must
be in solution before they can pass into it--experiment having
distinctly shown that the spongioles or apertures through which this
absorption takes place are too minute to admit even the smallest solid
particle.




CHAPTER II.

THE PROXIMATE CONSTITUENTS OF PLANTS.


The substances absorbed by the plant, which are of simple composition,
and contain only two elements, are elaborated within it, and converted
into the many complicated compounds of which its mass is composed. Some
of these, as, for example, the colouring matters of madder and indigo,
the narcotic principle of the poppy, &c., are confined to a single
species, or small group of plants, while others are found in all plants,
and form the main bulk of their tissues. The latter are the only
substances which claim notice in a treatise like the present. They have
been divided into three great classes, of widely different properties,
composition, and functions.

_1st. The Saccharine and Amylaceous Constituents._--These substances are
compounds of carbon, hydrogen, and oxygen, and all possess a certain
degree of similarity in composition, the quantities of hydrogen and
oxygen they contain being always in the proportion required to form
water, so that they may be considered as compounds of carbon and water;
not that it can be asserted that they actually do contain water, as
such, for of that there is no evidence, but only that its elements are
present in the proportion to form it.

_Cellulose._--This substance forms the fundamental part of all plants.
It is the principal constituent of woody fibre, and is found in a state
of purity in the fibre of cotton and flax, and in the pith of plants;
but in wood it is generally contaminated with another substance, which
has received the name incrusting matter, because it is deposited in and
around the cells of which the plant is in part composed. Cellulose is
insoluble in all menstrua, but, when boiled for a long time with
sulphuric acid, is converted into a substance called dextrine. Cellulose
consists of--

           From pith of Elder-tree.  Spongioles of roots.

Carbon              43·37                43·00
Hydrogen             6·04                 6·18
Oxygen              50·59                50·82
                  -------              -------
                   100·00               100·00

It is represented chemically by the formula, C_{24}H_{21}O_{21}, which
shows it to be a compound of 24 atoms of carbon with 21 of hydrogen and
21 of oxygen.

_Incrusting matter._--Large quantities of this substance enter into the
composition of all plants. Of its chemical nature little is known, as it
cannot be obtained separate from cellulose, but it is analogous to that
substance in its composition, and probably contains hydrogen and oxygen
in the proportion to form water.

_Starch._--Starch is one of the most abundant constituents of plants,
and is found in most seeds, as those of the cereals and the leguminous
plants; in the tubers of the potatoe, the bulbs of tulips, &c. &c. It is
obtained by placing a quantity of wheat flour in a bag, and kneading it
under a gentle stream of water. When the water is allowed to stand, it
deposits the starch as a fine white powder, which, when examined by the
microscope, is found to be composed of minute grains, formed of
concentric layers deposited on one another. These grains vary
considerably in size and structure in different plants; but in the same
plant they are generally so much alike as to admit of their recognition
by a practised observer. They were formerly believed to be composed of
an external coating of a substance insoluble in water, and containing in
their interior a soluble kernel; but this opinion has been refuted, and
distinct evidence been brought to show that the exterior and interior of
the globules are identical in chemical properties. Starch is insoluble
in cold water, but by boiling, it dissolves, forming a thick paste. By
long continued boiling with water containing a small quantity of acid,
it is completely dissolved and converted into dextrine, and eventually
into sugar. The same change is produced by the action of fermenting
substances, such as the extract of malt; when heated in the dry state to
a temperature of about 390 Fahr., it becomes soluble in cold water. It
is distinguished by giving a brilliant blue compound with iodine. Starch
contains--

     Carbon    44·47
     Hydrogen   6·28
     Oxygen    49·25
              ------
              100·00

and its composition is represented by the formula C_{12}H_{10}O_{10}, so
that it differs but little from cellulose in composition, although its
chemical functions in the plant are extremely different. It is connected
with some of the most important changes which occur in the growing
plants, and by a series of remarkable transformations is converted into
sugar and other important compounds.

_Lichen Starch_ is found in most species of lichens, and is
distinguished from common starch by producing a green colour with
iodine. Its composition is the same as that of ordinary starch.

_Inuline._--The species of starch to which this name is given is
characterised by its dissolving in boiling water, and giving a white
pulverulent deposit in cooling. It is found in the tuber of the dahlia,
in the dandelion, and some other plants. Its composition is identical
with that of cellulose, and its formula is C_{24}H_{21}O_{21}.

_Gum_ is excreted from various plants as a thick fluid, which dries up
into transparent masses. Its composition is identical with that of
starch. It dissolves readily in cold water, and is converted into sugar
by long continued boiling with acids. Its properties are best marked in
gum arabic, which is obtained from various species of acacia; that from
other plants differs to some extent, although its chemical composition
is the same.

_Dextrine._--When starch is exposed to a heat of about 400°, or when
treated with sulphuric acid, or with a substance extracted from malt
called _diastase_, it is converted into dextrine. It may also be
obtained from cellulose by a similar treatment. The dextrine so obtained
has the same composition as the starch from which it is produced, but
its properties more nearly resemble those of gum. It plays a very
important part in the process of germination, and may be converted into
sugar on the one hand, and apparently also into starch on the other.

_Sugar._--Under this name are included four or five distinct substances,
of which the most important are, cane sugar, grape sugar, and the
uncrystallisable sugar found in many plants.

_Cane Sugar._--This variety of sugar, as its name implies, is found most
abundantly in the sugar cane, but it occurs also in the maple,
beet-root, and various species of palms, from all of which it is
extracted on the large scale. It is extremely soluble in water, and can
be obtained in large transparent prismatic crystals, as in common
sugar-candy. It swells up, and is converted into a brown substance
called caramel, when heated, and by contact with fermenting substances,
yields alcohol and carbonic acid. It contains--

     Carbon    42·22
     Hydrogen   6·60
     Oxygen    51·18
              ------
              100·00

and its chemical formula is C_{12}H_{11}O_{11}.

_Grape Sugar_ is met with in the grape, and most other fruits, as well
as in honey. It is produced artificially when starch is boiled for a
long time with sulphuric acid, or treated with a large quantity of
diastase. It is less soluble in water than cane sugar, and crystallises
in small round grains. Its composition, when dried at 284°, is--

     Carbon    40·00
     Hydrogen   6·66
     Oxygen    53·34
              ------
              100·00

and its formula is C_{12}H_{12}O_{12}; but when crystallised it contains
two equivalents of water, and is then represented by the formula
C_{12}H_{12}O_{12} + 2H_{2}O.

The uncrystallisable sugar of plants is closely allied to grape sugar,
and, so far as at present known, has the same composition, although,
from the difficulty of obtaining it quite free from crystallised sugar,
this is still uncertain.

_Mucilage_ is the name applied to the substance existing in linseed,
and in many other seeds, and which communicates to them the property of
swelling up and becoming gelatinous when treated with water. It is found
in a state of considerable purity in gum tragacanth and some other gums.
Its composition is not known with absolute certainty, but it is either
C_{24}H_{19}O_{19}, or C_{12}H_{10}O_{10}; and in the latter case it
must be identical with starch and gum.

It will be observed that all the substances belonging to this class are
very closely related in chemical composition, some of them, as starch
and gum, though easily distinguished by their properties, being
identical in constitution, while others only differ in the quantity of
water, or of its elements which they contain. In fact, they may all be
considered as compounds of carbon and water, and their relations are,
perhaps, more distinctly seen when their formulæ are written so as to
show this, as is done in the following table, in the second column of
which those containing twelve equivalents of carbon are doubled, so as
to make them comparable with cellulose:--

                                                            Water.
Grape sugar,   C_{12}H_{12}O_{12}   C_{24}H_{24}O_{24}   C_{24} + 24
Cane sugar,    C_{12}H_{11}O_{11}   C_{24}H_{22}O_{22}   C_{24} + 22
Cellulose,     C_{24}H_{21}O_{21}   C_{24}H_{21}O_{21}   C_{24} + 21
Inuline,       C_{24}H_{21}O_{21}   C_{24}H_{21}O_{21}   C_{24} + 21
Starch,        C_{12}H_{10}O_{10}   C_{24}H_{20}O_{20}   C_{24} + 20
Dextrine,      C_{12}H_{10}O_{10}   C_{24}H_{20}O_{20}   C_{24} + 20
Gum,           C_{12}H_{10}O_{10}   C_{24}H_{20}O_{20}   C_{24} + 20
Mucilage,      C_{12}H_{10}O_{10}   C_{24}H_{20}O_{20}   C_{24} + 20

The relation between these substances being so close, it is not
difficult to understand how one may be converted into another by the
addition or subtraction of water. Thus, cellulose has only to absorb an
equivalent of water to become grape sugar, or to lose an equivalent in
order to be converted into starch, and we shall afterwards see that
such changes do actually occur in the plant during the process of
germination.

_Pectine and Pectic Acid._--These substances are met with in many fruits
and roots, as, for instance, in the apple, the carrot, and the turnip.
They differ from the starch group in containing more oxygen than is
required to form water along with their hydrogen; but their exact
composition is still uncertain, and they undergo numerous changes during
the ripening of the fruit.

_2d. Oily or Fatty Matters._--The oily constituents of plants form a
rather extensive group of substances all closely allied, but
distinguished by minor differences in properties and constitution. Some
of them are very widely distributed throughout the vegetable kingdom,
but others are almost peculiar to individual plants. They are all
compounds of carbon, hydrogen, and oxygen, and are at once distinguished
from the preceding class, by containing much less oxygen than is
required to form water with their hydrogen. The principal constituents
of the fatty matters and oils of plants are three substances, called
stearine, margarine, and oleine, the two former solids, the latter a
fluid; and they rarely, if ever, occur alone, but are mixed together in
variable proportions, and the fluidity of the oils is due principally to
the quantity of the last which they contain. If olive oil be exposed to
cold, it is seen to become partially solid; and if it be then pressed, a
fluid flows out, and a crystalline substance remains; the former is
oleine, though not absolutely pure, and the latter margarine. The
perfect separation of these substances involves a variety of troublesome
chemical processes; and when it has been effected, it is found that each
of them is a compound of a peculiar acid, with another substance having
a sweet taste, and which has received the name of glycerine, or the
sweet principle of oil. Glycerine, as it exists in the fats, appears to
be a compound of C_{3}H_{2}O, and its properties are the same from
whatever source it is obtained. The acids separated from it are known by
the names of margaric, stearic, and oleic acids.

_Margaric Acid_ is best obtained pure by boiling olive oil with an
alkali until it is saponified, and decomposing the soap with an acid,
expressing the margaric acid, which separates, and crystallising it from
alcohol. It is a white crystalline fusible solid, insoluble in water,
but soluble in alcohol and in solutions of the alkalies. Its composition
is--

     Carbon     75·56
     Hydrogen   12·59
     Oxygen     11·85
               ------
               100·00

and its formula C_{34}H_{34}O_{4}.

_Stearic Acid._--Although this acid exists in many plants, it is most
conveniently extracted from lard. It is a crystalline solid less fusible
than margaric acid, but closely resembling it in its other properties.
Its formula is C_{36}H_{36}O_{4}.

_Oleic Acid._--Under this name two different substances appear to be
included. It has been applied generally to the fluid acids of all oils,
while it would appear that the drying and non-drying oils actually
contain substances of different composition. The acid extracted from
olive oil appears to have the formula C_{36}H_{34}O_{4}, while that from
linseed oil is C_{46}H_{38}O_{6}, but this is still doubtful.

Other fatty acids have been detected in palm oil, cocoa-nut oil, &c.
&c., which so closely resemble margaric and stearic acids as to be
easily confounded with them. Though presenting many points of interest,
it is unnecessary to describe them in detail here.

_Wax_ is a substance closely allied to the oils. It consists of two
substances, cerine and myricine, which are separated from one another by
boiling alcohol, in which the former is more soluble. They are extremely
complex in composition, the former consisting principally of an acid
similar to the fatty acids, called cerotic acid, and containing
C_{54}H_{54}O_{4}. The latter has the formula C_{92}H_{92}O_{4}. The wax
found in the leaves of the lilac and other plants appears to consist of
myricine, while that extracted from the sugar-cane is said to be
different, and to have the formula C_{48}H_{50}O_{2}. It is probable
that other plants contain different sorts of wax, but their
investigation is still so incomplete, that nothing definite can be said
regarding them. Wax and fats appear to be produced in the plant from
starch and sugar; at least it is unquestionable that the bee is capable
of producing the former from sugar, and we shall afterwards see that a
similar change is most probably produced in the plant. The fatty matters
contained in animals are identical with those of plants.

_3d. Nitrogenous or Albuminous Constituents of Plants and Animals._--The
nitrogenous constituents of plants and animals are so closely allied,
both in properties and composition, that they may be most advantageously
considered together.

_Albumen._--Vegetable albumen is found dissolved in the juices of most
plants, and is abundant in that of the potato, the turnip, and wheat. In
these juices it exists in a soluble state, but when its solution is
heated to about 150°, it coagulates into a flocky insoluble substance.
It is also thrown down by acids and alcohol. Coagulated albumen is
soluble in alkalies and in nitric acid. Animal albumen exists in the
white of eggs, the serum of blood, and the juice of flesh; and from all
these sources is scarcely distinguishable in its properties from
vegetable albumen.

It is a substance of very complicated composition, and chemists are not
agreed as to the formula by which its constitution is to be expressed, a
difficulty which occurs also with most of the other nitrogenous
compounds. The results of the analyses of albumen from different sources
are however quite identical, as may be seen from those subjoined--

             From    From       From    From
             Wheat.  Potatoes.  Blood.  White of Egg.
Carbon        53·7    53·1        53·4      53·0
Hydrogen       7·1     7·2         7·0       7·1
Nitrogen      15·6     ...        15·5      15·6
Oxygen      }      {   ...        22·1      22·9
Sulphur     } 23·6 {  0·97         1·6       1·1
Phosphorus  }      {   ...         0·4       0·3
             -----               -----     -----
             100·0               100·0     100·0

Closely allied to vegetable albumen is the substance known by the name
of _glutin_, which is obtained by boiling the gluten of wheat with
alcohol. It appears to be a sort of coagulated albumen, with which its
composition completely agrees.

_Vegetable Fibrine._--If a quantity of wheat flour be tied up in a piece
of cloth, and kneaded for some time under water, the starch it contains
is gradually washed out, and there remains a quantity of a glutinous
substance called gluten. When this is boiled with alcohol, the _glutin_
above referred to is extracted, and vegetable fibrine is left. It
dissolves in dilute potash, and on the addition of acetic acid is
deposited in a pure state. Treated with hydrochloric acid, diluted with
ten times its weight of water, it swells up into a jelly-like mass.
When boiled or preserved for a long time under water, it cannot be
distinguished from coagulated albumen.

_Animal Fibrine_ exists in the blood and the muscles, and agrees in all
its characters and composition with vegetable fibrine, as is shown by
the subjoined analyses--

             Wheat Flour.     Blood.    Flesh.
Carbon         53·1            52·5      53·3
Hydrogen        7·0             6·9       7·1
Nitrogen       15·6            15·5      15·3
Oxygen         23·2            24·0      23·1
Sulphur         1·1             1·1       1·2
              -----           -----     -----
              100·0           100·0     100·0

_Caseine._--Vegetable caseine exists abundantly in most plants,
especially in the seeds, and remains in the juice after albumen has been
precipitated by heat, from which it may be separated in flocks by the
addition of an acid. It has been obtained for chemical examination,
principally from peas and beans, and from the almond and oats. When
prepared from the pea it has been called _legumine,_ from almonds
_emulsine_, and from oats _avenine_; but they are all three identical in
their properties, although formerly believed to be different, and
distinguished by these names. Vegetable caseine is best obtained by
treating peas or beans with hot water, and straining the fluid. On
standing, the starch held in suspension is deposited, and the caseine is
retained in solution in the alkaline fluid; by the addition of an acid
it is precipitated as a thick curd. Caseine is insoluble in water, but
dissolves readily in alkalies; its solution is not coagulated by heat,
but, on evaporation, becomes covered with a thin pellicle, which is
renewed as often as it is removed.

_Animal Caseine_ is the principal constituent of milk, and is obtained
by the cautious addition of an acid to skimmed milk, by which it is
precipitated as a thick white curd. It is also obtained by the use of
rennet, and the process of curding milk is simply the coagulation of its
caseine. It is soluble in alkalies, and precipitated from its solution
by acids, and in all other respects agrees with vegetable caseine.

The composition of animal caseine has been well ascertained, but
considerable doubt still exists as to that of vegetable caseine, owing
to the difficulty of obtaining it absolutely pure. The analyses of
different chemists give rather discordant results, but we have given
those which appear most trustworthy--

                      From Peas.
     Carbon          50·6    50·7
     Hydrogen         6·8     6·6
     Nitrogen        16·5    15·8
     Oxygen          25·6    23·8
     Sulphur          0·5     0·8
     Phosphorus       ...     2·3
                    -----   -----
                    100·0   100·0

Other results differ considerably from these, and some observers have
even obtained as much as eighteen per cent of nitrogen and fifty-three
of carbon.

The composition of animal caseine differs from this principally in the
amount of carbon. Its composition is--

     Carbon       53·6
     Hydrogen      7·1
     Nitrogen     15·8
     Oxygen       22·5
     Sulphur       1·0
                 -----
                 100·0

The most cursory examination of these analytical numbers is sufficient
to show that a very close relation subsists between the different
substances just described. Indeed, with the exception of vegetable
caseine, they may be said all to present the same composition; and, as
already mentioned, there are analyses of it which would class it
completely with the others. While, however, the quantities of carbon,
hydrogen, nitrogen, and oxygen are the same, differences exist in the
sulphur and phosphorus they contain, and which, though very small in
quantity, are indubitably essential to them. Much importance has been
attributed to these constituents by various chemists, and especially by
Mulder, who has endeavoured to make out that all the albuminous
substances are compounds of a substance to which he has given the name
of _proteine_, with different quantities of sulphur and phosphorus. The
composition of proteine, according to his newest experiments, is--

     Carbon       54·0
     Hydrogen      7·1
     Nitrogen     16·0
     Oxygen       21·4
     Sulphur       1·5
                 -----
                 100·0

and is exactly the same from whatever albuminous compound it is
obtained. Although the importance of proteine is probably not so great
as Mulder supposed, it affords an important illustration of the close
similarity of the different substances from which it is obtained, the
more especially as there is every reason to believe that the different
albuminous compounds are capable of changing into one another, just as
starch and sugar are mutually convertible; and the possibility of this
change throws much light on many of the phenomena of nutrition in plants
and animals. Indeed, it would seem probable that these compounds are
formed from their elements by plants only, and are merely assimilated by
animals to produce the nitrogenous constituents they contain.

_Diastase_ is the name applied to a substance existing in malt, and
obtained by macerating that substance with cold water, and adding a
quantity of alcohol to the fluid, when the diastase is immediately
precipitated in white flocks. It is produced during the malting process,
and is not found in the unmalted barley. Its chemical composition is
unknown, but it is nitrogenous, and is believed to be produced by the
decomposition of gluten. If a very small quantity of diastase be mixed
with starch suspended in hot water, the starch is found gradually to
dissolve, and to pass first into the state of dextrine, then into that
of sugar. The change thus effected takes place also in a precisely
similar manner in the plant, diastase being produced during the process
of germination of all seeds and tubers, for the purpose of effecting
this change, and to fulfil other functions less understood, but no doubt
equally important. Diastase is found in the seeds only during the period
when the starch they contain is passing into sugar; as soon as that
change has taken place, its function is ended, and it disappears.




CHAPTER III.

THE CHANGES WHICH TAKE PLACE IN THE FOOD OF PLANTS DURING THEIR GROWTH.


The simple compounds which the plant absorbs from the atmosphere and
soil are elaborated within its system, and converted into the various
complex substances of which its tissues are composed, by a series of
changes, the details of which are still in some respects imperfectly
known, although their general nature is sufficiently well understood.
They may be best rendered intelligible by reference, in the first
instance, to the changes occurring during germination, when the young
plant is nourished by a supply of food stored up in the seed, in
sufficient quantity to maintain its existence until the organs by which
it is afterwards to draw its nutriment from the air and soil are
sufficiently developed to serve that purpose.

_Changes occurring during Germination._--When a seed is placed in the
soil under favourable circumstances, it becomes the seat of an important
and remarkable series of chemical changes, which result in the
production of the young plant. Experiment and observation have shown
that heat, moisture, and air, are necessary to the production of these
changes, and though probably not absolutely essential, the absence of
light is favourable in the early stages. The temperature required for
germination varies greatly in different seeds, some germinating readily
at a few degrees above the freezing point, and others requiring a
tolerably high temperature. The rapidity with which it takes place
appears to increase with the temperature; but this is true only within
very narrow limits, for beyond a certain point heat is injurious, and
when it exceeds 120° or 130° Fahrenheit, entirely prevents the process.
The presence of oxygen is also essential, for it has been shown that if
seeds are placed in a soil exposed to an atmosphere deprived of that
element, or if they be buried so deep that the air does not reach them,
they may lie without change for an unlimited period; but so soon as they
are exposed to the air, germination immediately commences. Illustrations
of this fact are frequently observed where earth from a considerable
depth has been thrown up to the surface, when it often becomes covered
with plants not usually seen in the neighbourhood, which have sprung
from buried seeds. When all the necessary conditions for germination are
fulfilled, the seed absorbs moisture, swells up, and sends out a shoot
which rises to the surface, and a radicle which descends--the one
destined to develop the leaves, the other the roots, by which the plant
is afterwards to derive its nutriment from the air and the soil. But
until these organs are properly developed, the plant is dependent on the
matters contained in the seed itself. These substances are mostly
insoluble, but are brought into solution by the atmospheric oxygen
acting upon the gluten, and converting it into a soluble substance
called diastase, which in its turn reacts upon the starch, converting it
first into dextrine, and then into cellulose, and the latter is finally
deposited in the form of organised cells, and produces the first little
shoot of the plant. At the first moment of germination, the oxygen
absorbed appears simply to oxidize the constituents of the seed, but
this condition exists only for a very limited period, and is soon
followed by the evolution of carbonic acid, water being at the same time
formed from the organic constituents of the seed, which gradually
diminishes in weight. The amount of this diminution is different with
different plants, but always considerable. Boussingault found that the
loss of dry substance in the pea amounted in 26 days to 52 per cent, and
in wheat to 57 per cent in 51 days. Against this, of course, is to be
put the weight of the young plant produced; but this is never sufficient
to counterbalance the diminished weight of the seed, for Saussure found
that a horse bean and the plant produced from it weighed, after 16 days,
less by 29 per cent than the seed before germination. The same
phenomenon is observed in the process of malting, which is in fact the
artificial germination of barley, the malt produced always weighing
considerably less than the grain from which it was obtained. It was
believed by Saussure, and the older investigators, that the carbonic
acid evolved was entirely produced from starch and sugar; and as these
substances may be viewed as compounds of carbon and water, the change
was very simply explained by supposing that the carbon was oxidised and
converted into carbonic acid and its water eliminated. But this
hypothesis is incapable of explaining all the phenomena observed; for
woody fibre, which is one of the chief constituents of the young plant,
contains more carbon than the starch and sugar from which it must have
been produced, and we are, therefore, forced to admit that the action
must be more complicated. There is every reason to believe that the
nitrogenous constituents of the seed are most abundantly oxidized, for
they are remarkably prone to change; but the action of the air is not
confined to them, and it appears most probable that all the substances
take part in the decomposition, and the process of germination may, in
some respects, be compared to decay or putrefaction, which, like it, is
attended by the absorption of oxygen and evolution of carbonic acid; but
while in the latter case the residual substances remain in a useless
state, in the former they at once become part of a new organism.

_Changes occurring during the After-growth of the Plant._--When the
plant has developed its roots and leaves, and exhausted the store of
materials laid up for it in the seed, it begins to derive its
subsistence from the surrounding air, and to absorb carbonic acid,
water, ammonia, and nitric acid, and to decompose and convert them into
the different constituents of its tissues. These changes take place
slowly at first, and more rapidly as the organs fitted for the
elaboration of its food are developed. The roots and the leaves are
equally active in performing this duty, the former absorbing the mineral
matters along with the carbonic acid, ammonia, nitric acid, and moisture
in the soil, or the manure added to it; the latter gathering the gaseous
substances existing in the air. Each of these undergoes a series of
changes claiming our consideration.

_Decomposition of Carbonic Acid._--Carbonic acid, which appears to be
absorbed with equal readiness by the roots, leaves, and stems, undergoes
immediate decomposition, its carbon being retained, and its oxygen, in
whole or in part, evolved into the air. This decomposition occurs only
under the action of the sun's rays, and has been found to be
proportionate to the amount of light to which the plant is exposed. It
takes place only in the green parts of plants, for though the roots
absorb carbonic acid, they cannot decompose it, or evolve oxygen; and
the coloured parts, the flowers, fruits, etc., have an entirely opposite
effect, absorbing oxygen and giving off carbonic acid. The absorption of
carbonic acid and escape of oxygen has been proved by numerous direct
experiments by Saussure and others, in which both atmospheric air and
artificial mixtures containing an increased quantity of carbonic acid
have been employed. Saussure allowed seven plants of periwinkle (_Vinca
minor_) to vegetate in an atmosphere containing 7·5 per cent of carbonic
acid for six days, during each of which the apparatus was exposed for
six hours to the sun's rays. The air was analysed both before and after
the experiment, and the results obtained were--

                        Volume                              Carbonic
                        of the air.   Nitrogen.   Oxygen.   Acid.
Before the experiment,   5746           4199       1116       431
After       "            5746           4338       1408         0
                         ----           ----       ----      ----
Difference,                 0           +139       +292      -431

In this experiment the whole of the carbonic acid, amounting to 431
volumes, was absorbed, but only 292 volumes of oxygen were given off.
Had the carbonic acid been entirely decomposed, and all its oxygen
eliminated, its volume would have been equal to that of the acid, or
431, so that in this instance 139 volumes of the oxygen of the carbonic
acid have been retained to form part of the tissues of the plant. On the
other hand, the nitrogen is found to be increased after the experiment.
It might be supposed that the nitrogen evolved had been derived from the
decomposition of the nitrogenous constituents of the plant, but this
cannot be the true explanation, because in this particular case it
greatly exceeded the whole nitrogen contained in the plants
experimented on. Its source is not well understood, but Boussingault
supposes it to have existed in the interstices of the plant, and to have
escaped during the course of the experiment. Saussure found that the
oak, the horse-chesnut, and other plants, absorb oxygen and give off
carbonic acid in less volumes than the oxygen, while the house-leek and
the cactus absorb oxygen without evolving carbonic acid. The absorption
and decomposition of carbonic acid takes place only during the day, and
matters are entirely reversed during the night, when oxygen is absorbed
and carbonic acid eliminated from all parts of the plants.

Although the action occurring during the night is the reverse of that
which takes place during the day, it is in no degree to be attributed to
a re-oxidation of the carbon which had been deposited in the tissues of
the plant. It appears, on the contrary, to be a purely mechanical, and
not a chemical process. During the night the sap continues to circulate
through the vessels of the plant, and moisture, carrying with it
carbonic acid in solution, is absorbed by the roots; but when it reaches
the leaves, where the sun's light would have caused its decomposition
during the day, it is again exhaled unchanged. The oxygen absorbed
during the night must, however, take part in some chemical processes,
for if it were merely mechanical, the absorption would not be confined
to that gas alone, but would be participated in by the other
constituents of the air. Moreover, the amount of absorption varies
greatly in different plants--being scarcely appreciable in some, and
very abundant in others. Plants containing volatile oils, which are
readily converted into resins by the action of oxygen, or those
containing tannin or other readily oxidizable substances, take up the
largest quantity. This is remarkably illustrated by an experiment in
which the leaves of the Agave americana, after twenty-four hours'
exposure in the dark, were found to have absorbed only 0·3 of their
volume of oxygen, while those of the fir, in which volatile oil is
abundant, had taken up twice, and those of the oak, containing tannin,
eighteen times as much oxygen.

In the flowers, both by day and night, there is a constant absorption of
oxygen, and evolution of carbonic acid. In fact, an active oxidation is
going on, attended by the evolution of heat, which, in the _Arum
maculatum_ and some other plants, is so great as to raise the
temperature of the flower 10° or 12° above that of the surrounding air.

_Decomposition of Water in the Plant._--In addition to the function
which water performs in the plant, as the solvent of the different
substances which form its nutriment, and hence as the medium through
which they pass into its organs, it serves also as a direct food,
undergoing decomposition, and yielding hydrogen to the organic
substances. Its constituents, along with those of the carbonic acid
absorbed, undergo a variety of transformations, and form the principal
part of the non-nitrogenous constituents. It has been already observed
that starch, sugar, and the other allied substances, may be considered
as compounds of carbon with water; and they might be supposed to owe
their origin to the carbonic acid losing the whole of its oxygen, and
direct combination then ensuing between the residual carbon and a
certain proportion of water; but this would imply that the latter
substance undergoes no decomposition, and though undoubtedly the
simplest view of the case, it is by no means the most probable. It is
much more likely that the carbonic acid is only partially decomposed,
half its oxygen being separated, and replaced by hydrogen, produced by
the decomposition of a certain quantity of water into its elements.
Thus, for instance, sugar may be produced from twelve equivalents of
carbonic acid and twelve equivalents of water, twenty-four equivalents
of oxygen being eliminated, as thus represented:

12 equivalents of carbonic acid,  C_{12}O_{12}O_{12}
12  "  water,                     H_{12}O_{12}
 1  "  sugar, and 24 of ox.       C_{12}H_{12}O_{12} + O_{24}

It must not be supposed that we are in a condition to assert that sugar
is really produced in the manner here shown, the illustration being
given merely for the purpose of pointing out how it may be supposed to
occur, and on a similar principle it is possible to explain the
formation of most other vegetable compounds; and this subject has been
very fully discussed by the late Dr. Gregory, in his "Handbook of
Organic Chemistry." That water must be decomposed, is evident from the
fact, established by analysis, that the hydrogen of the plant generally
exceeds the quantity required to form water with its oxygen, so that
this excess at least must be produced by the decomposition of water. The
hydrogen of the volatile oils, many of which contain no oxygen, and that
of the fats, which contain only a small quantity, must manifestly be
obtained in a similar manner.

_Decomposition of Ammonia._--The nitrogenous or albuminous compounds of
vegetables must necessarily obtain their nitrogen from the decomposition
either of ammonia or nitric acid, experiment having distinctly shown
that they are incapable of absorbing it in the free state from the
atmosphere. It has been clearly ascertained that the albuminous
substances do not contain ammonia, and it is hence apparent that a
complete decomposition of that substance must take place in the plant.
No doubt carbonic acid and water take part with it in these changes,
which must be of a very complex character, and in the present state of
our knowledge it seems hopeless to attempt any explanation of them.

_Decomposition of Nitric Acid._--Chemists are not entirely at one as to
whether nitric acid is directly absorbed by the plant, or is first
converted into ammonia. But there are certain facts connected with the
chemistry of the soil, to be afterwards referred to, which seem to us to
leave no doubt that it may be directly absorbed; and in that case it
must be decomposed, its oxygen being eliminated, and the nitrogen taking
part with carbon and hydrogen in the formation of the organic compounds.
It must be clearly understood that while such changes as those described
manifestly must take place, the explanations of them which have been
attempted by various chemists are not to be accepted as determinately
established _facts_; they are at present no more than hypothetical views
which have been expressed chiefly with the intention of presenting some
definite idea to the mind, and are unsupported by absolute proof; they
are only inferences drawn from the general bearings of known facts, and
not facts themselves. Although, therefore, they are to be received with
caution, they have advantages in so far as they present the matter to us
in a somewhat more tangible form than the vague general statements which
are all that could otherwise be made.




CHAPTER IV.

THE INORGANIC CONSTITUENTS OF PLANTS.


When treating of the general constituents of plants, it has been already
stated that the older chemists and vegetable physiologists, misled by
the small quantity of ash found in them, entertained the opinion that
mineral matters were purely fortuitous components of vegetables, and
were present merely because they had been dissolved and absorbed along
with the humus, which was then supposed to enter the roots in solution,
and to form the chief food of the plant. This supposition, which could
only be sustained at a time when analysis was imperfect, has been long
since disproved and abandoned, and it has been distinctly shown by
repeated experiment that not only are these inorganic substances
necessary to the plant, but that every one of them, however small its
quantity, must be present if it is to grow luxuriantly and arrive at a
healthy maturity. The experiments of Prince Salm Horstmar, before
alluded to, have established beyond a doubt, that while a seed may
germinate, and even grow, to a certain extent, in absence of one or more
of the constituents of its ash, it remains sickly and stunted, and is
incapable of producing either flower or seed.

Of late years the analysis of the ash of different plants has formed the
subject of a large number of laborious investigations, by which our
knowledge of this subject has been greatly extended. From these it
appears that the quantity of ash contained in each plant or part of a
plant is tolerably uniform, differing only within comparatively narrow
limits, and that there is a special proportion belonging to each
individual organ of the plant. This fact may be best rendered obvious by
the subjoined table, showing the quantity of ash contained in a hundred
parts of the different substances dried at 212°. Most of these numbers
are the mean of several experiments:--


_Table showing the quantity of inorganic matters in 100 parts of
different plants dried at 212°._

SEEDS.

Wheat                                                       1·97
Barley                                                      2·48
Oats (with husk)                                            3·80
Oats (without husk)                                         2·06
Rye                                                         2·00
Millet                                                      3·60
Rice                                                        0·37
Maize                                                       1·20
Peas                                                        2·88
Beans                                                       3·22
Kidney Beans                                                4·09
Lentils                                                     2·51
Tares                                                       2·60
Buckwheat                                                   2·13
Linseed                                                     4·40
Hemp seed                                                   5·60
Rape seed                                                   4·35
Indian Rape-seed[A]                                         4·06
Sunflower                                                   3·26
Cotton seed                                                 5·93
Guinea Corn                                                 1·99
Gold of Pleasure                                            4·10
White Mustard                                               4·15
Black Mustard                                               4·31
Poppy                                                       6·56
Niger seed (_Guizotia oleifera_)                            7·00
Earth nut                                                   3·88
Sweet Almond                                                4·90
Horse-chesnut                                               2·81
Grape                                                       2·76
Clover                                                      6·19
Turnip                                                      3·98
Carrot                                                     10·03
Sainfoin                                                    5·27
Italian Ryegrass                                            6·91
Mangold-Wurzel                                              6·58

STRAWS AND STEMS.

Wheat                                                       4·54
Barley                                                      4·99
Oat                                                         7·24
Winter Rye                                                  5·15
Summer Rye                                                  5·78
Millet                                                      8·32
Maize                                                       3·60
Pea                                                         4·81
Bean                                                        6·59
Tares                                                       6·00
Lentil                                                      5·38
Buckwheat                                                   4·50
Hops                                                        4·42
Flax straw                                                  4·25
Hemp                                                        4·14
Gold of Pleasure                                            6·05
Rape                                                        4·41
Potato                                                     14·90
Jerusalem Artichoke                                         4·40

ENTIRE PLANT.

Potato                                                     17·70
Spurry                                                     10·06
Red Clover                                                  8·79
White Clover                                                8·72
Yellow Clover                                               8·56
Crimson Clover (_T. incarnatum_)                           10·81
Cow Grass (_T. medium_)                                    11·31
Sainfoin                                                    6·51
Ryegrass                                                    6·42
Meadow Foxtail (_Alopecurus pratensis_)                     7·81
Sweet-scented Vernal Grass (_Anthoxanthum odoratum_)        6·32
Downy Oat Grass (_Avena pubescens_)                         5·22
Bromus erectus                                              5·21
Bromus mollis                                               5·82
Cynosurus cristatus                                         6·38
Dactylis glomeratus                                         5·31
Festuca duriuscula                                          5·42
Holcus lanatus                                              6·37
Hordeum pratense                                            5·67
Lolium perenne                                              7·54
Poa annua                                                   2·83
Poa pratensis                                               5·94
Poa trivialis                                               8·33
Phleum pratense                                             5·29
Plantago lanceolata                                         8·68
Poterium Sanguisorba                                        7·97
Yarrow                                                     13·45
Rape Kale                                                   8·00
Cow Cabbage                                                10·00
Asparagus                                                   6·40
Parsley                                                     1·10
Furze                                                       3·11
Chamomile (_Anthemis arvensis_)                             9·66
Wild Chamomile (_Matricaria Chamomilla_)                    9·10
Corn Cockle (_Agrostemma Githago_)                         13·20
Corn Blue Bottle (_Centaurea Cyanus_)                       7·32
Foxglove                                                   10·89
Hemlock (_Conium maculatum_)                               12·80
Sweet Rush (_Acorus Calamus_)                               6·90
Common Reed (_Arundo Phragmites_)                           1·44
Celandine (_Chelidonium majus_)                             6·85
Equisetum fluviatile                                       23·60
Equisetum hyemale                                          11·80
     "     arvense                                         13·80
     "     linosum                                         15·50
Fucus nodosus                                              19·03
Fucus vesiculosus                                          27·63
Laminaria digitata                                         39·68

LEAVES.

Turnip                                                      9·37
Beet                                                       20·30
Kohl-rabi                                                  18·54
Carrot                                                     10·95
Jerusalem Artichoke                                        28·30
Hemp                                                       22·00
Hop                                                        17·25
Tobacco                                                    22·62
Spinach                                                    19·76
Chicory                                                    15·67
Poplar                                                     23·00
Red Beech                                                   6·00
White Beech                                                10·51
Oak                                                         9·80
Elm                                                        16·33
Horse-chesnut                                               9·08
Maple                                                      28·05
Ash                                                        14·76
Fir                                                         2·31
Acacia                                                     18·20
Olive                                                       6·45
Orange                                                     13·73
Potato                                                     15·10
Tussac Grass                                                7·15

ROOTS AND TUBERS.

Potato                                                      4·16
Jerusalem Artichoke                                         5·38
Turnip                                                     13·64
Beet                                                        8·27
Kohl-rabi                                                   6·08
Rutabaga                                                    7·34
Carrot                                                      5·80
Belgian White Carrot                                        6·22
Mangold-Wurzel                                              8·78
Parsnip                                                     5·52
Radish                                                      7·35
Chicory                                                     5·21
Madder                                                      8·33

WOODS.

Beech                                                       0·38
Apple                                                       1·29
Cherry                                                      0·28
Birch                                                       1·00
Oak                                                         2·50
Walnut                                                      1·57
Lime                                                        5·00
Horse-chesnut                                               1·05
Olive                                                       0·58
Mahogany                                                    0·81
Vine                                                        2·57
Larch                                                       0·32
Fir                                                         0·14
Scotch Fir                                                  0·17
Filbert                                                     0·50
Chesnut                                                     3·50
Poplar                                                      0·80
Hazel                                                       0·50
Orange                                                      2·74
Vine                                                        2·57

BARKS.

Beech                                                       6·62
Cherry                                                     10·37
Fir                                                         1·79
Oak                                                         6·00
Horse-chesnut                                               7·85
Filbert                                                     6·20
Cork                                                        1·12

FRUITS.

Plum                                                        0·40
Cherry                                                      0·43
Strawberry                                                  0·41
Pear                                                        0·41
Apple                                                       0·27
Chesnut                                                     0·99
Cucumber                                                    0·63
Vegetable Marrow                                            5·10

On examining this table it may be observed that, notwithstanding the
very great variety in the proportion of ash in different plants, some
general relations may be traced. A certain similarity may be observed
between those belonging to the same natural family, the seeds of all the
cereal grains, for instance, containing in round numbers two per cent of
inorganic matters. Leguminous seeds (peas and beans) contain about three
per cent, while in rape-seed, linseed, and the other oily seeds, it
reaches four per cent. In the stems and straws less uniformity exists,
but with the exception of a few extreme cases, the quantity of ash in
general approaches pretty closely to five per cent. Still more
diversified results are obtained from the entire plants; but this
diversity is probably much more apparent than real, and must be, in part
at least, dependent on the proportion existing between the stem and
leaves, for the leaves are peculiarly rich in ash, and a leafy plant
must necessarily yield a higher total percentage of ash, although, if
stems and leaves were separately examined, they might not show so
conspicuous a difference.

The leaves surpass all other parts of plants, in the proportion of
inorganic constituents they contain, the table showing that in some
instances, as in the maple and Jerusalem artichoke, they exceed
one-fourth of the whole weight of the dry matter. In other leaves, and
more especially in those of the coniferæ, the proportion is much
smaller. Taking the average of all the analyses hitherto made, it
appears that leaves contain about thirteen per cent of ash, but the
variations on either side are so large that little value is to be
attached to it except as an indication of the general abundance of
mineral matters.

In roots and tubers the variations are less, and all, except the potato
and the turnip, contain about seven per cent of ash.

The smallest proportion of mineral matter is found in wood. In one case
only does the proportion reach five per cent, while the average scarcely
exceeds one, and in the fir the quantity amounts to no more than one
six-hundredth of the dry matter. In the bark the quantity is much
larger, and may be stated at seven per cent.

The general proportion of ash found in different parts of plants is
given in round numbers in the subjoined table:--

Wood                1
Seeds               3
Stems and straws    5
Roots and tubers    7
Bark                7
Leaves             13

The differences in the quantity of ash contained in different parts of
plants are obviously intended to serve a useful purpose, and it is
interesting to observe that the wood which is destined to remain for a
long period, sometimes for several centuries, a part of the plant,
contains the smallest proportion, and it is not improbable that what it
does contain is really due, not to the actual woody matter itself, but
to the sap which permeates its vessels. By this arrangement but a small
proportion of these important mineral matters, which the soil supplies
in very limited quantity, is locked up within the plant, and those which
are absorbed, after circulating through it, and fulfilling their
allotted functions, are accumulated in the leaves, and annually returned
to the soil.

The different proportions of mineral matters contained in the individual
organs of plants is most strikingly illustrated when parallel
experiments are made on the same species; but the number of instances in
which a sufficiently extensive series of analyses has been made to show
this, is comparatively limited, and is confined to the oat, the
orange-tree, and the horse chesnut--each of which has formed the subject
of a very elaborate investigation. The following table gives the results
obtained on the oat:--

+-------------+-----------+---------+-----------+-------+-------+------+
|             | Hopetoun  | Hopetoun| Potato    | Black | Sandy |      |
|             | Oats,     | Oats,   | Oats,     | Oats, | Oats, | Mean.|
|             | North-    | Fife-   | North-    | Edin- | Fife- |      |
|             | umberland.| shire.  | umberland.| burgh.| shire.|      |
+-------------+-----------+---------+-----------+-------+-------+------+
| Grain       |    2·14   |  1·81   |    2·22   |  2·11 |  1·76 |  2·00|
| Husk        |    6·47   |  6·03   |    6·99   |  8·24 |  6·03 |  6·75|
| Chaff       |   16·53   | 17·23   |   15·59   | 19·19 | 18·97 | 16·06|
| Leaves      |    8·44   |  7·19   |   14·59   | 10·29 | 15·92 | 10·88|
| Upper part  |           |         |           |       |       |      |
|   of straw  |    4·95   |  5·44   |    9·22   |  8·25 | 11·0  |  7·77|
| Middle part |           |         |           |       |       |      |
|   of straw  |    6·11   |  5·23   |    7·41   |  6·53 |  9·01 |  6·66|
| Lower part  |           |         |           |       |       |      |
|   of straw  |    5·33   |  5·18   |    9·76   |  7·11 |  7·30 |  6·93|
+-------------+-----------+---------+-----------+-------+-------+------+

The specimens of oats on which these analyses were made were from
different districts of country, grown on soils of different quality, and
were, further, of different varieties; and yet they show, on the whole,
a remarkable similarity in the proportion of ash in each part, and
indicate that there is a normal quantity belonging to it. Such a series
of analyses also affords the most convincing proof that the inorganic
matters cannot be fortuitous, and merely absorbed from the soil along
with their organic food, as the old chemists supposed, because, in that
case, they ought to be uniformly distributed throughout the entire
plant, and not accumulated in particular proportions in each individual
organ.

Not only does the proportion of ash vary in the different parts of a
plant, but even in the same part it is greatly influenced by its period
of growth. The laws which regulate these variations are very imperfectly
known, but in general it is observed that during the period of active
growth the quantity of ash is largest. Thus, it has been found that in
early spring the wood of the young shoots of the horse-chesnut contains
9·9 per cent of ash. In autumn this has diminished to 3·4, and the last
year's twigs contain only 1·1 per cent, while in the old wood the
quantity does not exceed 0·5. Saussure has also observed that the
quantity of ash diminishes in certain plants when the seed has ripened.
Thus, he found that the percentages of ash, before flowering, and after
seeding, were as follows:--

               Before flowering.  With ripe seed.
Sunflower             14·7             9·3
Wheat                  7·9             3·3
Maize                 12·2             4·6

On the other hand, the quantity of ash in the leaves of trees increases
considerably in autumn, as shown by this table:--

                          PER-CENTAGE OF ASH IN
                          May.        September.
Oak leaves                5·3         5·5
Poplar                    6·6         9·3
Hazel                     6·1         7·0
Horse-chesnut             7·2         8·6

In general, the proportion of ash appears to increase as the plant
reaches maturity, and this is particularly seen in the oat, of which
very complete analyses have been made at different periods of its
growth:--


_Proportion of Ash in different parts of the Oat at different periods of
its growth._

+--------------+---------+---------+--------+------------+
|              |         |         |        | Grain      |
| Date.        | Stalks. | Leaves. | Chaff. | with husk. |
+--------------+---------+---------+--------+------------+
| 2d July      |  7·83   |  11·35  |   ...  |   4·91     |
| 9th July     |  7·80   |  12·20  |   ...  |   4·36     |
| 16th July    |  7·94   |  12·61  |  6·00  |   3·38     |
| 23d July     |  7·99   |  16·45  |  9·11  |   3·62     |
| 30th July    |  7·45   |  16·44  | 12·28  |   4·22     |
| 5th August   |  7·63   |  16·05  | 13·75  |   4·31     |
| 13th August  |  6·62   |  20·47  | 18·68  |   4·07     |
| 20th August  |  6·66   |  21·14  | 21·07  |   3·64     |
| 27th August  |  7·71   |  22·13  | 22·46  |   3·51     |
| 3d September |  8·35   |  20·90  | 27·47  |   3·65     |
+--------------+---------+---------+--------+------------+

The increase is here principally confined to the leaves and chaff, while
the stalks, which owe their strength to a considerable extent to the
inorganic matters they contain, are equally supplied at all periods of
their growth. In the grain only is there a diminution, but this is
apparent and not real, and is due to the fact that the determination of
the quantity of ash, as made on the grain with its husk, and the former,
which contains only a small quantity of mineral matters, increases much
more rapidly in weight than the latter, when it approaches the period of
ripening, and it is accordingly during the last three weeks of its
growth that this diminution becomes apparent.

The nature of the soil has also a very important influence on the
proportion of mineral matters, and of this an interesting illustration
is given in the following table, which shows the quantities found in the
grain and straw of the same variety of the pea grown on fourteen
different soils:--

+----+-------+--------+
|    | Seed. | Straw. |
+----+-------+--------+
|  1 |  2·30 |        |
|  2 |  3·25 |  3·43  |
|  3 |  4·27 |  3·62  |
|  4 |  3·40 |  3·39  |
|  5 |  2·99 |  3·90  |
|  6 |  3·19 |  6·80  |
|  7 |  2·53 |  3·90  |
|  8 |  2·27 |  6·59  |
|  9 |  2·69 |  3·49  |
| 10 |  1·61 |  3·91  |
| 11 |  3·11 |  5·28  |
| 12 |  3·34 |  7·57  |
| 13 |  2·78 |  3·76  |
| 14 |  3·01 |  3·38  |
+----+-------+--------+

Although those differences are very large, especially in the straw, and
must be attributed to the soil, it has hitherto been found impossible to
ascertain the nature of the relation subsisting between it and the crops
it yields; indeed, it must obviously be dependent on very complicated
questions, which cannot at present be solved, for it may be observed
that the increase in the grain does not occur simultaneously with that
in the straw, and in several cases a large proportion of ash in the
former is associated with an unusually small amount in the latter. _A
priori_, it might be expected that those soils which are especially rich
in the more important constituents of the ash should yield a produce
containing more than the average quantity, but this is very far from
being an invariable occurrence, and not unfrequently the very reverse
is the case. In some instances the variations may be traced to the soil,
as in the following analyses of the fruit of the horse-chesnut, grown on
an ordinary forest soil, and on a rich soil, produced by the
disintegration of porphyritic rock, in which the latter yields a much
larger quantity of ash:--

                  Kernel of seed.  Green husk.   Brown husk.
Forest soil        2·26             4·53          1·70
Porphyry soil      3·36             7·29          2·20

In the majority of instances we fail to establish any connection between
the nature of the soil and the plants it yields, chiefly because we are
still very deficient in analyses of those grown on uncultivated soils;
and on cultivated land it is impossible to draw conclusions, because the
nature of the manure exerts an influence quite as great, if not greater,
than that of the soil itself.

The relative proportion in which the different mineral matters enter
into the composition of the ash varies within very wide limits, as will
be apparent from the following table, containing a selection of the best
analyses of our common cultivated and a few uncultivated plants.


_Table of the Composition of the Ash of different Plants in 100 Parts._

_Note._--Alumina and oxide of manganese occur so rarely, that separate
columns have not been introduced for them, but their quantity is stated
in notes at the end of the table.

+---------------------+--------+------+-----------+---------+------+----------+
|                     | Potash.| Soda.| Chloride  | Chloride| Lime.| Magnesia.|
|                     |        |      | of        | of      |      |          |
|                     |        |      | Potassium.| Sodium. |      |          |
+---------------------+--------+------+-----------+---------+------+----------+
|Wheat, grain         | 30·02  | 3·82 |   ...     |   ...   |  1·15|  13·39   |
|  straw              | 17·98  | 2·47 |   ...     |   ...   |  7·42|   1·94   |
|  chaff              |  9·14  | 1·79 |   ...     |   ...   |  1·88|   1·27   |
|Barley, grain        | 21·14  |  ... |  5·65     |  1·01   |  1·65|   7·26   |
|  straw              | 11·22  |  ... |   ...     |  2·14   |  5·79|   2·70   |
|Oats, grain[B]       | 20·63  |  ... |  1·03     |   ...   | 10·28|   7·82   |
|  straw              | 19·46  | 1·93 |  2·71     |  4·27   |  7·01|   3·79   |
|  chaff[C]           |  6·33  | 3·93 |   ...     |  0·24   |  1·95|   0·38   |
|Rye, grain           | 33·83  | 0·39 |   ...     |   ...   |  2·61|  12·81   |
|  straw              | 17·20  |  ... |  0·30     |  0·60   |  9·10|   2·40   |
|Maize, grain         | 28·37  | 1·74 |   ...     | trace   |  0·57|  13·60   |
|  stalks and leaves  | 35·26  |  ... |   ...     |  2·29   | 10·53|   5·52   |
|Rice, grain          | 20·21  | 2·49 |   ...     |   ...   |  7·18|   4·26   |
|Buckwheat, straw     | 31·71  |  ... |  7·42     |  4·55   | 15·71|   1·66   |
|Peas (gray), seed    | 41·70  |  ... |  3·82     |  1·24   |  4·78|   5·78   |
|  straw              | 21·30  | 4·22 |   ...     |   ...   | 37·17|   7·17   |
|Beans (common field),|        |      |           |         |      |          |
|  grain              | 51·72  | 0·54 |   ...     |   ...   |  5·20|   6·90   |
|  straw              | 32·85  | 2·77 |   ...     | 11·54   | 19·85|   2·53   |
|Tare, straw          | 32·82  |  ... |  3·27     |  4·03   | 20·78|   5·31   |
|  straw              | 31·72  |  ... |  7·41     |  4·55   | 15·71|   1·66   |
+---------------------+--------+------+-----------+---------+------+----------+


+-----------------------+-------+------------+-----------+----------+---------+
|                       | Oxide | Phosphoric | Sulphuric | Carbonic | Silica. |
|                       | of    | Acid.      | Acid.     | Acid.    |         |
|                       | Iron. |            |           |          |         |
+-----------------------+-------+------------+-----------+----------+---------+
| Wheat, grain          | 0·91  |  46·79     |    ...    |   ...    |  3·89   |
|   straw               | 0·45  |   2·75     |   3·09    |   ...    | 63·89   |
|   chaff               | 0·37  |   4·31     |    ...    |   ...    | 81·22   |
| Barley, grain         | 2·13  |  28·53     |   1·91    |   ...    | 30·68   |
|   straw               | 1·36  |   7·20     |   1·09    |   ...    | 68·50   |
| Oats, grain           | 3·85  |  50·44     |    ...    |   ...    |  4·40   |
|   straw               | 1·49  |   5·07     |   3·35    |  1·36    | 49·56   |
|   chaff               | 1·58  |   1·04     |   9·61    |   ...    | 72·85   |
| Rye, grain            | 1·04  |  39·92     |   0·17    |   ...    |  9·22   |
|   straw               | 1·40  |   3·80     |   0·80    |   ...    | 64·50   |
| Maize, grain          | 0·47  |  53·69     |    ...    |   ...    |  1·55   |
|   stalks and leaves   | 2·28  |   8·09     |   5·16    |  2·87    | 27·98   |
| Rice, grain           | 2·12  |  62·23     |    ...    |   ...    |  1·37   |
| Buckwheat, straw      |  ...  |  10·34     |   4·67    | 20·37    |  3·57   |
| Peas (gray), seed     | 0·18  |  36·50     |   4·47    |  0·82    |  0·68   |
|   straw               | 1·07  |   4·65     |   8·68    | 12·48    |  3·23   |
| Beans (common field), |       |            |           |          |         |
|   grain               |  ...  |  28·72     |   3·05    |  3·42    |  0·42   |
|   straw               | 0·61  |   0·49     |   1·40    | 25·32    |  2·61   |
| Tare, straw           | 0·65  |  10·59     |   2·52    | 18·73    |  1·28   |
|   straw               |  ...  |  10·34     |   4·67    | 20·37    |  3·57   |
+-----------------------+-------+------------+-----------+----------+---------+

+---------------------+--------+------+-----------+---------+------+----------+
|                     | Potash.| Soda.| Chloride  | Chloride| Lime.| Magnesia.|
|                     |        |      | of        | of      |      |          |
|                     |        |      | Potassium.| Sodium. |      |          |
+---------------------+--------+------+-----------+---------+------+----------+
|Flax, seed           |  34·17 |  1·69|   ...     |   0·36  |  8·40|  13·11   |
|  straw              |  21·53 |  3·68|   ...     |   9·21  | 21·20|   4·20   |
|Rape, seed[D]        |  16·33 |  0·34|   ...     |   0·96  |  8·30|   8·80   |
|  straw[E]           |  16·63 | 10·57|   ...     |   2·53  | 21·51|   2·92   |
|Spurry               |  26·12 |  1·14|   ...     |   8·90  | 14·46|   8·88   |
|Chicory root         |  34·64 |   ...|  8·92     |   2·98  |   ...|    ...   |
|Red clover           |  25·60 |   ...|  9·08     |   6·02  | 21·57|   8·47   |
|Cow grass,           |        |      |           |         |      |          |
|  _Trifolium medium_ |  22·78 |   ...| 12·39     |   1·86  | 24·42|  8·86   |
|Yellow clover        |  27·48 |   ...| 11·72     |   8·16  | 17·26|   8·39   |
|Alsike clover        |  29·72 |   ...|  6·29     |   1·05  | 26·83|   4·01   |
|Lucerne              |  27·56 |   ...| 11·64     |   1·91  | 20·60|   5·22   |
|Anthoxanthum odoratum|  32·03 |   ...|  7·03     |   4·90  |  9·21|   2·53   |
|Alopecurus pratensis |  37·03 |   ...|  9·50     |    ...  |  3·90|   1·28   |
|Avena pubescens      |  31·21 |   ...|  4·05     |   5·66  |  4·72|   3·17   |
|Bromus erectus       |  20·33 |   ...| 10·63     |   1·38  | 10·38|   4·99   |
|Bromus mollis        |  30·09 |  0·33|   ...     |   3·11  |  6·64|   2·60   |
|Cynosurus cristatus  |  24·99 |   ...| 11·60     |    ...  | 10·16|   2·43   |
|Dactylis glomerata   |  29·52 |   ...| 17·86     |   3·09  |  5·82|   2·22   |
|Festuca duriuscula   |  31·84 |   ...|  8·17     |   0·62  | 10·31|   2·83   |
|Holcus lanatus       |  34·83 |   ...|  3·91     |   6·66  |  8·31|   3·41   |
|Lolium perenne       |  24·67 |   ...| 13·80     |   7·25  |  9·64|   2·85   |
|Annual ryegrass      |  28·99 |  0·87|   ...     |   5·11  |  6·82|   2·59   |
|Poa annua            |  41·86 |   ...|  0·47     |   3·35  | 11·69|   2·44   |
|Poa pratensis        |  31·17 |   ...| 11·25     |   1·31  |  5·63|   2·71   |
|Poa trivialis        |  29·40 |   ...|  6·90     |    ...  |  8·80|   3·22   |
|Phleum pratense      |  31·09 |   ...|  0·70     |   3·24  | 14·94|   5·30   |
|Plantago lanceolata  |  33·26 |  ... |   4·53    |   8·80  | 19·01|   3·51   |
|Poterium Sanguisorba |  30·26 |  ... |   3·27    |   1·35  | 24·82|   4·21   |
|Achillea Millefolia  |  30·37 |  ... |  20·49    |   3·63  | 13·40|   3·01   |
|Potato, tuber        |  43·18 |  0·09|    ...    |   7·92  |  1·80|   3·17   |
|  stem               |  39·53 |  3·95|    ...    |  20·43  | 14·85|   4·10   |
|  leaves             |  17·27 |  ... |   4·95    |  11·37  | 27·69|   7·78   |
|Jerusalem Artichoke  |  55·89 |  ... |   4·88    |    ...  |  3·34|   1·30   |
|  stem               |  38·40 |  0·69|    ...    |   4·68  | 20·31|   1·91   |
|  leaves             |   6·81 |  3·72|    ...    |   1·82  | 40·15|   1·95   |
|Turnip, seed         |  21·91 |  1·23|    ...    |    ...  | 17·40|   8·74   |
|  bulb               |  23·70 | 14·75|    ...    |   7·05  | 11·82|   3·28   |
|  leaves             |  11·56 | 12·43|    ...    |  12·41  | 28·49|   2·62   |
|Mangold Wurzel, root |  21·68 |  3·13|    ...    |  49·51  |  1·90|   1·79   |
|  leaves             |   8·34 | 12·21|    ...    |  37·66  |  8·72|   9·84   |
|Carrot, root         |  42·73 | 12·11|    ...    |    ...  |  5·64|   2·29   |
|  leaves             |  17·10 |  4·85|    ...    |   3·62  | 24·05|   0·89   |
|Kohl-rabi, bulb      |  36·27 |  2·84|    ...    |  11·90  | 10·20|   2·36   |
|  leaves             |   9·31 |  ... |   5·99    |   6·66  | 30·31|   3·62   |
|Cow cabbage, head    |  40·86 |  2·43|    ...    |    ...  | 15·01|   2·39   |
|  stalk              |  40·93 |  4·05|    ...    |   2·08  | 10·61|   3·85   |
|Poppy seed           |   9·10 |  ... |   7·15    |   1·94  | 35·36|   9·49   |
|  leaves             |  36·37 |  ... |   2·50    |   2·51  | 30·24|   6·47   |
|Mustard seed (white) |  25·78 |  0·33|    ...    |    ...  | 19·10|   5·90   |
|Radish root          |  21·16 |  ... |   1·29    |   7·07  |  8·78|   3·53   |
|Tobacco leaves       |  36·37 |  ... |   2·50    |   2·51  | 30·24|   6·47   |
|Fucus nodosus[F]     |  20·03 |  4·58|    ...    |  24·33  |  9·60|   6·65   |
|Fucus vesiculosus[G] |  20·75 |  6·09|    ...    |  24·81  |  8·92|   5·83   |
|Laminaria digitata[H]|  12·16 |  ... |   2·30    |  19·34  |  4·62|  10·94   |
+---------------------+--------+------+-----------+---------+------+----------+

+---------------------+-------+------------+-----------+----------+---------+
|                     | Oxide | Phosphoric | Sulphuric | Carbonic | Silica. |
|                     | of    | Acid.      | Acid.     | Acid.    |         |
|                     | Iron. |            |           |          |         |
+---------------------+-------+------------+-----------+----------+---------+
|Flax, seed           |  0·50 |  38·54     |  1·56     |  0·22    |  1·45   |
|  straw              |  5·58 |   7·53     |  3·39     | 15·75    |  7·92   |
|Rape, seed           |  1·79 |  31·90     |  5·38     |  5·44    | 19·98   |
|  straw              |  1·30 |   4·68     |  3·90     | 23·04    | 11·80   |
|Spurry               |   ... |  10·20     |  1·79     | 27·38    |  1·14   |
|Chicory root         |   ... |    ...     |   ...     |   ...    |   ...   |
|Red clover           |  1·26 |   4·09     |  2·96     | 18·05    |  1·95   |
|Cow grass,           |       |            |           |          |         |
|  _Trifolium medium_ |  1·09 |   4·94     |  2·66     | 20·16    |  1·12|
|Yellow clover        |  1·40 |    ...     |  4·82     |  4·31    |  1·76   |
|Alsike clover        |  0·71 |   5·64     |  3·25     | 20·74    |  1·73   |
|Lucerne              |  2·23 |   6·47     |  4·80     | 15·94    |  2·63   |
|Anthoxanthum odoratum|  1·18 |  10·09     |  3·39     |  1·26    | 28·35   |
|Alopecurus pratensis |  0·47 |   6·25     |  2·16     |  0·65    | 38·75   |
|Avena pubescens      |  0·72 |  10·82     |  3·37     |   ...    | 36·28   |
|Bromus erectus       |  0·26 |   7·53     |  5·46     |  0·55    | 38·48   |
|Bromus mollis        |  0·28 |   9·62     |  4·91     |  9·07    | 33·34   |
|Cynosurus cristatus  |  0·18 |   7·24     |  3·20     |   ...    | 40·11   |
|Dactylis glomerata   |  0·59 |   8·60     |  3·52     |  2·09    | 26·65   |
|Festuca duriuscula   |  0·78 |  12·07     |  3·45     |  1·38    | 28·53   |
|Holcus lanatus       |  0·31 |   8·02     |  4·41     |  1·82    | 28·31   |
|Lolium perenne       |  0·21 |   8·73     |  5·20     |  0·49    | 27·13   |
|Annual ryegrass      |  0·28 |  10·07     |  3·45     |   ...    | 41·79   |
|Poa annua            |  1·57 |   9·11     | 10·18     |  3·29    | 16·03   |
|Poa pratensis        |  0·28 |  10·02     |  4·26     |  0·40    | 32·93   |
|Poa trivialis        |  0·29 |   9·13     |  4·47     |  0·29    | 37·50   |
|Phleum pratense      |  0·27 |  11·29     |  4·86     |  4·02    | 31·09   |
|Plantago lanceolata  |  0·90 |   7·08     |  6·11     |  14·40   |  2·37   |
|Poterium Sanguisorba |  0·86 |   7·81     |  4·84     |  21·72   |  0·83   |
|Achillea Millefolia  |  0·21 |   7·13     |  2·44     |   9·36   |  9·92   |
|Potato, tuber        |  0·44 |   8·61     | 15·24     |  18·29   |  1·94   |
|  stem               |  1·34 |   6·68     |  6·56     |    ...   |  2·56   |
|  leaves             |  4·50 | 13·60      |  6·37     |    ...   |  6·47   |
|Jerusalem Artichoke  |  0·45 | 16·99      |  3·77     |  11·80   |  1·52   |
|  stem               |  0·88 |  2·97      |  3·23     |  25·40   |  1·51   |
|  leaves             |  1·14 |  6·61      |  2·21     |  24·31   | 17·25   |
|Turnip, seed         |  1·95 | 40·17      |  7·10     |   0·82   |  0·67   |
|  bulb               |  0·47 |  9·31      | 16·13     |  10·74   |  2·69   |
|  leaves             |  3·02 |  4·85      | 10·36     |   6·18   |  8·04   |
|Mangold Wurzel, root |  0·52 |  1·65      |  3·14     |  15·23   |  1·40   |
|  leaves             |  1·46 |  5·89      |  6·54     |   6·92   |  2·35   |
|Carrot, root         |  0·51 | 12·31      |  4·26     |  18·00   |  1·11   |
|  leaves             |  3·43 |  6·21      |  5·08     |  23·15   | 11·61   |
|Kohl-rabi, bulb      |  0·38 | 13·45      | 11·43     |  10·24   |  0·83   |
|  leaves             |  5·50 |  9·43      | 10·63     |   8·97   |  9·57   |
|Cow cabbage, head    |  0·77 | 12·53      |  7·27     |  16·68   |  1·66   |
|  stalk              |  0·41 | 19·57      | 11·11     |   6·33   |  1·04   |
|Poppy seed           |  0·41 | 31·38      |  1·92     |    ...   |  3·24   |
|  leaves             |  2·14 |  3·28      |  5·09     |    ...   | 11·40   |
|Mustard seed (white) |  0·39 | 44·97      |  2·19     |    ...   |  1·31   |
|Radish root          |  1·19 | 41·09      |  7·71     |    ...   |  8·17   |
|Tobacco leaves       |  2·18 |  3·24      |  5·09     |    ...   | 11·40   |
|Fucus nodosus        |  0·26 |  1·71      | 21·97     |   6·39   |  0·38   |
|Fucus vesiculosus    |  0·35 |  2·14      | 28·01     |   2·20   |  0·67   |
|Laminaria digitata   |  0·45 |  1·75      |  7·26     |  15·23   |  1·20   |
+---------------------+-------+------------+-----------+----------+---------+

A simple inspection of this table leads to various interesting
conclusions. It is particularly to be observed that some of the
constituents of the ash are not invariably present, and two at
least--namely, alumina and manganese--are found so rarely as to justify
the inference that they are not indispensable. Of the other substances,
iodine is restricted exclusively to sea-plants, but to them it appears
to be essential. Oxide of iron, which occurs only in small quantities,
has sometimes been considered fortuitous, but it is almost invariably
present, and the experiments of Prince Salm Horstmar leave no doubt that
it is essential to the plant. Its function is unknown, but it is an
important constituent of the blood of herbivorous animals, and may be
present in the plant, less for its own benefit than for that of the
animal of which it is destined to become the food.

Soda appears to be a comparatively unimportant constituent of the ash,
of which it generally forms but a small proportion, although the
instances of its entire absence are rare. In the cruciferous plants
(turnip, rape, etc.) it is found abundantly, and to them it appears
indispensable, but in most other plants it admits of replacement by
potash. It seems probable that where the soil is rich in the latter
substance, plants will select that alkali in preference to soda; but as
they must have a certain quantity of alkali, the latter may supply the
place of the former where it is deficient. Cultivation, probably by
enriching the soil in that element, increases the proportion of potash
found in the ash of plants, as is remarkably seen in the asparagus,
which gave the following quantities of alkalies and chlorine:--

                Wild.   Cultivated.
     Potash     18·8     50·5
     Soda       16·2     trace.
     Chlorine   16·5      8·3

The soda having almost entirely disappeared in the cultivated plant,
while a corresponding increase had taken place in the quantity of
potash.

Potash is one of the most important elements of the ash of all plants,
rarely forming less than 20, and sometimes more than 50 per cent of its
weight. The latter proportion occurs chiefly in the roots and tubers,
but it is also abundant in all seeds and in the grasses. The straw, and
particularly the chaff of the cereals, and the leaves of most plants,
contain it in smaller quantity, although exceptions to this are not
unfrequent, one of the most curious being the case of poppy-seed, which
contains only about 12 per cent, while the leaves yield upwards of 37
per cent.

The proportion of lime varies within very wide limits, being sometimes
as low as 1, and in other plants reaching 40 per cent of their ash. The
former proportion occurs in the grains of the cerealia, and the latter
in the leaves of some plants, and more especially in the Jerusalem
artichoke. The turnip and some of the leguminous plants also contain it
abundantly.

Magnesia is generally found in small quantity. It is largest in the
grains, amounting in them to about 12 or 13 per cent of the ash, but in
other plants it varies from 2 to 4 per cent. Although small in quantity,
it is an important substance, and apparently cannot be dispensed with;
at least there is no instance known of its entire absence.

_Chlorine_ is by no means an invariable constituent of the ash, although
it is generally present, and sometimes in considerable quantity. It is
most abundant when the proportion of soda is large, and exists in the
ash principally in combination with that base as common salt. The
relation between these two elements may be traced more or less
distinctly throughout the whole table of analyses, and conspicuously in
that of mangold-wurzel, where the common salt amounts to almost exactly
one-half of the whole mineral matter. The analyses of the cultivated and
uncultivated asparagus also show that a diminution in the soda is
accompanied by a reduction in the proportion of chlorine.

_Sulphuric Acid_ is an essential constituent of the ash. But it is to be
observed that it is in some instances entirely, and in all partially, a
product of the combustion to which the plant has been submitted in order
to obtain the ash. It is partly derived from the sulphur contained in
the albuminous compounds, which is oxidised and converted into sulphuric
acid during the process of burning the organic matter, and remains in
the ash. The quantity of sulphuric acid found in the ash is, however, no
criterion of that existing in the plant, for a considerable quantity of
it escapes during burning. The extent to which this occurs in particular
instances is well illustrated by reference to the case of white mustard,
which yields an ash containing only 2·19 of sulphuric acid, equivalent
to 0·9 of sulphur; and if calculated on the seed itself, this will
amount to no more than 0·039 per cent, while experiments made in another
manner prove it to contain about thirty times as much, or more than 1
per cent. For the purpose of determining the total quantity of sulphur
which the plants contain in their natural state, it is necessary to
oxidise them by means of nitric acid; and from such experiments the
following table, showing the _total_ amount of sulphur contained in 100
parts of different plants, dried at 212°, has been constructed:--

     Poa palustris         0·165
     Lolium perenne        0·310
     Italian Ryegrass      0·329
     Trifolium pratense    0·107
               repens      0·099
     Lucerne               0·336
     Vetch                 0·178
     Potato tuber          0·082
            tops           0·206
     Carrot, root          0·092
         tops              0·745
     Mangold-Wurzel, root  0·058
                    tops   0·502
     Swede, root           0·435
            tops           0·458
     Rape                  0·448

     Drumhead Cabbage      0·431
     Wheat, grain          0·068
            straw          0·245
     Barley, grain,        0·053
             straw         0·191
     Oats, grain           0·103
           straw           0·289
     Rye, grain            0·051
     Beans                 0·056
     Peas                  0·127
     Lentils               0·110
     Hops                  1·063
     Gold of Pleasure      0·253
     Black Mustard         1·170
     White Mustard         1·050

_Phosphoric acid_, which may be looked upon as the most important
mineral constituent of plants, is found to be present in very variable
proportions. The straws, stems, and leaves contain it in comparatively
small quantity, but in the seeds of all plants it is very abundant. In
these of the cereals it constitutes nearly half of their whole mineral
components, and it rarely falls below 30 per cent.

_Carbonic acid_ occurs in very variable quantities in the ash. It is of
comparatively little importance in itself, and is really produced by the
oxidation of part of the carbonaceous matters of the plant; but it has a
special interest, in so far as it shows that part of the bases contained
in the plant must in its natural state have been in union with organic
acids, or combined in some way with the organic constituents of the
plant.

_Silica_ is an invariable constituent of the ash, but in most plants
occurs but in small quantity. The cereals and grasses form an exception
to this rule, for in them it is an abundant and important element. It is
not, however, uniformly distributed through them, but is accumulated to
a large extent in the stem, to the strength and rigidity of which it
greatly contributes. The hard shining layer which coats the exterior of
straw, and which is still more remarkably seen on the surface of the
bamboo, consists chiefly of silica; and in the latter plant this element
is sometimes so largely accumulated, that concretions resembling opal,
and composed entirely of it, are found loose within its joints. The
necessity for a large supply of silica in the stems of other plants does
not exist, and in them it rarely exceeds 5 or 6 per cent, but in some
leaves it is more abundant.

A knowledge of the composition of the ash of plants is of considerable
importance in a practical point of view, and enables us in many
instances to explain why some plants will not grow upon particular soils
on which others flourish. Thus, for instance, a plant which contains a
large quantity of lime, such as the bean or turnip, will not grow in a
soil in which that element is deficient, although wheat or barley, which
require but little lime, may yield excellent crops. Again, if the soil
be deficient in phosphoric acid, those plants only will grow luxuriantly
which require but a small quantity of that element, and hence it follows
that on such a soil plants cultivated for the sake of their stems,
roots, or leaves, in which the quantity of phosphoric acid is small, may
yield a good return; while others, cultivated for the sake of their
seed, in which the great proportion of that constituent of the ash is
accumulated, may yield a very small crop. It is obvious also that even
where a soil contains a proper quantity of all its ingredients, the
repeated cultivation of a plant which removes a large quantity of any
individual element, may, in the course of time, so far reduce the amount
of that substance as to render the soil incapable of any longer
producing that plant, although, if it be replaced by another which
requires but little of the element thus removed, it may again produce an
abundant crop. On this principle also, attempts have been made to
explain the rotation of crops, which has been supposed to depend on the
cultivation in successive years of plants which abstract from the soil
preponderating quantities of different mineral matters. But though this
has unquestionably a certain influence, we shall afterwards see reason
to doubt whether it affords a sufficient explanation of all the observed
phenomena.

It may be observed, on examining the table of the percentage and
position of the ash, that some plants are especially rich in alkalies,
while in others lime or silica preponderate, and it would therefore be
the object of the farmer to employ, in succession, crops containing
these elements in different proportions. In carrying out this view,
attempts have been made to classify different plants under the heads of
silica plants, lime plants, and potash plants; and the following table,
extracted from Liebig's _Agricultural Chemistry_, in which the
constituents of the ash are grouped under the three heads of salts of
potash and soda, lime and magnesia, and silica, gives such a
classification as far as it is at present possible:--

----------------------------------------------------------------------
|                                  | Salts of   | Salts of  | Silica.
|                                  | Potash and | Lime and  |
|                                  | Soda.      | Magnesia. |
|----------------------------------|------------|-----------|---------
|Silica  {  Oat straw with seeds   |  34·00     |   4·00    |  62·00
|Plants. {  Wheat straw            |  22·50     |   7·20    |  61·50
|        {  Barley straw with seeds|  19·00     |  25·70    |  55·30
|        {  Rye straw              |  18·65     |  16·52    |  63·89
|        {  Good hay               |   6·00     |  34·00    |  60·00
|Lime   {  Tobacco                 |  24·34     |  67·44    |   8·30
|Plants {  Pea straw               |  27·82     |  63·74    |   7·81
|       {  Potato plant            |   4·20     |  59·40    |  36·40
|       {  Meadow Clover           |  39·20     |  56·00    |   4·90
|Potash  {  Maize straw            |  72·45     |   6·50    |  18·00
|Plants. {  Turnips                |  81·60     |  18·40    |    --
|        {  Beet root              |  88·00     |  12·00    |    --
|        {  Potatoes               |  85·81     |  14·19    |    --
|        {  Jerusalem Artichoke    |  84·30     |  15·70    |    --
----------------------------------------------------------------------

The special application of these facts must be reserved till we come to
treat of the rotation of crops.

It is manifest that, as the crops removed from the soil all contain a
greater or less amount of inorganic matters, they must be continually
undergoing diminution, and at length be completely exhausted unless
their quantity is maintained from some external source. In many cases
the supply of these substances is so large that ages may elapse before
this becomes apparent, but where the quantity is small, a system of
reckless cropping may reduce a soil to a state of absolute sterility. A
remarkable illustration of this fact is found in the virgin soils of
America, from which the early settlers reaped almost unheard-of crops,
but, by injudicious cultivation, they were soon exhausted and abandoned,
new tracts being brought in and cultivated only to be in their turn
abandoned. The knowledge of the composition of the ash of plants assists
us in ascertaining how this exhaustion may be avoided, and indicates the
mode in which such soils may be preserved in a fertile state.

FOOTNOTES:

[Footnote A: Apparently a species of Sinapis.]

[Footnote B: Oxide of Manganese, 0·42.]

[Footnote C: Oxide of Manganese, 0·92.]

[Footnote D: Alumina, 1·02.]

[Footnote E: Alumina, 0·63.]

[Footnote F: Iodide of Potassium, 0·44; Sulphuret of Sodium, 3·66.]

[Footnote G: Iodide of Potassium, 0·23.]

[Footnote H: Iodide of Potassium, 1·68.]




CHAPTER V.

THE SOIL--ITS CHEMICAL AND PHYSICAL CHARACTERS.


No department of agricultural chemistry is surrounded with greater
difficulties and uncertainties than that relating to the properties of
the soil. When chemistry began to be applied to agriculture, it was not
unnaturally supposed that the examination of the soil would enable us to
ascertain with certainty the mode in which it might be most
advantageously improved and cultivated, and when, as occasionally
happened, analysis revealed the absence of one or more of the essential
constituents of the plant in a barren soil, it indicated at once the
cause and the cure of the defect. But the expectations naturally formed
from the facts then observed have been as yet very partially fulfilled;
for, as our knowledge has advanced, it has become apparent that it is
only in rare instances that it is possible satisfactorily to connect
together the composition and the properties of a soil, and with each
advancement in the accuracy and minuteness of our analysis the
difficulties have been rather increased than diminished. Although it is
occasionally possible to predicate from its composition that a
particular soil will be incapable of supporting vegetation, it not
unfrequently happens that a fruitful and a barren soil are so similar
that it is impossible to distinguish them from one another, and cases
even occur in which the barren appears superior to the fertile soil. The
cause of this apparently anomalous phenomenon lies in the fact that
analysis, however minute, is unable to disclose all the conditions of
fertility, and that it must be supplemented by an examination of its
physical and other chemical properties, which are not indicated by
ordinary experiments. Of late years very considerable progress has been
made in the investigation of the properties of the soil, and many facts
of great importance have been discovered, but we are still unable to
assert that all the conditions of fertility are yet known, and the
practical application of those recently discovered is still very
imperfectly understood.

It must not be supposed that a careful analysis of a soil is without
value, for very important practical deductions may often be drawn from
it, and when this is not practicable it is not unfrequently due to its
being imperfect or incomplete, for it is so complex that the cases in
which all the necessary details have been eliminated are even now by no
means numerous. In fact, the want of a large number of thorough analyses
of soils of different kinds is a matter of some difficulty, and so soon
as a satisfactory mode of investigation can be determined upon, a full
examination of this subject would be of much importance.

_Origin of Soils._--The constituents of the soil, like those of the
plant, may be divided into the great classes of organic and inorganic.
The origin of the former has been already discussed: they are derived
from the decay of plants which have already grown upon the soil, and
which, in various stages of decomposition, form the numerous class of
substances grouped together under the name of humus. The organic
substances may therefore be considered as in a manner secondary
constituents of the soil, which have been accumulated in it as the
consequence of the growth and decay of successive generations of plants,
while the primeval soil consisted of inorganic substances only.

The inorganic constituents of the soil are obtained as the result of a
succession of chemical changes going on in the rocks which protrude
through the surface of the earth. We have only to examine one of these
rocks to observe that it is constantly undergoing a series of important
changes. Under the influence of air and moisture, aided by the powerful
agency of frost, it is seen to become soft, and gradually to
disintegrate, until it is finally converted into an uniform powder, in
which the structure of the original rock is with difficulty, if at all
distinguishable. The rapidity with which these changes take place is
very variable; in the harder rocks, such as granite and mica slate it is
so slow as to be scarcely perceptible, while in others, such as the
shales of the coal formation, a very few years' exposure is sufficient
for the purpose. These actions, operating through a long series of
years, are the source of the inorganic constituents of all soils.

Geology points to a period at which the earth's surface must have been
altogether devoid of soil, and have consisted entirely of hard
crystalline rocks, such as granite and trap, by the disintegration of
which, slowly proceeding from the creation down to the present time, all
the soils which now cover the surface have been formed. But they have
been produced by a succession of very complicated processes; for these
disintegrated rocks being washed away in the form of fine mud, or at
least of minute particles, and being deposited at the bottom of the
primeval seas, have there hardened into what are called sedimentary
rocks, which being raised above the surface by volcanic action or other
great geological forces, have been again disintegrated to yield
different soils. Thus, then, all soils are directly or indirectly
derived from the crystalline rocks, those overlying them being formed
immediately by their decomposition, while those found above the
sedimentary rocks may be traced back through them to the crystalline
rocks from which they were originally formed.

Such being the case, the composition of different soils must manifestly
depend on that of the crystalline rocks from which they have been
derived. Their number is by no means large, and they all consist of
mixtures in variable proportions of quartz, felspar, mica, hornblende,
augite, and zeolites. With the exception of quartz and augite, these
names are, however, representatives of different classes of minerals.
There are, for instance, several different minerals commonly classified
under the name of felspar, which have been distinguished by
mineralogists by the names of orthoclase, albite, oligoclase, and
labradorite; and there are at least two sorts of mica, two of
hornblende, and many varieties of zeolites.

Quartz consists of pure silica, and when in large masses is one of the
most indestructible rocks. It occurs, however, intermixed with other
minerals in small crystals, or irregular fragments, and forms the entire
mass of pure sand.

The four kinds of felspar which have been already named are compounds of
silica with alumina, and another base which is either potash, soda, or
lime. Their composition is as follows, two examples of each being
given--

----------------------------------------------------------------------------
                  |  Orthoclase. |    Albite.  | Oligoclase. | Labradorite. |
------------------|--------------|-------------|-------------|--------------|
Silica            |  65·72| 65·00| 67·99| 68·23| 62·70| 63·51| 54·66|  54·67|
Alumina           |  18·57| 18·64| 19·61| 18·30| 23·80| 23·09| 27·87|  27·89|
Peroxide of iron  | traces|  0·83|  0·70|  1·01|  0·62|   -- |  --  |   0·31|
Oxide of manganese| traces|  0·13|  --  |   -- |   -- |   -- |  --  |    -- |
Lime              |   0·34|  1·23|  0·66|  1·26|  4·60|  2·44| 12·01|  10·60|
Magnesia          |   0·10|  1·03|  --  |  0·51|  0·02|  0·77|  --  |   0·18|
Potash            |  14·02|  9·12|  --  |  2·53|  1·05|  2·19|  --  |   0·49|
Soda              |   1·25|  3·49| 11·12|  7·99|  8·00|  9·37|  5·46|   5·05|
----------------------------------------------------------------------------|
                  | 100·00| 99·47|100·08| 99·83|100·79|101·37|100·00| 99·19 |
-----------------------------------------------------------------------------

It is obvious that soils produced by the disintegration of these
minerals must differ materially in quality. Those yielded by orthoclase
must generally abound in potash, while albite and labradorite,
containing little or none of that element, must produce soils in which
it is deficient. The quality of the soil they yield is not however
entirely dependent on the nature of the particular felspar which yields
it, but is also intimately connected with the extent to which the
decomposition has advanced. It is observed that different felspars
undergo decomposition with different degrees of rapidity but after a
certain time they all begin to lose their peculiar lustre, acquire a
dull and earthy appearance, and at length fall into a more or less white
and soft powder. During this change water is absorbed, and, by the
decomposing action of the air, the alkaline silicate is gradually
rendered soluble, and at length entirely washed away, leaving a
substance which, when mixed with water, becomes plastic, and has all the
characters of common clay. The nature of this change will be best seen
by the following analysis of the clay produced during this composition,
which is employed in the manufacture of porcelain under the name of
kaolin, or china clay--

     Silica             46·80
     Alumina            36·83
     Peroxide of iron    3·11
     Carbonate of lime   0·55
     Potash              0·27
     Water              12·44
                         ----
                       100·00

In this instance the decomposition of the felspar had reached its limit,
a mere trace of potash being left, but if taken at different stages of
the process, variable proportions of that alkali are met with. This
decomposition of felspar is the source of the great deposits of clay
which are so abundantly distributed over the globe, and it takes place
with nearly equal rapidity with potash and soda felspar. It is rarely
complete, and the soils produced from it frequently contain a
considerable proportion of the undecomposed mineral, which continues for
a long period to yield a supply of alkalies to the plants which grow on
them.

Mica is a very widely distributed mineral, and two varieties of it are
distinguished by mineralogists, one of which is characterised by the
large quantity of magnesia it contains. Different specimens are found to
vary very greatly in composition, but the following analyses may
represent their most usual composition:

                                 MICA.
                           |----------------|
                           Potash.  Magnesia.
     Silica                46·36    42·65
     Alumina               36·80    12·96
     Peroxide of iron       4·53      ---
     Protoxide of iron       ---     7·11
     Oxide of manganese     0·02     1·06
     Magnesia                ---    25·75
     Potash                 9·22     6·03
     Hydrofluoric acid      0·70     0·62
     Water                  1·84     3·17
                            ----     ----
                           99·47    99·35

Mica undergoes decomposition with extreme slowness, as is at once
illustrated by the fact that its shining scales may frequently be met
with entirely unchanged in the soil. Its persistence is dependent on the
small quantity of alkaline constituents which it contains; and for this
reason it is observed that the magnesian micas undergo decomposition
less rapidly than those containing the larger quantity of potash.
Eventually, however, both varieties become converted into clay, their
magnesia and potash passing gradually into soluble forms.

Hornblende and augite are two widely distributed minerals, which are so
similar in composition and properties that they may be considered
together. Of the former two varieties, basaltic and common have been
distinguished, and their composition is given below:--

                      Hornblende.
                  |----------------|
                  Common.  Basaltic.    Augite.

Silica             41·50     42·24      50·12
Alumina            15·75     13·92       4·20
Protoxide of iron   7·75     14·59      11·60
Oxide of manganese  0·25      0.33        --
Lime               14·09     12·24      20·55
Magnesia           19·40     13·74      13·70
Water               0·50       --         --
                    ----      ----       ----
                   99·24     97·05      99·67

In these minerals alkalies are entirely absent, and their decomposition
is due to the presence of protoxide of iron, which readily absorbs
oxygen from the air, when the magnesia is separated and a ferruginous
clay left.

The minerals just referred to, constitute the great bulk of the mountain
masses, but they are associated with many others which take part in the
formation of the soil. Of these the most important are the zeolites
which do not occur in large masses but are disseminated through the
other rocks in small quantity. They form a large class of minerals of
which Thomsonite and natrolite may be selected as examples--

        Thomsonite.  Natrolite.

Silica     38·73       48·68
Alumina    30·84       26·36
Lime       13·43         --
Potash      0·54        0·23
Soda        3·85       16·00
Water      13·09        9·55
            ----        ----
          100·48      100·83

They are chiefly characterized by containing their silica in a soluble
state, and hence may yield that substance to the plants in a condition
particularly favourable for absorption.

It is obvious from what has been stated that all these minerals are
capable, by their decomposition, of yielding soft porous masses having
the physical properties of soils, but most of them would be devoid of
many essential ingredients, while not one of them would yield either
phosphoric acid, sulphuric acid, or chlorine. It has, however, been
recently ascertained that certain of these minerals, or at least the
rocks formed from them, contain minute, but distinctly appreciable
traces of phosphoric acid, although in too small quantity to be detected
by ordinary analysis; and small quantities of chlorine and sulphuric
acid may also in most instances be found.

Still it will be observed that most of these minerals would yield a soil
containing only two or three of those substances, which, as we have
already learned, are essential to the plant. Thus, potash felspar, while
it would give abundance of potash, would be but an inefficient source
of lime and magnesia; and labradorite, which contains abundance of lime,
is altogether deficient in magnesia and potash.

Nature has, however, provided against this difficulty, for she has so
arranged it that these minerals rarely occur alone, the rocks which form
our great mountain masses being composed of intimate mixtures of two or
more of them, and that in such a manner that the deficiencies of the one
compensate those of the other. We shall shortly mention the composition
of these rocks.

Granite is a mixture of quartz, felspar, and mica in variable
proportions, and the quality of the soil it yields depends on whether
the variety of felspar present be orthoclase or albite. When the former
is the constituent, granite yields soils of tolerable fertility,
provided their climatic conditions be favourable; but it frequently
occurs in high and exposed situations which are unfavourable to the
growth of plants. Gneiss is a similar mixture, but characterised by the
predominance of mica, and by its banded structure. Owing to the small
quantity of felspar which it contains, and the abundance of the
difficulty decomposable mica, the soils formed by its disintegration are
generally inferior. Mica slate is also a mixture of quartz, felspar, and
mica, but consisting almost entirely of the latter ingredient, and
consequently presenting an extreme infertility. The position of the
granite, gneiss, and mica slate soils in this country is such that very
few of them are of much value; but in warm climates they not
unfrequently produce abundant crops of grain. Syenite is a rock similar
in composition to granite, but having the mica replaced by hornblende,
which by its decomposition yields supplies of lime and magnesia more
readily than they can be obtained from the less easily disintegrated
mica. For this reason soils produced from the syenitic rocks are
frequently possessed of considerable fertility.

The series of rocks of which greenstone and trap are types, and which
are very widely distributed, differ greatly in composition from those
already mentioned. They are divisible into two great classes, which have
received the names of diorite and dolerite, the former a mixture of
albite and hornblende, the latter of augite and labradorite, sometimes
with considerable quantities of a sort of oligoclase containing both
soda and lime, and of different kinds of zeolitic minerals. Generally
speaking, the soils produced from diorite are superior to those from
dolerite. The albite which the former contains undergoes a rapid
decomposition, and yields abundance of soda along with some potash,
which is seldom altogether wanting, while the hornblende supplies both
lime and magnesia. Dolerite, when composed entirely of augite and
labradorite, produces rather inferior soils; but when it contains
oligoclase and zeolites, and comes under the head of basalt, its
disintegration is the source of soils remarkable for their fertility;
for these latter substances undergoing rapid decomposition furnish the
plants with abundant supplies of alkalies and lime, while the more
slowly decomposing hornblende affords the necessary quantity of
magnesia. In addition to these, the basaltic rocks are found to contain
appreciable quantities of phosphoric acid, so that they are in a
condition to yield to the plant almost all its necessary constituents.

The different rocks now mentioned, with a few others of less general
distribution, constitute the whole of our great mountain masses; and
while their general composition is such as has been stated, they
frequently contain disseminated through them quantities of other
minerals which, though in trifling quantity, nevertheless add their
quota of valuable constituents to the soils. Moreover, the exact
composition of the minerals of which the great masses of rocks are
composed is liable to some variety. Those which we have taken as
illustrations have been selected as typical of the minerals; but it is
not uncommon to find albite containing 2 or 3 per cent of potash,
labradorite with a considerable proportion of soda, and zeolitic
minerals containing several per cent of potash, the presence of which
must of course considerably modify the properties of the soils produced
from them. They are also greatly affected by the mechanical influences
to which the rocks are exposed; and being situated for the most part in
elevated positions, they are no sooner disintegrated than they are
washed down by the rains. A granite, for instance, as the result of
disintegration, has its felspar reduced to an impalpable powder, while
its quartz and mica remain, the former entirely, the latter in great
part, in the crystalline grains which existed originally in the granite.
If such a disintegrated granite remains on the spot, it is easy to see
what its composition must be; but if exposed to the action of running
water, by which it is washed away from its original site, a process of
separation takes place, the heavy grains of quartz are first deposited,
then the lighter mica, and lastly the felspar. Thus there may be
produced from the same granite, soils of very different nature and
composition, from a pure and barren sand to a rich clay formed entirely
of felspathic debris.

The sedimentary or stratified rocks are formed of particles carried down
by water and deposited at the bottom of the primeval seas from which
they have been upheaved in the course of geological changes. The
process of their formation may be watched at the present day at the
mouths of all great rivers, where a delta composed of the suspended
matters carried down by the waters is slowly formed. The nature of these
rocks must therefore depend entirely on that of the country through
which the river flows. If its course runs through a country in which
lime is abundant, calcareous rocks will be deposited, and if it passes
through districts of different geological characters the deposit must
necessarily consist of a mixture of the disintegrated particles of the
different rocks the river has encountered. For this reason it is
impossible to enter upon a detailed account of their composition. It is
to be observed, however, that the particles of which they are composed,
though originally derived from the crystalline rocks, have generally
undergone a complex series of changes, geology teaching that, after
deposition, they may in their turn undergo disintegration and be carried
away by water, to be again deposited. Their composition must therefore
vary not merely according to the nature of the rock from which they have
been formed, but also according to the extent to which the decomposition
has gone, and the successive changes to which they have been exposed.
They may be reduced to the three great classes of clays, including the
different kinds of clay slates, shales, etc., sandstone and limestone.
It must be added also, that many of them contain carbonaceous matters
produced by the decomposition of early races of plants and animals, and
that mixtures of two or more of the different classes are frequent.

The purest clays are produced by the decomposition of felspar, but
almost all the crystalline rocks may produce them by the removal of
their alkalies, iron, lime, etc. Where circumstances have been
favourable, the whole of these substances are removed, and the clay
which remains consists almost entirely of silica and alumina, and yields
a soil which is almost barren, not merely on account of the deficiency
of many of the necessary elements of plants, but because it is so stiff
and impenetrable that the roots find their way into it with difficulty.
It rarely happens, however, that decomposition has advanced so far as to
remove the whole of the alkalies, which is exemplified by the following
analyses of the fire clay of the coal formation, and of transition clay
slate:--

                  Transition   Fire Clay.
                  Clay Slate.

Silica              60·03        54·77
Alumina             14·91        28·61
Peroxide of iron     8·94         4·92
Lime                 2·08         0·58
Magnesia             4·22         1·14
Potash               3·87         1·00
Soda                  --          0·24
Carbonic acid }      5·67         8·24
Water         }
                     ----         ----
                    99·72        99·50

The sandstones are derived from the siliceous particles of granite and
other rocks, and consist in many cases of nearly pure silica, in which
case their disintegration produces a barren sand, but they more
frequently contain an admixture of clay and micaceous scales, which
sometimes form a by no means inconsiderable portion of them. Such
sandstones yield soils of better quality, but they are always light and
poor. Where they occur interstratified with clays, still better soils
are produced, the mutual admixture of the disintegrated rocks affording
a substance of intermediate properties, in which the heaviness of the
clay is tempered by the lightness of the sandstone.

Limestone is one of the most widely distributed of the stratified rocks,
and in different localities occurs of very different composition.
Limestones are divided into two classes, common and magnesian; the
former a nearly pure carbonate of lime, the latter a mixture of that
substance with carbonate of magnesia. But while these are the principal
constituents, it is not uncommon to find small quantities of phosphate
and sulphate of lime, which, however trifling their proportions, are not
unimportant in an agricultural point of view. The following analyses
will serve to illustrate the general composition of these two sorts of
limestone as they occur in the early geological formations:--

                          COMMON.                     MAGNESIAN.
                 |-------------------------|   |----------------------|
                  Mid-Lothian.   Sutherland.   Sutherland.   Dumfries.

Silica                2·00          7·42           6·00        2·31
Peroxide of iron }    0·45          0·76           1·57        2·00
  and alumina    }
Carbonate of lime    93·61         84·11          50·21       58·81
Carbonate of }        1·62          7·45          41·22       36·41
  magnesia   }
Phosphate of lime     0·56          ...            ...         ...
Sulphate of lime      0·92          ...            ...         0·10
Organic matter        0·20          ...            ...         ...
Water                 0·50          ...            0·69        ...
                      ----          ----           ----        ----
                     99·86         99·74          99·69       99·63

These limestones are hard and possess to a greater or less extent a
crystalline texture. They are replaced in later geological periods by
others which are much softer, and often purer, of which the oolitic
limestones, so called from their resemblance to the roe of a fish, and
chalk are the most important. Other limestones are also known which
contain an admixture of clay. The soils produced by the disintegration
of limestone and chalk are generally light and porous, but when mixed
with clay, possess a very high degree of fertility, and this is
particularly the case with chalk, which yields some of the most valuable
of all soils. But it is true only of the common limestones, for
experience has shown that those which contain magnesia in large quantity
are often prejudicial to vegetation, and sometimes yield barren or
inferior soils.

Such are the general characters of the three great classes of stratified
rocks; any attempt to particularise the numerous varieties of each would
lead us far beyond the limits of the present work. It is necessary,
however, to remark, that in many instances one variety passes into the
other, or, more correctly speaking, sedimentary rocks occur, which are
mixtures of two or more of the three great classes. In fact, the name
given to each really expresses only the preponderating ingredient, and
many sandstones contain much clay, shales and clay slates abound in
lime, and limestones in sand or clay, so that it may sometimes be a
matter of some difficulty to decide to which class they belong. Such
mixtures usually produce better soils than either of their constituents
separately, and accordingly, in those geological formations in which
they occur, the soils are generally of excellent quality. The same
effect is produced where numerous thin beds of members of the different
classes are interstratified, the disintegrated portions being gradually
intermixed, and valuable soils formed.

The fertility of the soils formed from the stratified rocks is also
increased by the presence of organic remains which afford a supply of
phosphoric acid, and which are sometimes so abundant as to form a by no
means unimportant part of their mass. They do not occur in the oldest
sedimentary rocks, but as we ascend to the more recent geological
epochs, they increase in abundance, until, in the greensands and other
recent formations, whole beds of coprolites and other organic remains
are met with. Great differences are observed in the quality of the soils
yielded by different rocks. In general, those formed by the
disintegration of clay slates are cold, heavy, and very difficult and
expensive to work; those of sandstone light and poor, and of limestone
often poor and thin. These statements must, however, be considered as
very general; for individual cases occur in which some of these
substances may produce good soils, remarkable exceptions being offered
by the lower chalk and some of the shales of the coal formation. Little
is at present known regarding the peculiar nature of many of these
rocks, or their composition; and the cause of the differences in the
fertility of the soil produced from them is a subject worthy of minute
investigation.

_Chemical Composition of the Soil._--Reference has been already made to
the division of the constituents of the soil into the two great classes
of organic and inorganic. And when treating of the sources of the
organic constituents of plants, we entered with some degree of
minuteness into the composition and relations of the different members
of the former class, and expressed the opinion that they did not admit
of being directly absorbed by the plant. But though the parts then
stated lead to the inference that, as a direct source of these
substances, humus is unimportant, it has other functions to perform
which render it an essential constituent of all fertile soils. These
functions are dependent partly on the power which it has of absorbing
and entering into chemical composition with ammonia, and with certain of
the soluble inorganic substances, and partly on the effect which the
carbonic acid produced by its decomposition exerts on the mineral
matters of the soil. In the former way, its effects are strikingly seen
in the manner in which ammonia is absorbed by peat; for it suffices
merely to pour upon some dried peat a small quantity of a dilute
solution of ammonia to find its smell immediately disappear. This
peculiar absorptive power extends also to the fixed alkalies, potash and
soda, as well as to lime and magnesia, and has an important effect in
preventing these substances being washed out of the soil--a property
which, as we shall afterwards see, is possessed also by the clay
contained in greater or less quantity in most soils. On the other hand,
the air and moisture which penetrate the soil cause its decomposition,
and the carbonic acid so produced attacks the undecomposed minerals
existing in it, and liberate the valuable substances they contain.

In considering the composition of a soil, it is important to bear in
mind that it is a substance of great complexity, not merely because it
contains a large number of chemical elements, but also because it is
made up of a mixture of several minerals in a more or less decomposed
state. The most cursory examination shows that it almost invariably
contains sand and scales of mica, and other substances can often be
detected in it. Now it has been already observed that the minerals of
which soils are composed, differ to a remarkable extent in the facility
with which they undergo decomposition, and the bearing of this fact on
its fertility is a matter of the highest importance, for it has been
found that the mere presence of an abundant supply of all the essential
constituents of plants is not always sufficient to constitute a fertile
soil. Two soils, for instance, may be found on analysis to have exactly
the same composition, although in practice one proves barren and the
other fertile. The cause of this difference lies in the particular
state of combination in which the elements are contained in them, and
unless this be such that the plant is capable of absorbing them, it is
immaterial in what quantity they are present, for they are thus locked
up from use, and condemn the soil to hopeless infertility.

It is admitted that unless the substances be present in a state in which
they can be dissolved, the plant is incapable of absorbing them; but it
is a matter of doubt whether it is necessary that they be actually
dissolved in the water which permeates the soil, or whether the plant is
capable of exercising a directly solvent action. The latter view is the
most probable, but at the same time it cannot be doubted, that if they
are presented to the plant in solution, they will be absorbed in that
state in preference to any other. Hence it has been considered important
in the analysis of a soil, not to rest content with the determination of
the quantity of each element it contains, but to obtain some indication
of the state of combination in which it exists, so as to have some idea
of the ease or difficulty with which they may be absorbed. For this
purpose it is necessary to determine, _1st_, The substances soluble in
water; _2d_, Those insoluble in water, but soluble in acids; _3d_, Those
insoluble both in water and acids; and if to these the organic
constituents be added, there are four separate heads under which the
components of a soil ought to be classified. This classification is
accordingly adopted in the most careful and minute analyses; but the
difficulty and labour attending them has hitherto precluded the
possibility of making them except in a few instances; and, generally
speaking, chemists have been contented with treating the soil with an
acid, and determining in the solution all that is dissolved. Such
analyses are often useful for practical purposes, as for example, when
they show the absence of lime, or any other individual substance, by the
addition of which we may rectify the deficiency of the soil; but they
are of comparatively little scientific value, and throw but little light
on the true constitution of the soil, and the sources of its fertility.
Nor is it likely that much satisfactory information will be obtained
until the number of minute analyses is so far extended as to establish
the fundamental principles on which the various properties of the soil
depends.

The separation of the constituents of a soil into the four great groups
already mentioned, is effected in the following manner:--A given
quantity of the soil is boiled with three or four successive quantities
of water, which dissolves out all the soluble matters. These generally
amount to about one-half per cent of the whole soil, and consist of
nearly equal proportions of organic and inorganic substances. In very
light and sandy soils, it occasionally happens that not more than one or
two-tenths per cent dissolve in water, and in peaty soils, on the other
hand, the proportion is sometimes considerably increased, principally
owing to the abundance of soluble organic matters.

When the residue of this operation is treated with dilute hydrochloric
acid, the matters soluble in acids are obtained in the fluid. The
proportion of these substances is liable to very great variations, and
in some soils of excellent quality, and well adapted to the growth of
wheat, it does not exceed 3 per cent; while in calcareous soils, such as
those of the chalk formation, it may reach as much as 50 or 60 per cent.
In general, however, it amounts to about 10 per cent. The organic
constituents are also very variable in amount; ordinary soils of good
quality containing from 2 to 10 per cent, while in peat soils they not
unfrequently reach 30 or even 50 per cent. But these cannot be
considered _fertile_ soils. The insoluble constituents are likewise
subject to great variations, but, in the ordinary clay and sandy soils
of this country, they generally form from 70 to 85 per cent of the
whole.

The distribution of the constituents under these different heads will be
best illustrated by a few analyses of soils of good quality, and for
this purpose we shall select two, noted for the excellent crops of wheat
they produce, and for their general fertility. The analyses were made
from the upper 10 inches, and a quantity of the 10 inches immediately
subjacent was analysed as subsoil. The first is the ordinary wheat soil
of the county of Mid-Lothian, the other the alluvial soil of the Carse
of Gowrie in Perthshire, so celebrated for the abundance and luxuriance
of the crops it produces.

|------------------------------------------|-------------------
|                      |    Mid-Lothian.   |    Perthshire.
|----------------------|-------------------|-------------------
|                      |  Soil. | Subsoil. |   Soil. | Subsoil.
|----------------------|--------|----------|---------|---------
|SUBSTANCES SOLUBLE    |        |          |         |
|  IN WATER.           |        |          |         |
|Silica                | 0·0149 | 0·0104   |  0·0072 | 0·0461
|Lime                  | 0·0300 | 0·0072   |  0·0184 | 0·0306
|Magnesia              | 0·0097 | 0·0016   |  0·0040 | 0·0034
|Chlor. of magnesium   |   --   |   --     |    --   | 0·0033
|Potash                | 0·0034 | 0·0037   |    --   |   --
|Soda                  | 0·0065 | 0·0049   |    --   |   --
|Chloride of potassium |   --   |   --     |  0·0088 | 0·0080
|Chloride of sodium    |   --   |   --     |  0·0110 | 0·0166
|Sulphuric acid        | 0·0193 | 0·0124   |  0·0089 | 0·0239
|Chlorine              |  trace |  trace   |    --   |   --
|Organic matters       | 0·1481 | 0·2228   |  0·0608 | 0·1342
|                      | -------|----------|-------------------
|                      | 0·2319 | 0·2630   |  0·1191 | 0·2661
|----------------------|--------|----------|---------|---------
|SOLUBLE IN ACIDS.     |        |          |         |
|Silica                |  0·1490|   0·0680 |  0·0482 | 0·1697
|Peroxide of iron      |  5·1730|   3·4820 |  4·8700 | 4·6633
|Alumina               |  2·1540|   1·8130 |  2·6900 | 3·9070
|Lime                  |  0·4470|   0·3810 |  0·3616 | 0·5050
|Magnesia              |  0·4120|   0·2850 |  0·3960 | 0·9420
|Potash                |  0·0650|   0·1650 |  0·3445 | 0·1670
|Soda                  |  0·0050|   0·0560 |  0·1242 | 0·1920
|Sulphuric acid        |  0·0250|   0·0850 |  0·0911 | 0·0160
|Phosphoric acid       |  0·4300|   0·1970 |  0·2400 | 0·2680
|Carbonic acid         |    --  |     --   |  0·0500 |   --
|                      |--------|----------|---------|-------
|                      |  8·8600|   6·5320 |  9·2156 |10·8300
|                      |--------|----------|---------|-------
|INSOLUBLE IN ACIDS.   |        |          |         |
|Silica                | 71·3890|  82·5090 | 63·1400 |61·4200
|Alumina               |  4·7810|   3·5120 | 11·3500 |10·3400
|Peroxide of iron      |  trace |   trace  |   --    | 1·5670
|Lime                  |  0·7520|   0·5500 |  0·4500 | 0·7400
|Magnesia              |  0·6610|   0·5500 |  0·6200 | 0·4450
|Potash                |  0·2860|     --   |  2·4500 | 2·0030
|Soda                  |  0·4220|     --   |  1·3100 | 0·8440
|                      |--------|----------|---------|-------
|                      | 78·2910|  87·1210 | 79·3200 |77·3590
|                      |--------|----------|---------|-------
|ORGANIC MATTERS.      |        |          |         |
|                      |        |          |         |
|Insoluble organic }   |        |          |         |
|  matter          }   |  8·8777|   4·2370 |  7·7400 | 6·2910
|Humine                |  0·8850|   0·3450 |  0·0700 | 0·0840
|Humic acid            |  0·1340|   0·0310 |  0·6800 | 0·3600
|Apocrenic acid        |  0·1533|     --   |   --    | 0·0929
|Water                 |  2·6840|   1·7670 |  2·7000 | 4·5750
|                      |--------|----------|---------|-------
|                      | 12·7340|   6·3800 | 11·1900 |11·4020
|                      |========|==========|=========|=======
|Sum of all the        |        |          |         |
|  constituents        |100·1169| 100·2960 | 99·8447 |99·8571
|
|         AMOUNT OF CARBON, HYDROGEN, NITROGEN, AND OXYGEN
|                CONTAINED IN 100 PARTS OF EACH SOIL.
|
|Carbon                |  4·510 |   1·3060 |  2·55   | 2·03
|Hydrogen              |  0·550 |   0·3324 |  0·71   | 0·53
|Nitrogen              |  0·220 |   0·0973 |  0·21   | 0·17
|Oxygen                |  4·918 |   3·1001 |  5·08   | 4·09
|                      |--------|----------|---------|-------
|                      | 10·198 |   4·8358 |  8·55   | 6·82
---------------------------------------------------------------

In examining these analyses, it is particularly worthy of notice that by
far the larger proportion of the substances soluble in water consists of
organic matter, lime, and sulphuric acid, the two last being in
combination as sulphate of lime, while some of those substances which
are usually considered to be the most important mineral constituents of
plants are present in very small quantity--potash, for instance, forming
not more than 1-25,000th of the whole soil, and phosphoric acid being
entirely absent. On the other hand, this portion contains the whole of
the chlorine which exists in the soil, and this might be anticipated
from the ready solubility in water of the compounds of that substance.

The portion soluble in acids consists of alumina and oxide of iron, both
of which are comparatively unimportant to the plant, but very important,
as we shall afterwards see, in relation to the physical properties of
the soil. The remainder of the substances soluble in acids, amounting to
from 1 and 2 per cent, is composed of some of the most essential
constituents of plants. Lime, magnesia, potash, and soda, appear again
in larger quantity than in the soluble part, and along with them we have
the phosphoric acid to the amount of from 0·2 to 0·4 per cent of the
whole soil, and sulphuric acid in much smaller quantity.

The insoluble matters differ remarkably in the two soils, that from the
Carse of Gowrie being characterised by a large quantity of potash and
soda, indicating an important difference in the materials from which
they have been formed. In the Perthshire soil it is obvious that the
felspathic element has been abundant, and that its decomposition has
been arrested at a time, when it still contained a large quantity of
alkalies. And this difference is of great practical importance, because
those soils, which contain a large quantity of potash in their insoluble
portion, have within them a source of permanent fertility, the alkali
being gradually liberated by the decomposition which is constantly in
progress, owing to the air and moisture permeating the soil. As regards
the special distribution of the inorganic matters, it is to be observed
that some of them occur in each of the three heads under which they are
arranged, while others are confined to one or two. Silica and the
alkalies occur generally, though not invariably, in all three. Chlorine
is met with only in the part soluble in water, phosphoric acid only in
that soluble in acids, while sulphuric acid occurs in both the
last-named divisions.

The greater part of the organic matters are insoluble both in water and
acids. At least it is generally believed that any portion dissolved by
strong acids, in the course of analysis, has been entirely decomposed,
and is in a completely different state from that in which it existed
actually in the soil.

As an example of a calcareous soil, forming a striking contrast to those
given above, we select one from the island of Antigua, from which very
large crops of sugar-cane are obtained. The soil is of great depth, and
analyses of the subsoil at the depth of 18 inches and 5 feet are given.
These last analyses are not so minute as that of the soil itself, the
soluble matters not having been separately determined, but included in
that soluble in acids.

+-----------------------------+---------+-----------+--------+
|                             | Surface | 18 inches | 5 feet |
|                             | Soil.   | deep.     | deep.  |
+-----------------------------+---------+-----------+--------+
| SOLUBLE IN WATER.           |         |           |        |
|                             |         |           |        |
| Lime                        |   0·07  |     ...   |    ... |
| Magnesia                    |  trace  |     ...   |    ... |
| Potash                      |   0·06  |     ...   |    ... |
| Soda                        |   0·04  |     ...   |    ... |
| Chlorine                    |   0·05  |     ...   |    ... |
| Organic matter              |   0·15  |     ...   |    ... |
|                             |   ----  |           |        |
|                             |   0·37  |           |        |
| SOLUBLE IN ACIDS.           |         |           |        |
|                             |         |           |        |
| Silica                      |   0·74  |     ...   |    ... |
| Peroxide of iron            |   2·22  |    1·67   |   1·87 |
| Protoxide of iron           |   0·77  |    9·05   |   3·10 |
| Alumina                     |   1·90  |    2·52   |   4·21 |
| Lime                        |  10·43  |    3·04   |  25·75 |
| Magnesia                    |   0·20  |    0·54   |   0·51 |
| Potash                      |   0·03  |    0·29   |   0·28 |
| Soda                        |   0·02  |    0·11   |   0·16 |
| Sulphuric acid              |  trace  |    0·02   |   0·13 |
| Phosphoric acid             |   0·14  |   trace   |   0·04 |
| Carbonic acid               |   7·38  |    0·82   |  20·23 |
|                             |  -----  |   -----   |  ----- |
|                             |  23·83  |   18·06   |  56·28 |
|                             |         |           |        |
| INSOLUBLE IN ACIDS.         |         |           |        |
|                             |         |           |        |
| Silica                      |  41·44  |   51·24   |  27·67 |
| Protoxide of iron           |   3·24  |    0·26   |   1·40 |
| Alumina                     |   9·00  |    1·50   |   1·00 |
| Lime                        |   0·08  |    0·88   |  trace |
| Magnesia                    |   0·80  |    0·54   |  trace |
| Potash                      |    ...  |    0·74   |    ... |
| Soda                        |    ...  |    0·25   |    ... |
|                             |  -----  |   -----   |  ----- |
|                             |  54·56  |   55·41   |  30·07 |
| ORGANIC MATTERS.            |         |           |        |
|                             |         |           |        |
| Humine                      |   1·58 }|           |        |
| Humic acid                  |   1·15 }|   12·05   |  7·49  |
| Insoluble organic matters   |   7·66 }|           |        |
| Water                       |  11·13  |   14·69   |  6·06  |
|                             |  -----  |   -----   | -----  |
|                             |  21·52  |   26·74   | 13·55  |
|                             +---------+-----------+--------+
| Sum of all the constituents | 100·28  |  100·21   | 99·90  |
|                             |=========|===========|========|
+-----------------------------+---------+-----------+--------+

In this soil there is a general resemblance in the composition of the
portion soluble in water to those of the wheat soils. But the part
soluble in acids is distinguished by the great abundance of carbonate of
lime.

The subsoil contains also a large quantity of protoxide of iron, a
substance frequently found in subsoils containing much organic matter,
and to which the air has imperfect access. Under these circumstances
peroxide of iron is reduced to protoxide; and when present abundantly in
the soil in that form, iron has been found to exercise a very injurious
influence on vegetation; and it has frequently happened that when
subsoils containing it have been brought up to the surface, they have in
the first instance caused a manifest deterioration of the soil, although
after some time, when it had become peroxidised by the action of the
air, it ceased to be injurious.

The soil of Holland, from the neighbourhood of the Zuider Zee, which is
an alluvial deposit from the waters of the Rhine, and produces large
crops, gave the results which follow--

+-----------------------------+----------+-----------+-----------+
|                             | Surface. | 15 inches | 30 inches |
|                             |          | deep.     | deep.     |
+-----------------------------+----------+-----------+-----------+
| Insoluble silica            | 57·646   |  51·706   |   55·372  |
| Soluble silica              |  2·340   |   2·496   |    2·286  |
| Alumina                     |  1·830   |   2·900   |    2·888  |
| Peroxide of iron            |  9·039   |  10·305   |   11·864  |
| Protoxide of iron           |  0·350   |   0·563   |    0·200  |
| Oxide of manganese          |  0·288   |   0·354   |    0·284  |
| Lime                        |  4·092   |   5·096   |    2·480  |
| Magnesia                    |  0·130   |   0·140   |    0·128  |
| Potash                      |  1·026   |   1·430   |    1·521  |
| Soda                        |  1·972   |   2·069   |    1·937  |
| Ammonia                     |  0·060   |   0·078   |    0·075  |
| Phosphoric acid             |  0·466   |   0·324   |    0·478  |
| Sulphuric acid              |   0·896  |    1·104  |    0·576  |
| Carbonic acid               |   6·085  |    6·940  |    4·775  |
| Chlorine                    |   1·240  |    1·302  |    1·418  |
| Humic acid                  |   2·798  |    3·991  |    3·428  |
| Crenic acid                 |   0·771  |    0·731  |    0·037  |
| Apocrenic acid              |   0·107  |    0·160  |    0·152  |
| Other organic matters and } |          |           |           |
|   Combined water          } |   8·324  |    7·700  |    9·348  |
| Loss                        |   0·540  |    0·611  |    0·753  |
|                             | -------  |  -------  |  -------  |
|                             | 100·000  |  100·000  |  100·000  |
|                             |==========|===========|===========|
+-----------------------------+----------+-----------+-----------+

It is unnecessary to multiply analyses of fertile soils, those now given
being sufficient to show their general composition. They are all
characterised by the presence, in considerable quantity, of all the
essential constituents of plants, in a state in which they may be
readily absorbed. The absence of one or more of these substances
immediately diminishes or altogether destroys the fertility of the soil;
and the extent to which this occurs is illustrated by the following
analysis of a soil from Pumpherston, Mid-Lothian, forming a small patch
in the lower part of a field, and on which nothing would grow. Being
naturally wet, it had been drained and sowed with oats, which died out
about six weeks after sowing, and left a bare soil on which weeds did
not show the slightest disposition to grow.

        SOLUBLE IN ACIDS.

     Soluble silica     0·173
     Peroxide of iron   6·775
     Alumina            1·150
     Oxide of manganese  trace
     Carbonate of lime   0·856
     Magnesia            0·099
     Potash              0·132
     Soda                0·123
     Phosphoric acid     trace
     Chlorine            trace
                         ----   9·308
     Silica             73·096
     Peroxide of iron    1·371
     Alumina             4·263
     Lime                0·858
     Magnesia            0·520
                         ----  80·108
     Organic matter      8·012
     Water               2·391
                         ----  10·403
                               ------
                               99·819

In this instance the barrenness of the soil is distinctly traceable to
the deficiency of phosphoric acid, sulphuric acid, and chlorine. There
is also a remarkably large quantity of oxide of iron, which, when acted
on by the humic acid, is well known to be highly prejudicial to
vegetation, and that this took place was shown by the fact that the
drains, a couple of months after being laid, were almost stopped up by
humate of iron. Still more striking are the following analyses:--

+----------------------+-----------------+------------+-------------+
|                      | Moorland soil   | Sandy soil | Soil from   |
|                      | near Aurich,    | near       | near        |
|                      | East Friesland. | Wettingen. | Muhlhausen. |
+----------------------+-----------------+------------+-------------+
| Silica and sand      |  70·576         |    96·000  |    77·490   |
| Alumina              |   1·050         |     0·500  |     9·490   |
| Oxide of iron        |   0·252         |     2·000  |     5·800   |
| Oxide of manganese } |   trace         | {   trace  |     0·105   |
| Lime               } |                 | {   0·001  |     0·866   |
| Magnesia             |   0·012   }     |            |     0·728   |
| Potash             } |           }     |            | {   trace   |
| Soda               } |   trace   }     |     trace  | {           |
| Phosphoric acid    } |           }     |            |     0·003   |
| Sulphuric acid     } |           }     |            |     trace   |
| Carbonic acid        |     ...         |       ...  |     0·200   |
| Chlorine             |   trace         |     trace  |     trace   |
| Humic acid           |  11·910         |     0·200  |     0·732   |
| Insoluble humus      |  16·200         |     1·299  |     0·200   |
| Water                |     ...         |       ...  |     4·096   |
|                      |-----------------+------------|-------------|
|                      | 100·000         |   100·000  |   100·000   |
+----------------------+-----------------+------------+-------------+

The results contained in these analyses are peculiarly remarkable, for
they indicate the almost total absence of all those substances which the
plant requires. They must, however, be considered as in a great measure
exceptional cases, as it is but rarely that so large a number of
constituents is absent, and it is much more frequent to find the
deficiency restricted to one or two substances. They are illustrations
of barrenness dependent on different circumstances. The first shows the
unimportance of the organic matters of the soil, which are here
unusually abundant, without in any way counteracting the infertility
dependent on the absence of the other constituents. The second is that
of a nearly pure sand; and the third, though it contains a greater
number of the essential ingredients of the ash, is still rendered
unfruitful by the deficiency of alkalies, sulphuric acid, and chlorine.

An examination of the foregoing analyses indicates pretty clearly some
of the conditions of fertility of the soil, which must obviously
contain all the constituents of the plants destined to grow upon it. But
it by no means exhausts the subject, for numerous instances are known of
soils containing all the essential elements of plants in abundance, but
on which they nevertheless refuse to grow. In these instances the defect
is due either to the presence of some substance injurious to the plant,
or to the state of combination of those it requires being such as to
prevent their absorption. Reference has been already made to the bad
effects of protoxide of iron, and it would appear that organic matter is
sometimes injurious. Even water, by excluding air, and so preventing
those decompositions which play so important a part in liberating the
essential elements from their more permanent compounds, although it
cannot render a soil absolutely barren, not unfrequently materially
diminishes its fertility.

The state of combination of the soil constituents unquestionably
exercise a most important influence on its fertility. That this must be
the case is an inference which may be easily drawn from the statements
already made regarding the different minerals from which it is directly
or indirectly produced. If, for instance, a soil consist to a large
extent of mica, it would be found on analysis to contain abundance of
potash and some other matters, and yet our knowledge of the difficulty
with which that mineral is decomposed, would enable us to pronounce
unfavourably of the soil; and practical experience here fully confirms
the scientific inference.

The forms of combination most favourable to fertility is a subject on
which our information is at present comparatively limited. It was at one
time believed that solubility in water was an indispensable requisite,
but recent investigations appear to lead to a directly contrary
conclusion. The analyses of soils already given, show that the part
directly soluble in water embraces only a certain number of the
constituents of the plant, and of those dissolved the quantity is very
small. This becomes still more apparent if we estimate from the analyses
the actual quantities of those substances contained in an acre of soil.
It is generally assumed that the soil on an imperial acre of land 10
inches deep weighs in round numbers about 1000 tons; and calculating
from this, we find that the quantity of potash soluble in water in the
Mid-Lothian wheat soil, amounts to no more than 70 lb. per acre. But a
crop of hay carries off from the soil about 38 lb. of potash, and one of
turnips, including tops, not less than 200 lb., so that if only the
matters soluble in water could be taken up by the plant, such soils
could not possess the amount of fertility which they are actually found
to have.

It is to be remembered, also, that in these analyses the experiment is
made under the most favourable circumstances for ascertaining the whole
quantity of matters which are capable of dissolving in water; that
practically dissolved is very different. The recent analysis by Krocker
and Way of the drainage water of soils afford a means of estimating
this. Way found in one gallon of the drainage water from seven different
fields, collected in the end of December--

+-------------------+------+------+------+------+------+------+------+
|                   |  1   |  2   |  3   |  4   |  5   |  6   |  7   |
+-------------------+------+------+------+------+------+------+------+
| Potash,           |trace |trace | 0·02 | 0·05 |trace | 0·22 |trace |
| Soda,             | 1·00 | 2·17 | 2·26 | 0·87 | 1·42 | 1·40 | 3·20 |
| Lime,             | 4·85 | 7·19 | 6·05 | 2·26 | 2·52 | 5·82 |13·00 |
| Magnesia,         | 0·68 | 2·32 | 2·48 | 0·41 | 0·21 | 0·93 | 2·50 |
| Iron and Alumina, | 0·40 | 0·05 | 0·10 | none | 1·30 | 0·35 | 0·50 |
| Silica,           | 0·95 | 0·45 | 0·55 | 1·20 | 1·80 | 0·65 | 0·85 |
| Chlorine,         | 0·70 | 1·10 | 1·27 | 0·81 | 1·26 | 1·21 | 2·62 |
| Sulphuric acid,   | 1·65 | 5·15 | 4·40 | 1·71 | 1·29 | 3·12 | 9·51 |
| Phosphoric acid,  |trace | 0·12 |trace |trace | 0·08 | 0·06 | 0·12 |
| Ammonia,          | 0·018| 0·018| 0·018| 0·012| 0·018| 0·006| 0·018|
| Nitric acid,      | 7·17 |14·74 |12·72 | 1·95 | 3·45 | 8·05 |11·45 |
| Organic matter,   | 7·00 | 7·40 |12·50 | 5·60 | 5·70 | 5·80 | 7·40 |
+-------------------+------+------+------+------+------+------+------+

Some of the soils from which these waters were obtained had been manured
with unusually large quantities of nitrogenous matters, which accounts
for the large amount of nitric acid, as well as the lime which that acid
has extracted. Dr. Krocker's analyses were made on soils less highly
manured, and the water was collected in summer.

+-----------------------+------------------------------------------------+
|                       |             IN 10,000 PARTS.                   |
|                       +-------+-------+-------+-------+-------+--------|
|                       | 1     | 2     | 3     | 4     | 5     | 6      |
+-----------------------+-------+-------+-------+-------+-------+--------+
| Organic matter        | 0·25  | 0·24  | 0·16  | 0.06  | 0·63  | 0·56   |
| Carbonate of lime     | 0·84  | 0·84  | 1·27  | 0·79  | 0·71  | 0·84   |
| Sulphate of lime      | 2·08  | 2·10  | 1·14  | 0·17  | 0·77  | 0·72   |
| Nitrate of lime       | 0·02  | 0·02  | 0·01  | 0·02  | 0·02  | 0·02   |
| Carbonate of magnesia | 0·70  | 0·69  | 0·47  | 0·27  | 0·27  | 0·16   |
| Carbonate of iron     | 0·04  | 0·04  | 0·04  | 0·02  | 0·02  | 0·01   |
| Potash                | 0·02  | 0·02  | 0·02  | 0·02  | 0·04  | 0·06   |
| Soda                  | 0·11  | 0·15  | 0·13  | 0·10  | 0·05  | 0·04   |
| Chloride of sodium    | 0·08  | 0·08  | 0·07  | 0·03  | 0·01  | 0·01   |
| Silica                | 0·07  | 0·07  | 0.06  | 0·05  | 0·06  | 0·05   |
+-----------------------+-------+-------+-------+-------+-------+--------+

In order to obtain from these experiments an estimate of the quantity of
the substances actually dissolved, we shall select the results obtained
by Way. The average rainfall in Kent, where the waters he examined were
obtained, is 25 inches. Now, it appears that about two-fifths of all the
rain which falls escapes through the drains, and the rest is got rid of
by evaporation. An inch of rain falling on an imperial acre weighs
rather more than a hundred tons; hence, in the course of a year, there
must pass off by the drains about 1000 tons of drainage water, carrying
with it, out of the reach of the plants, such substances as it has
dissolved, and 1500 tons must remain to give to the plant all that it
holds in solution. These 1500 tons of water must, if they have the same
composition as that which escapes, contain only two and a half pounds of
potash, and less than a pound of ammonia. It may be alleged that the
water which remains, lying longer in contact with the soil, may contain
a larger quantity of matters in solution; but even admitting this to be
the case, it cannot for a moment be supposed that they can ever amount
to more than a very small fraction of what is required for a single
crop. It may therefore be stated with certainty that solubility in water
is not essential to the absorption of substances by the plant, which
must possess the power of itself directly attacking, acting chemically
on, and dissolving them. The mode in which it does this is entirely
unknown, but it in all probability depends on very feeble chemical
actions, and hence the importance of having the soil constituents, not
in solution, but in such a state that they may be readily made soluble
by the plants. Many of the minerals from which fertile soils are formed
are probably not attackable by plants when in their natural condition,
and even after disintegration the quantity of the essential elements of
their food, which are present in an easily assimilable state, is at no
one time very large. But this is of comparatively little importance, for
the soil is not an inert unchangeable substance; it is the theatre of an
important series of chemical changes effected by the action of air and
moisture, and producing a continued liberation of its constituents. This
decomposition is effected partly by the carbonic acid of the atmosphere,
but to a much larger extent by its oxygen acting upon the organic
matters of the soil, and causing a constant though slow evolution of
that acid, which in its turn attacks the mineral matters. Boussingault
and Levy have illustrated the extent of this action by examining the
composition of the air contained in the pores of different soils, and
have obtained the following results:--

+--------------------+------------+---------------+---------------------------+
| Nature of          | Crop.      | No. of cubic  |100 VOLUMES OF AIR CONTAIN |
|       Soil.        |            | inches of air +--------+--------+---------+
|                    |            | in 34 cubic   |Carbonic| Oxygen.|Nitrogen.|
|                    |            | inches of soil| acid.  |        |         |
+--------------------+------------+---------------+--------+--------+---------+
|Light sandy soil,   |            |               |        |        |         |
|   newly manured    |   ...      |   8·0         |  2·17  |    ... |    ...  |
|  Do. manured       |            |               |        |        |         |
|    8 days before   |   ...      |   ...         |  1·54  |   18·80|  79·66  |
|  Do. long          |Yellow      |               |        |        |         |
|    after manuring  |  turnip    |   7·9         |  0·93  |   19·50|  79·57  |
|Very sandy          |Vineyard    |   9·6         |  1·06  |   19·72|  79·22  |
|Sandy, with many    |            |               |        |        |         |
|  stones            |Forest      |   4·0         |  0·87  |   19·61|  79·52  |
|Loamy               |   ...      |   2·4         |  0·46  |    ... |    ...  |
|Sandy, subsoil      |            |               |        |        |         |
|  of the last       |   ...      |   3·0         |  0·24  |    ... |    ...  |
|Sandy soil, long    |            |               |        |        |         |
|  after manuring    |Trefoil     |   7·6         |  0·74  |   19·02|  80·24  |
|  Do.  Recently     |            |               |        |        |         |
|    manured         |   ...      |   ...         |  0·85  |   19·41|  79·74  |
|  Do. manured 8     |            |               |        |        |         |
|    days before     |   ...      |   ...         |   1·54 |   18·80|  79·66  |
|Heavy clay          |Jerusalem   |   7·0         |   0·66 |   19·99|  79·35  |
|                    |  artichoke |               |        |        |         |
|Fertile soil (moist)|Meadow      |   5·5         |   1·79 |   19·41|  78·80  |
+--------------------+------------+---------------+--------+--------+---------+

From these analyses it appears that the air contained in the pores of
the soil is much richer in carbonic acid than the atmosphere, the
poorest soil containing about 25 times, and a recently manured soil 250
times as much. This carbonic acid, which is obviously produced by the
decomposition of the vegetable matters and manure, acting partly as gas
and partly dissolved in the soil water, exerts a solvent action on its
constituents. And, though a very feeble acid, its continuous action
produces in the course of time a large effect; while, during the
interval, the constituents of the soil are safely stored up, and
liberated only as the plant requires them, by which bountiful provision
of nature they are exposed to fewer risks of loss than if they had been
all along in a state in which they could be absorbed. Carbonic acid not
only assists in effecting the decomposition of the minerals of the soil,
but its aqueous solution acts as a solvent of many substances, which
are quite insoluble in pure water. It is in this way that much of the
lime contained in natural waters is held in solution, and it has been
ascertained that magnesia, iron, and even phosphate of lime, may also be
dissolved by it. It is probable that when these substances are
dissolved, the plants will take them from solution in place of
themselves attacking the insoluble matters; but of the extent to which
this may occur nothing is yet known--the action of solvents on the soil
being a subject which is as yet scarcely examined.

Carbonic acid is, however, a most important agent in producing the
chemical changes in the soil, and the particular value of humus lies in
its affording a supply of that substance exactly when it is wanted; but
the carbonic acid of the atmosphere also takes part in these changes,
although with different degrees of rapidity according to the character
of the soil, acting rapidly in light, and slowly in stiff, clay soils.
The solvent action of the carbonic acid is, no doubt, principally
exerted on the substances soluble in acids, but not entirely, for it is
known that the insoluble part is gradually being disintegrated and made
soluble; and hence it is that the composition of that part of the soil
which resists the action of acids, and which at first sight might appear
of no moment, is really important. It is obvious that this circumstance
must at once confer on the soil of the Carse of Gowrie a great
superiority over those of Mid-Lothian and most other districts; for it
contains in its insoluble part a quantity of alkalies which must
necessarily form a source of continued fertility. Accordingly,
experience has all along shown the great superiority of that soil, and
of alluvial soils generally, which are all more or less similar to it.
The facility with which these matters are attackable by carbonic acid is
also an important element of the fertility of a soil, and it is to the
existence of compounds which are readily decomposed by it that we
attribute the high fertility of the trap soils.

By a further examination of the analyses of fertile soils, it is at once
apparent that the most essential constituents of plants are by no means
very abundant in them. In fact, phosphoric and sulphuric acids, lime,
magnesia, and the alkalies, which in most instances make up nine-tenths
of the ash of plants, form but a small portion of even the most fertile
soils; while silica, which, except in the grasses, occurs in small
quantity, oxide of iron which is a limited, and alumina a rare,
constituent of the ash, constitute by far their larger part. Thus the
total amount of potash, soda, lime, magnesia, phosphoric and sulphuric
acids and chlorine, contained in the Mid-Lothian wheat soil amounts only
to 3·5888 per cent, and in the Perthshire to 6·4385, the entire
remainder being substances which enter into the plant for the most part
in much smaller quantity. And, as these small quantities of the more
important substances are capable of supplying the wants of the plant, it
must be obvious that a very small fraction of the silica, oxide of iron,
and alumina, which the soils contain, would afford to it the whole
quantity of these substances it requires, and that the remainder must
have some other functions to perform.

The soil must be considered not merely as the source of the inorganic
food of plants, for it has to act also as a support for them while
growing, and to retain a sufficient quantity of moisture to support
their life; and unless it possess the properties which fit it for this
purpose, it may contain all the elements of the food of plants, and yet
be nearly or altogether barren.

The adaptation of the soil to this function is dependent to a great
extent on its mechanical texture, and on this considerable light is
frequently thrown by a kind of mechanical analysis.

If a soil be shaken up with water and allowed to stand for a few
minutes, it rapidly deposits a quantity of grains which are at once
recognised as common sand; and if the water be then poured off into
another vessel and allowed to stand for a longer time, a fine soft
powder, having the properties and composition of common clay, is
deposited, while the clear fluid retains the soluble matters. By a more
careful treatment it is possible to distinguish and separate humus, and
in soils lying on chalk or limestone, calcareous matter or carbonate of
lime.

In this way the components can be classified into four groups, a mixture
of two or more of which in variable proportions is found in all soils.

The relative proportions in which these substances exist in soils are,
as we shall afterwards see, the foundation of their classification into
the light, heavy, calcareous, and other sub-divisions. But they are also
intimately connected with certain chemical and mechanical peculiarities
which have an important bearing on its fertility. It is a familiar fact,
that particular soils are specially adapted to the growth of certain
crops; and we talk of a wheat or a turnip soil as readily
distinguishable. It is to be observed, however, that in many such
instances the mere analysis may show no difference, or, at least, none
sufficient to account for the peculiarity. A remarkable illustration is
offered by the following analyses of two soils, on one of which red
clover grows luxuriantly, while on the other it invariably fails.

                           Clover fails.  Clover succeeds.

Insoluble silicates           83·90            81·34
Soluble silica                 0·08             0·02
Peroxide of iron               4·45             6·68
Alumina                        2·40             3·00
Lime                           1·23             1·33
Magnesia                       0·45             0·25
Potash                         0·20             0·22
Soda                           0·07             0·09
Sulphuric acid                 0·05             0·08
Phosphoric acid                0·38             0·07
Carbonic acid                  0·09             0·34
Chlorine                       trace            trace
Humic acid                     0·42             0·43
Humine                         ...              0·10
Insoluble organic matters      3·70             3·61
Water                          2·54             2·52
                               ----             ----
                              99·96           100·08
Nitrogen                       0·15             1·15

In this instance such difference as exists is rather in favour of the
soil on which clover fails, but it is exceedingly trifling; and it is
necessary to seek an explanation in the special properties of its
mechanical constituents.

These properties are partly mechanical and partly chemical, and in both
ways exercise an important influence on the fertility of the soil.

Sand and clay, the most important of the mechanical constituents, confer
on the soil diametrically opposite properties; the former, when present
in large quantity, producing what are designated as light, the latter
stiff or heavy soils. The hard indestructible siliceous grains, of which
sand is composed, form a soil of an open texture, through which water
readily permeates; while clay, from its fine state of division, and
peculiar adhesiveness or plasticity, gives it a close-textured and
retentive character, and their proper intermixture produces a light
fertile loam, each tempering the peculiar properties of the other.
Indeed, their mixture is manifestly essential, for sand alone contains
little or none of the essential ingredients of plants; and if present in
large quantity, the openness of the soil is excessive, water flows
through it with rapidity, manures are rapidly wasted, and on the
accession of drought, the plants growing upon it soon languish and die.
Clay, on the other hand, is by itself equally objectionable; the
closeness of its texture prevents the spreading of the roots of plants,
and the access of carbonic acid, which, as we have already seen, is so
important an agent in the changes occurring in the soil. In fact, a pure
clay, that is to say, a clay unmixed with sand, even though it may
contain all the essential constituents of the plant, is for this reason
unfertile. Practically, of course, these extreme cases rarely occur; the
heaviest clay soils being mixtures of true clay with sand, and the most
sandy containing their proportion of clay; but frequently the
preponderance of the one over the other is so great, as to produce soils
greatly inferior to those in which the mixture is more uniform.

It is easy to understand how the proportions in which sand and clay are
mixed must affect the suitability of soils to particular crops, and that
an open soil must be favourable to the turnip, and a heavy clay, owing
to the resistance it offers to the expansion of the bulbs, unfavourable.
But these substances also exercise an important chemical action on the
soluble constituents of the food of plants, combining with them, and
converting them into an insoluble, or nearly insoluble state, so as to
prevent their being washed away by the rain or other water which
percolates through the soil. It has long been known to chemists that
clay has a tendency to absorb a small proportion of ammonia, and even
when brought up from a great depth frequently contains that substance.
It is to Mr. Thompson of Moat Hall, however, that we owe the important
observation, that arable soils rapidly remove ammonia from solution, and
Way, who pursued this investigation, showed that not only ammonia, but
potash, and several of the other important elements of the food of
plants, are thus absorbed. The removal of these substances from solution
is easily illustrated by a simple experiment. It suffices to take a tall
cylindrical vessel open at both ends, and filled with the soil to be
operated upon, which is retained by a piece of rag tied over its lower
end. A quantity of a dilute solution of ammonia being then poured upon
the surface of the soil, and allowed to percolate, the first quantity
which flows away is found to have entirely lost its peculiar smell and
taste; and in a similar manner the removal of potash may be illustrated.
This action is by no means confined to those substances when in the free
state, but is equally marked when they are combined with acids in the
form of salts, and in the latter case the absorption is attended with a
true chemical decomposition, the base only being retained, and the acid
escaping most commonly in combination with lime. Thus, if sulphate of
ammonia be employed, the water which flows from the soil contains
sulphate of lime, and if muriate of ammonia be used, it is muriate of
lime which escapes.

This absorbent action is most remarkably manifested in the case of
ammonia and potash, but it takes place also with magnesia and soda. With
the latter, however, it is incomplete, only a half or a fourth of the
soda being removed from solution, the difference depending to some
extent on the acid with which it is in combination. The extent to which
absorption takes place varies also with the nature of the soil, and the
state of combination of the substance used. Exact experiments have
hitherto been chiefly confined to ammonia, potash, and lime in the free
state, and as bicarbonate; and the following table gives the results
obtained by Way, with solutions containing about 1 per cent of these
substances in solution:--

-----------------------------------------------------------------------------
|                       |     Loamy  |  Red soil,|   Pure   | Subsoil       |
|                       |     soil,  | Berkshire.|    clay. |   clay,       |
|                       |Dorsetshire.|           |          | Somersetshire.|
------------------------|------------------------|----------|---------------|
|Ammonia, caustic       |     0·3438 |   0·1570  |     ...  |   ...         |
|   "     from muriate  |     0·3478 |   0·1966  |    0.2847|  0·0818       |
|Potash, caustic        |       ...  |     ...   |    1·050 |  2·087        |
|  "     from nitrate   |       ...  |     ...   |    0·4980|    ...        |
|Lime, caustic          |       ...  |     ...   |    1·468 |    ...        |
|  "   from bicarbonate |       ...  |     ...   |    0·731 |    ...        |
------------------------|------------|-----------|----------|---------------

From these numbers it appears that very great differences exist in the
absorbent power of different soils, the first of those experimented on
being capable of taking more than twice as much ammonia as the second,
and nearly four times as much as the subsoil clay. It appears also, as
far as absorption goes, to be immaterial whether the ammonia is free or
combined. But it is different with potash, which is absorbed from the
nitrate to the extent of about O·6 per cent, and from a caustic solution
of potash to double that amount.

The circumstances under which absorption takes place modify, in a manner
which cannot well be explained, the amount absorbed by the same soil. It
is found generally to be most complete with very dilute solutions, and
if a soil be agitated with a quantity of ammonia larger than it can
take up, it will absorb only a certain amount of that substance, but by
a further increase of the amount of ammonia a still larger quantity will
be absorbed.

It is important to observe that when a salt is used, the base only is
absorbed, and the acid escapes in combination with lime; even nitric
acid, notwithstanding its importance as a food of plants, being in this
predicament. From this it may be gathered that lime is not readily
absorbed from solutions of its salts; indeed, it would appear that the
only salt of that substance liable to absorption is the bicarbonate,
from which it is taken to the extent of 1·4 per cent by the soil. The
absorption of lime from this salt, and that of phosphoric acid, which
takes place to a considerable extent, probably occurs, however, quite
independently of the clay present in the soil, and is occasioned by its
_lime_, which forms an insoluble compound with phosphoric acid, and by
removing half the carbonic acid of the bicarbonate of lime converts it
also into an insoluble state.

In addition to these mineral substances, organic matters are also
removed from solution. This is conspicuously seen in the case of putrid
urine, which not only loses its ammonia, but also its smell and colour,
when allowed to percolate through soil; and an equally marked result was
obtained with flax water, from which the organic matter was entirely
abstracted.

The cause of this absorptive power is still very imperfectly known. Mr.
Way having observed that sand has no such property, while clay, even
when obtained from a considerable depth, always possesses it, supposed
that the absorption was entirely due to that substance. A difficulty,
however, presents itself in explaining how it should happen that while a
pure clay absorbs only 0·2847 of ammonia, a loamy soil, of which
one-half probably is sand, should absorb a larger quantity. The
inference is, that the effect cannot be due to the clay as a whole, and
Mr. Way has sought to explain it by supposing that there exist in the
soil particular double silicates of alumina and lime. He has shown that
felspar and the other minerals from which the soil is produced have no
absorbent power, but that artificial compounds can be formed which act
upon solutions of ammonia and potash in a manner very similar to the
soil; but there is not the slightest evidence that these compounds exist
in the soil, and in the year 1853[I] I pointed out the probability that
clay is not the only agent at work, but that the organic matters take
part in the process. So powerful indeed is the affinity of these
substances for ammonia, that chemists are at one as to the difficulty of
obtaining humic and other similar acids pure, owing to the obstinacy
with which they retain it; and there cannot be a doubt that in many
soils these substances are in this point of view of much importance.
This is particularly the case in peat soils, which, though naturally
barren, may be made to produce good crops by the application of sand or
gravel; and as neither of these can cause any absorption of the valuable
matters, we must attribute this effect to the organic matter. Referring
to an earlier series of experiments made in 1850, I showed that, if a
quantity of dry peat be taken and ammonia poured on it, its smell
disappears; and this may be continued until upwards of 1·5 per cent of
dry ammonia has been absorbed, and this quantity is _retained_ by the
peat.

In this case pure ammonia was used, but Way's experiments having shown
that this alkali is not absorbed from its salts by organic matters, I
expressed the opinion that humate of lime (which certainly exists in
most soils) ought on chemical grounds to decompose the salts of ammonia
and cause the retention of their base. The recent researches of
Brustlein have shown that lime does cause the organic matters to absorb
ammonia from its salts. He confirms the fact that pure ammonia is
absorbed by peat, and shows that decayed wood has the same effect,
although both are without action on solutions of its salts. A stiff
clay, on the other hand, containing organic matters and much carbonate
of lime, readily absorbed ammonia, both when pure and combined; but
after extracting the lime by means of a dilute acid, it lost the power
of taking it from its salts, although it retained the free alkali as
completely as before. On the addition of a small quantity of lime, it
again acquired the power of withdrawing ammonia from its compounds.
These experiments may be explained, either on the supposition of the
presence of humate of lime, or by supposing that the carbonate of lime
first decomposed the salts of ammonia, and that the liberated alkali
combined with the organic matter. It must be admitted, however, that it
is very doubtful whether the ammonia and other substances are fixed in
the soil by a true chemical combination. They are certainly retained by
a very feeble attraction, for it appears from Brustlein's experiments
that ammonia may be, to a considerable extent, removed by washing with
abundance of water, and that if the soil which has absorbed ammonia be
allowed to become dry in the air, it loses half its ammonia, and after
four times moistening and drying, three-fourths have disappeared. These
facts are certainly not incompatible with the presence of a true
chemical compound, for the humate of ammonia is not absolutely
insoluble, and many cases occur of actions taking place in the presence
of water, which are entirely reversed when that fluid is removed; and it
is quite possible that when humate of ammonia is dried in contact with
carbonate of lime, it may be decomposed, and carbonate of ammonia
escape. There are other circumstances, however, which render it, on the
whole, most probable that the combination is not wholly chemical, but
rather of a physical character, among which may be more especially
mentioned the fact, that the quantity of the substances retained by the
soil is dependent on the degree of dilution of the fluid from which they
are taken; and that the quantity absorbed never exceeds a very small
fraction of the weight of the soil.

The practical inferences to be drawn from these facts regarding the
value of soils are of the highest importance. It is obvious that two
soils having exactly the same chemical composition may differ widely in
absorptive power, and that which possesses it most largely must have the
highest agricultural value. The examination of different soils, in this
point of view, is a subject of much importance, and deserves the best
attention of both farmers and chemists, although little has as yet been
done in regard to it, and the results which have been obtained are not
of a very satisfactory character. Liebig states, that in his
experiments, all the arable soils examined possessed the same absorptive
power, whether they contained a large or a small proportion of lime or
alumina. It can scarcely be expected, however, that this should be true
in all cases, and there are many facts which seem to indicate that
differences must exist. It is well known that there are some soils in
which the manure is very rapidly exhausted, and it is more than probable
that this effect is due to deficient absorptive power, which leaves the
soluble matters at the mercy of the weather, and liable at any moment
to be washed out by a heavy fall of rain.

The more strictly mechanical properties of the soil, such as its
relations to heat and moisture, are not less important than its chemical
composition. It is known that soils differ so greatly in these respects
as sometimes materially to affect their productive capacity. Thus, for
instance, two soils may be identical in composition, but one may be
highly hygrometric, that is, may absorb moisture readily from the air,
while the other may be very deficient in that property. Under ordinary
circumstances no difference will be apparent in their produce, but in a
dry season the crop upon the former may be in a flourishing condition,
while that on the latter is languishing and enfeebled, merely from its
inability to absorb from the air, and supply to the plant the quantity
of water required for its growth. In the same way, a soil which absorbs
much heat from the sun's rays surpasses another which has not that
property; and though in many cases this effect is comparatively
unimportant, in others it may make the difference between successful and
unsuccessful cultivation in soils which lie in an unfavourable climate
or exposure.

The investigation of the physical characters of soils has attracted
little attention, and we owe all our present knowledge of the subject to
a very elaborate series of researches on this subject, published by
Schübler, nearly thirty years ago. He determined _1st_, The specific
gravity of the soils; _2d_, The quantity of water which they are capable
of imbibing; _3d_, The rapidity with which they give off by evaporation
the water they have imbibed; that is, their tendency to become dry;
_4th_, The extent to which they shrink in drying; _5th_, Their
hygrometric power; _6th_, The extent to which they are heated by the
sun's rays; _7th_, The rapidity with which a heated soil cools down,
which indicates its power of _retaining_ heat; _8th_, Their tenacity, or
the resistance they offer to the passage of agricultural implements;
_9th_, Their power of absorbing oxygen from the air. Each of these
experiments was performed on several different soils, and on their
mechanical constituents. Schübler's experiments are undoubtedly
important, and though the methods employed are some of them not
altogether beyond cavil, they have apparently been performed with great
care. It is nevertheless desirable that they should be repeated, for
such facts ought not to rest on the authority of one experimenter,
however skilful and conscientious, nor on a single series of soils,
which may not give a fair representation of their general physical
properties. In fact, Schübler appears to imagine that having once
determined the extent to which the sand, clay, and other mechanical
constituents of the soil possess these properties, we are in a condition
to predicate the effect of their mixture in variable proportions,
although this is by no means probable.

In examining these properties, Schübler selected for experiment, pure
siliceous sand, calcareous sand (carbonate of lime in coarse grains),
finely powdered carbonate of lime, pure clay, humus, and powdered
gypsum. He used also a heavy clay consisting of 11 per cent of sand and
89 of pure clay, a somewhat stiff clay containing 24 per cent of sand
and 76 of clay, a light clay with 40 per cent of sand and 60 of pure
clay, a garden soil consisting of 52·4 per cent of clay, 36·5 of
siliceous sand, 1·8 of calcareous sand, 2 per cent of finely divided
carbonate of lime, and 7·2 of humus, and two arable soils, one from
Hoffwyl, and one from a valley in the Jura, the former a somewhat stiff,
the latter a light soil.


+-------------------+----------+-----------+-----------+------------+
|                   |          |           | Of 100    |            |
|                   |          |           | parts of  | Diminution |
|                   |          | Water     | water     | in bulk    |
|                   |          | absorbed  | absorbed  | during     |
|                   |          | by 100    | there     | drying of  |
|                   | Specific | parts     | evaporate | 100 parts  |
|                   | gravity. | per cent. | in four   | moist      |
|                   |          |           | hours     | soil       |
|                   |          |           | at 66°    |            |
+-------------------+----------+-----------+-----------+------------+
| Siliceous sand    | 2·753    |  25       | 88·4      |  0·0       |
| Calcareous sand   | 2·822    |  29       | 75·9      |  0·0       |
| Light clay        | 2·701    |  40       | 52·0      |  6·0       |
| Stiff clay        | 2·652    |  50       | 45·7      |  8·9       |
| Heavy clay        | 2·603    |  61       | 34·9      | 11·4       |
| Pure clay         | 2·591    |  70       | 31·3      | 18·3       |
| Carbonate of lime | 2·468    |  85       | 28·0      |  5·0       |
| Humus             | 1·225    | 190       | 20·5      | 20·0       |
| Gypsum            | 2·358    |  27       | 71·7      |  0·0       |
| Garden soil       | 2·332    |  96       | 24·5      | 14·9       |
| Soil from Hoffwyl | 2·401    |  52       | 32·0      | 12·0       |
| Soil from Jura    | 2·526    |  47       | 40·1      |  9·5       |
+-------------------+----------+-----------+-----------+------------+

+-------------------+-------------------------------------------+------------+
|                   | Quantity                                  | Power of   |
|                   | of                                        | retaining  |
|                   | hygrometric water absorbed                | heat.      |
|                   | by 77·165 grains of the soil spread on    | Calcareous |
|                   | a surface of 141·48 square inches.        | sand,      |
--------------------+----------+----------+----------+----------+ 100.       |
|                   | 12 hours.| 24 hours.| 48 hours.| 72 hours.|            |
--------------------+----------+----------+----------+----------+-------------
| Siliceous sand    | 0        |  0       |   0      |   0      |  95·6      |
| Calcareous sand   | 0·154    | 0·231    | 0·231    | 0·231    | 100·0      |
| Light clay        | 1·617    | 2·002    | 2·156    | 2·156    |  76·9      |
| Stiff clay        | 1·925    | 2·310    | 2·618    | 2·695    |  71·1      |
| Heavy clay        | 2·310    | 2·772    | 3·080    | 3·157    |  68·4      |
| Pure clay         | 2·849    | 3·234    | 3·696    | 3·773    |  66·7      |
| Carbonate of lime | 2·002    | 2·387    | 2·695    | 2·695    |  61·8      |
| Humus             | 6·160    | 7·469    | 8·470    | 9·240    |  49·0      |
| Gypsum            | 0·077    | 0·077    | 0·077    | 0·077    |  73·2      |
| Garden soil       | 2·695    | 3·465    | 3·850    | 4·004    |  64·8      |
| Soil from Hoffwyl | 1·232    | 1·771    | 1·771    | 1·771    |  70·1      |
| Soil from Jura    | 1·078    | 1·463    | 1·540    | 1·540    |  74·3      |
--------------------+----------+----------+----------+----------+------------+

|------------------+------------+-----------------|
|                  |            | Quantity of     |
|                  |            | oxygen absorbed |
|                  |            | by              |
|                  |            | 77·165 grains   |
|                  |            | of the moist    |
|                  |            | soil in 30      |
|                  | Tenacity   | days, from 15   |
|                  | of the     | cubic inches    |
|                  | soils.     | of atmospheric  |
|                  | Pure clay, | air.            |
|                  |  100.      | Expressed in    |
|                  |            | cubic inches.   |
+-----------------+|------------|-----------------|
|Siliceous sand    |   0        | 0·24            |
|Calcareous sand   |   0        | 0·84            |
|Light clay        |  57·3      | 1·39            |
|Stiff clay        |  68·8      | 1·65            |
|Heavy clay        |  83·3      | 2·04            |
|Pure clay         | 100·0      | 2·29            |
|Carbonate of lime |   5·0      | 1·62            |
|Humus             |   8·7      | 3·04            |
|Gypsum            |   7·3      | 0·40            |
|Garden soil       |   7·6      | 2·60            |
|Soil from Hoffwyl |  33·0      | 2·43            |
|Soil from Jura    |  22·0      | 2·25            |
---------------------------------------------------

The experiments detailed in the preceding table speak in a great measure
for themselves, and scarcely require detailed comment. It may be
remarked, however, that the columns illustrating the relations of the
soil to water are probably more important than the others. The
superiority of a retentive over an open soil is sufficiently familiar in
practice, and though this is no doubt partly due to the former absorbing
and retaining more completely the ammonia and other valuable
constituents of the manures applied to it, it is also dependent to an
equal if not greater extent upon the power it possesses of retaining
moisture. A reference to the table makes it apparent that this power is
presented under three different heads, which are certainly related to
one another, but are not identical. In the second column of the table is
given the quantity of water absorbed by the soil, determined by placing
a given weight of the perfectly dry soil in a funnel, the neck of which
is partially stopped with a small piece of sponge or wool, pouring water
upon it, and weighing it after the water has ceased to drop from it.
This may be considered as representing the quantity of water retained by
these different soils when thoroughly saturated by long continued rains.
The column immediately succeeding gives the quantity of that water which
escapes by evaporation from the same soil after exposure for four hours
to dry air at the temperature of 66°. The fifth, sixth, seventh, and
eighth columns indicate the quantity of moisture absorbed, when the
soil, previously artificially dried, is exposed to moist air for
different periods. These characters are dependent principally, though
not entirely, on the porosity of the soil. The last may also be in some
measure due to the presence of particular salts, such as common salt,
which has a great affinity for moisture, but is chiefly occasioned by
their peculiar structure. It is to be remarked that clay and humus are
two of the most highly hygrometric substances known, and it is
peculiarly interesting to observe, that by a beneficent provision of
nature, they also form a principal part of all fertile soils. The
quantity of water imbibed by the soil is important to its fertility, in
so far as it prevents it becoming rapidly dry after having been
moistened by the rains. It is valuable also in another point of view,
because if the soil be incapable of absorbing much water, it becomes
saturated by a moderate fall of rain, and when a larger quantity falls,
the excess of necessity percolates through the soil, and carries off
with it a certain quantity of the soluble salts. Important as this
property is, however, it must not be possessed in too high a degree, but
must permit the _evaporation_ of the water retained with a certain
degree of rapidity. Soils which do not admit of this taking place are
the cause of much inconvenience and injury in practice. By becoming
thoroughly saturated with moisture during winter, they remain for a long
time in a wet and unworkable condition, in consequence of which they
cannot be prepared and sown until late in the season, and though
chemically unexceptionable, they are always disadvantageous, and in some
seasons greatly disappoint the hopes of the farmer.

The extent to which the imbibition and evaporation of water takes place
is very variable, but they are obviously related to one another, the
soils which absorb it least abundantly parting with it again with the
greatest, facility; for it appears that siliceous sand absorbs only
one-fourth of its weight of water, and again gives off in the course of
four hours four-fifths of that it had taken up, while humus, which
imbibes nearly twice its weight, retains nine-tenths of that quantity
after four hours' exposure. Long-continued and slow evaporation of the
water absorbed by a soil is injurious in another way, for it makes the
soil "cold"--a term of practical origin, but which very correctly
expresses the peculiarity in question. It is due to the fact, that when
water evaporates it absorbs a very large quantity of heat, which
prevents the soil acquiring a sufficiently high temperature from the
sun's rays. The soils which have absorbed a large quantity of moisture
shrink more or less in the process of drying, and form cracks, which
often break the delicate fibres of the roots of the plants, and cause
considerable injury: the extent of this shrinking is given in the fourth
column.

The relation of the soils to heat divides itself into two
considerations: the amount of heat absorbed by the soil, and the degree
in which it is retained. Of these the latter only is illustrated in the
table. The former is dependent on so many special considerations, that
the results cannot be tabulated in a satisfactory manner. It is
independent of the chemical nature of the soil, but varies to a great
extent according to its colour, the angle of incidence of the sun's
rays, and its state of moisture. It is, however, an important character,
and has been found by Girardin to exercise a considerable influence on
the rapidity with which the crop ripens. He found in a particular year
that, on the 25th of August, 26 varieties of potatoes were ripe on a
very dark-coloured sandy vegetable mould, 20 on an ordinary sandy soil,
19 on a loamy soil, and only 16 on a nearly white calcareous soil.

The tenacity of the soil is very variable, and indicates the great
differences in the amount of power which must be expended in working
them. According to Schübler, a soil whose tenacity does not exceed 10,
is easily tilled, but when it reaches 40 it becomes very difficult and
heavy to work.

On examining the table it becomes manifest, that as far as its
mechanical properties are concerned, humus is a substance of the very
highest importance, for it confers on the soil, in a high degree, the
power of absorbing and retaining water, diminishes its tenacity and
permits its being more easily worked, adds to its hygrometric power and
property of absorbing oxygen from the air, and finally, from its dark
colour, causes the more rapid absorption of heat from the sun's rays. It
will be thus understood, that though it does not directly supply food to
the plant, it ministers indirectly in a most important manner to its
well-being, and that to so great an extent that it must be considered an
indispensable constituent of a fertile soil. But it is important to
observe that it must not be present in too large a quantity, for an
excess does away with all the good effects of a smaller supply, and
produces soils notorious for their infertility.

Such are the important physical properties of the soil, and it is
greatly to be desired that they should be more extensively examined. The
great labour which this involves has, however, hitherto prevented its
being done, and will, in all probability, render it impossible except in
a limited number of cases. Some of these characters are, however, of
minor importance, and for ordinary purposes it might be sufficient to
determine the specific gravity of the soil in the dry and moist state,
the power of imbibing and retaining water, its hygrometric power, its
tenacity, and its colour. With these data we should be in a condition to
draw probable conclusions regarding the others; for the higher the
specific gravity in the dry state, the greater is the power of the soil
to retain heat, and the darker its colour the more readily does it
absorb it. The greater its tenacity the more difficult is it to work,
and the greater difficulty will the roots of the young plant find in
pushing their way through it. The greater the power of imbibing water,
the more it shrinks in drying; and the more slowly the water evaporates,
the colder is the soil produced. The hygrometric power is so important a
character that Davy and other chemists have even believed it possible to
make it the measure of the fertility of a soil; but though this may be
true within certain limits, it must not be too broadly assumed, the
results of recent experiments by no means confirming the opinion in its
integrity, but indicating only some relation between the two.

_The Subsoil._--The term soil is strictly confined to that portion of
the surface turned over by the plough working at ordinary depth; which,
as a general rule, may be taken at 10 inches. The portion immediately
subjacent is called the subsoil, and it has considerable agricultural
importance, and requires a short notice. In many instances, soil and
subsoil are separated by a purely imaginary line, and no striking
difference can be observed either in their chemical or physical
characters. In such cases it has been the practice with some persons not
to limit the term soil to the upper portion, but to apply it to the
whole depth, however great it may be, which agrees in characters with
the upper part, and only to call that subsoil which manifestly differs
from it. This principle is perhaps theoretically the more correct, but
great practical advantages are derived from limiting the name of soil to
the depth actually worked in common agricultural operations. The subsoil
is always analogous in its general characters to a soil, but it may be
either identical with that which overlies it or not. Of the former,
striking illustrations are seen in the wheat subsoils, the analyses of
which have been already given. In the latter case great differences may
exist, and a heavy clay is often found lying on an open and porous sand,
or on peat, and _vice versa_. Even where the characters of the subsoil
appear the same as those of the soil, appreciable chemical differences
are generally observed, especially in the quantity of organic matter,
which is increased in the soil by the decay of plants growing upon it
and by the manure added. In general, then, all that we have said
regarding the characters of soils both chemically and physically, will
apply to the subsoils, except that, owing to the difficulty with which
the air reaches the latter, some minor peculiarities are observed. The
most important is the effect of the decay of vegetable matter, without
access of air, which is attended by the reduction of the peroxide of
iron to the state of protoxide, and not unfrequently by the production
of sulphuret of iron, compounds which are extremely prejudicial to
vegetation, and occasionally give rise to some difficulties when the
subsoil is brought to the surface, as we shall afterwards have to
notice.

The physical characters of the subsoil are often of much importance to
the soil itself. As, for instance, where a light soil lies on a clay
subsoil, in which case its value is much higher than if it reposed on an
open or sandy subsoil. And in many similar modes an important influence
is exerted; but these belong more strictly to the practical department
of agriculture, and need not be mentioned here.

_Classification of Soils._--Numerous attempts have been made to form a
classification of soils according to their characters and value, but
they have not hitherto proved very successful; and the result of more
recent chemical investigations has not been such as to encourage a
farther attempt. We have not at present data sufficient for the purpose,
nor, if we had, would it be possible to arrange any soil in its class
except after an elaborate chemical examination. The only classification
at present possible must be founded on the general physical characters
of the soil; and the ordinary mode followed in practice of dividing them
into clays, loams, etc. etc., which we need not here particularize,
fulfils all that can be done until we have more minute information
regarding a large number of soils. Those of our readers who desire more
full information on this point are referred to the works of Thaer,
Schübler, and others, where the subject is minutely discussed.

FOOTNOTES:

[Footnote I: Transactions of the Highland and Agricultural Society, vol.
vi., p. 317.]




CHAPTER VI.

THE IMPROVEMENT OF THE SOIL BY MECHANICAL PROCESSES.


Comparatively few uncultivated soils possess the physical properties or
chemical composition required for the production of the most abundant
crops. Either one or more of the substances essential to the growth of
plants are absent, or, if present, they are deficient in quantity, or
exist in some state in which they cannot be absorbed. Such defects,
whether mechanical or chemical, admit of diminution, or even entire
removal, by certain methods of treatment, the adaptation of which to
particular cases is necessarily one of the most important branches of
agricultural practice, as the elucidation of their mode of action is of
its theory. The observations already made with regard to the characters
of fertile soils must have prepared the reader for the statement that
these defects may be removed, either by mechanical or chemical
processes. The former method of improvement may at first sight appear to
fall more strictly under the head of practical agriculture, of which the
mechanical treatment of the soil forms so important a part, and that
their improvement by chemical means should form the sole subject of our
consideration in a treatise on agricultural chemistry. But the line of
demarcation between the mechanical and the chemical, which seems so
marked, disappears on more minute observation, and we find that the
mechanical methods of improvement are frequently dependent on chemical
principles; and those which, at first sight, appear to be entirely
chemical, are also in reality partly mechanical. It will be necessary
for us, therefore, to consider shortly the mechanical methods of
improving the soil.

_Draining._--By far the most important method of mechanically improving
the soil is by draining--a practice the beneficial action of which is
dependent on a great variety of circumstances. It is unnecessary to
insist on the advantage derived from the rapid removal of moisture,
which enables the soil to be worked at times when this used to be almost
impossible, and other direct practical benefits. Of its more strictly
chemical effects, the most important is probably that which it produces
on the temperature of the soil. It has been already remarked that the
germination of a seed is dependent on the soil in which it is sown
acquiring a certain temperature, and the rapidity of the after-growth of
the plant is, in part at least, dependent on the same circumstance. The
necessary temperature is speedily attained by the heating action of the
sun's rays, when the soil is dry; but when it is wet, the heat is
expended in evaporating the moisture with which it is saturated; and it
is only after this has been effected that it acquires a sufficiently
high temperature to produce the rapid growth of the seeds committed to
it.

The extent to which this effect occurs may be best illustrated by
reference to some experiments made by Schübler, in which he determined
the temperature attained by different soils, in the wet and dry state,
when exposed to the sun's rays, from 11 till 3 o'clock, in the latter
part of August, when the temperature in the shade varied from 73° to
77°.

+-----------------------+-------+-------+
| Description of Soil.  | Wet.  | Dry.  |
+-----------------------+-------+-------+
|                       | Degs. | Degs. |
| Siliceous sand        |  99·1 | 112·6 |
| Calcareous sand       |  99·3 | 112·1 |
| Sandy clay            |  98·2 | 111·4 |
| Loamy clay            |  99·1 | 112·1 |
| Stiff clay            |  99·3 | 112·3 |
| Fine bluish-grey clay |  99·5 | 113·0 |
| Garden mould          |  99·5 | 113·5 |
| Arable soil           |  97·7 | 111·7 |
| Slaty marl            | 101·8 | 115·3 |
+-----------------------+-------+-------+

In a soil which is naturally dry or has been drained, the superfluous
moisture escapes by the drains, and only that comparatively small
quantity which is retained by capillary attraction is evaporated, and
hence the soil is more frequently and for a longer period in a condition
to take advantage of the heating effect of the sun's rays, and in this
way the period of germination, and, by consequence also, that of
ripening is advanced. The extent of this influence is necessarily
variable, but it is generally considerable, and in some districts of
Scotland the extensive introduction of draining has made the harvest, on
the average of years, from ten to fourteen days earlier than it was
before. It is unnecessary to insist on the importance of such a change,
which in upland districts may make cultivation successful when it was
previously almost impossible. The removal of moisture by drainage
affects the physical characters of the soil in another manner; it makes
it lighter, more friable, and more easily worked; and this change is
occasioned by the downward flow of the water carrying with it to the
lower part of the soil the finer argillaceous particles, leaving the
coarser and sandy matters above, and in this way a marked improvement is
produced on heavy and retentive clays. The access of air to the soil is
also greatly promoted by draining. In wet soils the pores are filled
with water, and hence the air, which is so important an agent in their
amelioration, is excluded; but so soon as this is removed, the air is
enabled to reach and act upon the organic matters and other decomposable
constituents present. In this way also provision is made for the
frequent change of the air which permeates the soil; for every shower
that falls expels from it a quantity of that which it contains, and as
the moisture flows off by the drains, a new supply enters to take its
place, and thus the important changes which the atmospheric oxygen
produces on the soil are promoted in a high degree. The air which thus
enters acts on the organic matters of the soil, producing carbonic acid,
which we have already seen is so intimately connected with many of its
chemical changes. In its absence the organic matters undergo different
decompositions, and pass into states in which they are slowly acted on,
and are incapable of supplying a sufficient quantity of carbonic acid to
the soil; and they thus exercise an action on the peroxide of iron,
contained in all soils, reduce it to the state of protoxide, or, with
the simultaneous reduction of the sulphuric acid, they produce sulphuret
of iron, forms of combination which are well known to be most injurious
to vegetation.

The removal of water from the lower part of the soil, and the admission
of air, which is the consequence of draining, submits that part of it to
the same changes which take place in its upper portion, and has the
effect of practically deepening the soil to the extent to which it is
thus laid dry. The roots of the plants growing on the soil, which stop
as soon as they reach the moist part, now descend to a lower level, and
derive from that part of it supplies of nourishment formerly
unavailable. The deepening of the soil has further the effect of making
the plants which grow upon it less liable to be burned up in seasons of
drought, a somewhat unexpected result of making a soil drier, but which
manifestly depends on its permitting the roots to penetrate to a greater
depth, and so to get beyond the surface portion, which is rapidly dried
up, and to which they were formerly confined.

It may be added also that the abundant escape of water from the drains
acts chemically by removing any noxious matters the soil may contain,
and by diminishing the amount of soluble saline matters, which sometimes
produce injurious effects. It thus prevents the saline incrustation
frequently seen in dry seasons on soils which are naturally wet, and
which is produced by the water rising to the surface by capillary
attraction, and, as it evaporates, depositing the soluble substances it
contained, as a hard crust which prevents the access of air to the
interior of the soil.

It is thus obvious that the drainage of the soil modifies its properties
both mechanically and chemically. It exerts also various other actions
in particular cases which we cannot here stop to particularize. It
ameliorates the climate of districts in which it is extensively carried
out, and even affects the health of the population in a favourable
manner. The sum of its effects must necessarily differ greatly in
different soils, and in different districts; but a competent
authority[J] has estimated, that, on the average, land which has been
drained produces a quarter more grain per acre than that which is
undrained. But this by no means exhausts the benefits derived from it,
draining being merely the precursor of further improvement. It is only
after it has been carried out that the farmer derives the full benefit
of the manures which he applies. He gains also by the increased facility
of working the soil, and by the rapidity with which it dries after
continued rain, thus enabling him to proceed at their proper season with
agricultural operations, which would otherwise have to be postponed for
a considerable time.

It would be out of place to enlarge here upon the mode in which draining
ought to be carried out; it may be remarked, however, that much
inconvenience and loss has occasionally been produced by too close
adherence to particular systems. No rules can be laid down as to the
depth or distance between the drains which can be universally
applicable, but the intelligent drainer will seek to modify his practice
according to the circumstances of the case. As a general rule, the
drains ought to be as deep as possible, but in numerous instances it may
be more advantageous to curtail their depth and increase their number.
If, for instance, a thick impervious pan resting on a clay were found at
the depth of three feet below the surface, it would serve no good
purpose to make the drains deeper; but if the pan were thin, and the
subjacent layer readily permeable by water, it might be advantageous to
go down to the depth of four feet, trusting to the possible action of
the air which would thus be admitted, gradually to disintegrate the pan,
and increase the depth of soil above it. It is a common opinion that if
we reach, at a moderate depth, a tenacious and little permeable clay, no
advantage is obtained by sinking the drains into it; but this is an
opinion which should be adopted with caution, both because no clay is
absolutely impermeable, even the most tenacious permitting to a certain
extent the passage of water, and because the clay may have been brought
down by water from the upper part of the soil, and may have stopped
there merely for want of some deeper escape for the water, and which
drains at a lower level might supply. In some cases it may even be
advisable to vary the depth of the drains in different parts of the same
field, and the judicious drainer may sometimes save a considerable sum
by a careful observation of the peculiarities of the different parts of
the ground to be drained.

_Subsoil and Deep Ploughing._--It frequently happens, when a soil is
drained, that the subsoil is so stiff as to permit the passage of water
imperfectly, and to prevent the tender roots of the plant from
penetrating it, and reaching the new supplies of nourishment which are
laid open to them. In such cases the benefits of subsoil ploughing and
deep ploughing are conspicuous. The mode of action of these two methods
of treatment is similar but not identical. The subsoil plough merely
stirs and opens the subsoil, and permits the more ready passage of water
and the access of air and of the roots of plants--the former to effect
the necessary decompositions, the latter to avail themselves of the
valuable matters set free. But deep ploughing produces more extensive
changes; it raises new soil to the surface, mixes it with the original
soil, and thus not only brings up fresh supplies of valuable matters to
it, but frequently changes its chemical and mechanical characters,
rendering a heavy soil lighter by the admixture of a light subsoil, and
_vice versa_. Both are operations which are useless unless they are
combined with draining, for it must manifestly serve no good purpose to
attempt to open up a soil unless the water which lies in it be
previously removed. In fact, subsoiling is useless unless the subsoil
has been made thoroughly dry; and it has been found by experience that
no good effects are obtained if it be attempted immediately after
draining, but that a sufficient time must elapse, in order to permit the
escape of the accumulated moisture, which often takes place very slowly.
Without this precaution, the subsoil, after being opened by the plough,
soon sinks together, and the good effects anticipated are not realized.
The necessity for allowing some time to elapse between draining and
further operations is still more apparent in deep ploughing, when the
soil is actually brought to the surface. In that case it requires to be
left for a longer period after draining, in order that the air may
produce the necessary changes on the subsoil; for if it be brought up
after having been for a long time saturated with moisture, and
containing its iron as protoxide, and the organic matter in a state in
which it is not readily acted upon by the air, the immediate effect of
the operation is frequently injurious in place of being advantageous.
One of the best methods of treating a soil in this way is to make the
operation a gradual one, and by deepening an inch or two every year
gradually to mix the soil and subsoil; as in this way from a small
quantity being brought up at a time no injurious effects are produced.
Deep ploughing may be said to act in two ways, _firstly_, by again
bringing to the surface the manures which have a tendency to sink to the
lower part of the soil, and, _secondly_, by bringing up a soil which has
not been exhausted by previous cropping--in fact a virgin soil.

The success which attends the operation of subsoiling or deep ploughing
must manifestly be greatly dependent on the character of the subsoil,
and good effects can only be obtained when its chemical composition is
such as to supply in increased quantity the essential constituents of
the plant; and it is no doubt owing to this that the opinions
entertained by practical men, each of whom speaks from the results of
his own experience, are so varied. The effects produced by deep
ploughing on the estates of the Marquis of Tweeddale, are familiarly
known to most Scottish agriculturists, and they are at once explained by
the analyses of the soil and subsoil here given, which show that the
latter, though poor in some important constituents, contains more than
twice as much potash as the soil.

                                 Soil.  Subsoil.

Insoluble silicates             87·623  82·72
Soluble silica                   0·393   0·12
Alumina and peroxide of iron     4·129   8·60
Lime                             0·341   0·18
Magnesia                         0·290   0·24
Sulphuric acid                   0·027   0·03
Phosphoric acid                  0·240   trace
Potash                           0·052   0·12
Soda                             0·050   0·04
Water                            1·956   3·26
Organic matter                   5·220   4·02
                                ------  -----
                               100·321  99·33

In addition to the difference in the amount of potash, something is
probably due to the large proportion of alumina and oxide of iron in the
subsoil, which for this reason must be more tenacious than the soil
itself, which appears to be rather light. In other instances, the use of
the subsoil plough has occasioned much disappointment, and has led to
its being decried by many practical men; but of late years its use
having become better understood, its merits are more generally
admitted. We believe, that in all cases in which the soil is deep, more
or less marked good effects must be produced by its use, but of course
there must be cases in which, from the defective composition of the
subsoil or other causes, it must fail. It may sometimes be possible _a
priori_ to detect these cases, but in a large majority of them our
knowledge is still too limited to admit of satisfactory conclusions
being arrived at.

_Improving the Soil by Paring and Burning._--It has long been familiarly
known, that a decided improvement has been produced on some soils by
burning. Its advantages have chiefly been observed on two sorts, heavy
clays and peat soils, on both of which it has been practised to a great
extent. The action of heat on the heavy clays appears to be of a twofold
character, depending partly on the change effected in its physical
properties, and partly on a chemical decomposition produced by the heat.
The operation of burning is effected by mixing the clay with brushwood
and vegetable refuse, and allowing it to smoulder in small heaps for
some time. It is a process of some nicety, and its success is greatly
dependent on the care which has been taken to keep the temperature as
low as possible during the whole course of the burning.

Experience has shown that burning is by no means equally advantageous to
all clays, but is most beneficial on those containing a considerable
quantity of calcareous matter, and of silicates of potash. In such clays
heat operates by causing the lime to decompose the alkaline silicates,
and liberate a quantity of the potash which was previously in an
unavailable state. Its effect may be best illustrated by the following
analyses by Dr. Voelcker of a soil, and the red ash produced in burning
it.

                                  Soil.    Red Ash.
Water                              0·93     1·18
Organic matter                    10·67     3·32
Oxides of iron and alumina        13·40    18·42
Carbonate of lime                 23·90     8·83
Sulphate of lime                  trace     1·15
Carbonate of magnesia              1·10      "
Magnesia                            "       1·76
Phosphoric acid                   trace     0·71
Potash                             0·38     1·08
Soda                               0·13      "
Chloride of sodium                  "       1·03
Insoluble matter, chiefly clay    49·66    62·52
                                  -----    -----
                                 100·17   100·00

In this instance the quantity of burned soil amounted to about fifteen
tons per acre, and it is obvious that the quantity of potash which had
been liberated from the insoluble clay and the phosphoric acid are equal
to that contained in a considerable manuring. In order to obtain these
results, it is necessary, as has been already observed, to keep the
temperature as low as possible during the process of burning, direct
experiment having shown that when this precaution is not observed
another change occurs, whereby the potash, which at low temperatures
becomes soluble, passes again into an insoluble state. A part of the
beneficial effect is no doubt also due to the change produced in the
physical characters of the clay by burning, which makes it lighter and
more friable, and by mixture with the unburnt clay ameliorates the
whole. This improvement in the physical characters of the clay also
requires that it shall be burnt with as low a heat as possible; for if
it rises too high, the clay coheres into hard masses which cannot again
be reduced to powder, and the success of the operation of burning may
always be judged of by the readiness with which it falls into a uniform
friable powder.

The improvement of peat by burning has been practised to some extent in
Scotland, though less frequently of late years than formerly; but it is
still the principal method of reclaiming peat soils in many countries,
and particularly in Finland, where large breadths of land have been
brought into profitable cultivation by means of it. The _modus operandi_
of burning peat is very simple; it acts by diminishing the superabundant
quantity of humus or other organic matters, which, in the previous
section we have seen to be so injurious to the fertility of the soil. It
_may_ act also in the same way as it does on clay, by making part of the
inorganic constituents more really soluble, although it is not probable
that its effect in this way can be very marked. Its chief action is
certainly by destroying the organic matters, and by thus improving the
physical character of the peat, and causing it to absorb and retain a
smaller quantity of water than it naturally does. For this reason it is
that it proves successful only on thin peat bogs, for if they be deep,
the inorganic matters soon sink into the lower part, and the surface
relapses into its old state of infertility. It is probably for this
reason that the practice has been so much abandoned in Scotland, more
especially as other and more economical modes of treating peat soils
have come into use.

_Warping._--This name has been given to a method of improving soils by
causing the water of rivers to deposit the mud it carries in suspension
upon them, and which has been largely practised in the low lying lands
of Lincoln and Yorkshire, where it was introduced about a century ago.
It is most beneficial on sandy or peaty soil, and by its means large
tracts of worthless land have been brought under profitable cultivation.
It requires that the land to be so treated shall be under the level of
the river at full tide, and it is managed by providing a sluice through
which the river water is allowed to flood the land at high tide, and
again to escape at ebb, leaving a layer of mud generally about a tenth
of an inch in thickness, which it brought along with it. By the
repetition of this process, a layer of several feet in thickness, of an
excellent soil, is accumulated on the surface. Herapath, who has
carefully examined this subject chemically, has shown that in one
experiment where the water used contained 233 grains of mud per gallon,
210 were deposited during the warping. The following analyses will show
the general nature of the matters deposited, and the change which they
produce on the soil:--No. 1 is the mud from the Humber in its natural
state, No. 2 a specimen of warp of average quality artificially dried,
No. 3 a sandy soil before warping, and No. 4, the same fifteen years
after having received a coating of 11 inches of mud.

+---------------------------------+---------+--------+---------+--------+
|                                 |   1     |   2    |   3     |  4     |
|                                 +---------+--------+---------+--------+
| Water                           |   47·49 |    ... |    1·06 |   2·00 |
| Organic matter                  |    5·94 |   6·93 |    2·20 |   7·61 |
| Chloride of Calcium             |     ... |    ... | }   ... |    ... |
|     Magnesium                   | }   ... |   0·10 | }   ... |    ... |
|     Sodium                      | }  1·66 | } 0·94 | }  0·14 |   0·16 |
|     Potassium                   | }       | }      | }       |        |
| Sulphate of Soda                | }   ... |   0·31 | }   ... |    ... |
|     Magnesia                    | }   ... |   1·18 | }   ... |    ... |
|     Lime                        |   trace |   1·10 |   trace |  trace |
| Carbonate of Magnesia           |    2·60 |   0·31 |   trace |   0·29 |
|     Lime                        |    3·59 |   8·18 |   trace |   0·46 |
| Potash and Soda                 |    0·18 |   0·47 |   trace |   0·17 |
| Magnesia                        |    1·69 |   2·60 |   trace |   0·26 |
| Lime                            |    0·39 |   0·68 |   trace |   0·14 |
| Peroxide of Iron                | }  6·63 |   5·05 |    0·08 |   1·17 |
| Alumina                         | }       |   8·18 |    0·39 |   0·41 |
| Phosphate of Iron               |    0·58 |   1·04 |   trace |   0·28 |
| Silica                          |     ... |   9·05 |    0·14 |   2·77 |
| Sand and Stones                 |   29·15 |  55·87 |   95·91 |  84·97 |
|                                 |---------|--------|---------|--------|
|                                 |  100·00 | 100·00 |  100·00 | 100·00 |
+---------------------------------+---------+--------+---------+--------+

It is easy to understand the importance of the effects produced by
adding to any soil large quantities of a mud containing upwards of one
per cent of phosphate of iron; and in point of fact, Herapath has
calculated that in one particular instance the quantity of phosphoric
acid brought by warping upon an acre of land, exceeded seven tons per
acre. As, moreover, the matters are all in a high state of division,
they must exist in a condition peculiarly favourable to the plant. The
overflow of the Nile is only an instance of warping on the large scale,
with this difference, that it is repeated once only in every year,
whereas, in this country, the operation is repeated at every tide until
a deposit sometimes of several feet in thickness is obtained, after
which it is stopped, and the soil brought under ordinary cultivation.

An operation which is, in some respects, the converse of warping, has
been carried out on Blair-Drummond Moss, where the peat has been
dislodged and carried off by the action of water, leaving the subjacent
soil in a state fitted for cropping. Of course both this and warping are
restricted to special localities, but they are most important means of
ameliorating the soil when circumstances admit of their being carried
out.

_Mixing of Soils._--When soils possess conspicuous defects in their
physical, and even in their chemical properties, great advantages may,
in some instances, be derived from their proper admixture. A light sandy
soil, for instance, is greatly improved by the addition of clay, and
_vice versa_; so that, when two soils of opposite properties occur near
to one another, both may be improved by mixture. It has been applied to
the improvement of heavy clay soil and of peat, the former being mixed
with sand or marl so as to diminish its tenacity; the latter with clay
or gravel to add to its inorganic matters, and in both instances it has
proved successful.

The process of chalking, which has been carried out on a large scale in
some parts of England, and which consists in bringing up the chalk from
pits, penetrating through the overlying tenacious clay, and mixing it
with the soil, operates, to some extent, in a similar manner, though no
doubt the lime also exercises a strictly chemical action. It is probable
that the mixing of soils might be advantageously extended, and it merits
more minute study than it has yet obtained. Its use is obviously limited
by the expense, because, of course, where good effects are to be
obtained, it is necessary to remove large quantities of soil, in some
instances as much as 50 or 100 tons per acre, but the expense might be
much diminished if it were carried out methodically, and on a
considerable scale. The admixture of highly fertile soils with others of
inferior quality is also worthy of attention; indeed, it is understood
that this has been done, to some extent, with the rich trap soils of
some parts of Scotland, but the extent of the benefit derived from it
has not been made public.

FOOTNOTES:

[Footnote J: Mr. Dudgeon, Spylaw. Transactions of the Highland Society,
vol. cxxix., p. 505.]




CHAPTER VII.

THE GENERAL PRINCIPLES OF MANURING.


In their natural condition all soils not absolutely barren are capable
of supporting a certain amount of vegetation, and they continue to do so
for an unlimited period, because the whole of the substances extracted
from them are again restored, either directly by the decay of the
plants, or indirectly by the droppings of the wild animals which have
browzed upon them. Under these circumstances, a soil yields what may be
called its normal produce, which varies within comparatively narrow
limits, according to the nature of the season, temperature, and other
climatic conditions. But the case is completely altered if the crop, in
place of being allowed to decay on the soil, is removed from it, for,
though the air will continue to afford an undiminished supply of those
elements of the food of plants which may be derived from it, the fixed
substances, which can only be obtained from the soil, decrease in
quantity, and are at length entirely exhausted. In this way a gradual
diminution of the fertility of the soil takes place, until, after the
lapse of a period, longer or shorter, according to its natural
resources, it will become entirely incapable of maintaining a crop, and
fall into absolute infertility unless the substances removed from it are
restored from some other source in the form of manure. When this is
done, the fertility of the soil may not only be sustained but greatly
increased, and, in point of fact, all cultivated soils, by the use of
manure, are made to yield a much larger crop than they can do in their
natural condition.

The fundamental principle upon which a manure is employed is that of
adding to the soil an abundant supply of the elements removed from it by
plants in the condition best fitted for absorption by their roots; but
looked at in its broadest point of view, it acts not merely in this way,
but also by promoting the decomposition of the already partially
disintegrated rocks of which the soil is composed, setting free those
substances it already contains, and facilitating their absorption by the
plants.

In considering the practical applications of the broad general principle
just stated, it might be assumed that a manure ought invariably to
contain all the elements of plants in the quantities in which they are
removed by the crops, and that when this has been accurately ascertained
by analysis, it would only be necessary to use the various substances in
the proportions thus indicated. But this, though a very important, and
no doubt in many cases essential condition, is by no means the only
matter which requires to be taken into consideration in the economical
application of manures. And this becomes sufficiently obvious when the
circumstances attending the exhaustion of the soil are minutely
examined. When a soil is cropped during a succession of years with the
same plant, and at length becomes incapable of longer maintaining it,
the exhaustion is rarely, if ever, due to the simultaneous consumption
of all its different constituents, but generally depends upon that of
one individual substance, which, from its having originally existed in
the soil in comparatively small quantity, is removed in a shorter time
than the others. To restore the fertility of a soil in this condition,
it is by no means necessary to supply all the different substances
required by the plant, for it will suffice to add that which has been
entirely removed. On the other hand, if an ordinary soil be supplied
with a manure containing a very small quantity of one of the elements of
plant food, along with abundance of all the others, the amount of
increase which it yields must obviously be measured, not by those which
are abundant, but by that which is deficient; for the crop which grows
luxuriantly so long as it obtains a supply of all its constituents, is
arrested as effectually by the want of one as of all, as has been proved
by the experiments of Prince Salm Horstmar and others, referred to in a
previous chapter; and hence, in order to obtain a good crop, it would be
necessary to use the manure in such abundance as to supply a sufficiency
of the deficient element for that purpose. If this course were
persevered in for a succession of years, the other substances which
would have been used in much more than the quantity required by the
crops, must either have been entirely lost or have accumulated in the
soil. In the latter case it is sufficiently obvious that the soil must
have been gradually acquiring an amount of resources which must remain
dormant until the system of manuring is changed. To render them
available, it is only necessary to add to it a quantity of the
particular substance in which the manure hitherto employed has been
deficient, so as to restore the lost balance, and enable the plant to
make use of those which have been stored up within it. The substance so
used is called a _special_ manure; that containing all the constituents
of the crop is a _general_ manure.

The distinction of these two classes of manures is very important in a
practical point of view, because a special manure is not by itself
capable of maintaining the life of plants, but is only a means of
bringing into use the natural and acquired resources of the soil. In
place of preventing or retarding its exhaustion, it rather accelerates
it by causing the increased crops to consume more abundantly, and within
a shorter period of time, those substances which it contains. On the
other hand, a general manure prevents or diminishes the consumption of
the elements of plant-food contained in the soil, and if added in
sufficient abundance, may cause them to accumulate in it, and even
enable an almost absolutely barren soil to yield a tolerable crop.
General manures must therefore always be the most important and
essential, and no others would be used if it were possible to obtain
them of a composition exactly suited to the requirements of the crop to
be raised. Practically, however, this condition cannot be fulfilled,
because all the substances available for the purpose, and particularly
farm-yard manure, are refuse matters, the exact composition of which is
not under our control, and they do not necessarily contain their
constituents either in the most suitable proportions, or the most
available forms, and consequently when they are used during a succession
of years, certain of their constituents may accumulate in the soil, and
it is under such circumstances that special manures are both necessary
and advantageous.

Several different substances, but more especially farm-yard manure,
fulfil in a very remarkable manner the conditions of a general manure,
and supply abundantly, not merely the mineral, but also the carbonaceous
and nitrogenous matters necessary for building up the organic part of
the plant; and hence its use is governed by principles of comparative
simplicity, and really resolves itself into determining the best mode of
managing it so as effectually to preserve its useful constituents, and,
at the same time, to bring them into those forms of combination in which
they are most available to the plant. But the employment of a special
manure opens up nice questions as to the relative importance of the
different elements of plants which have given rise to much controversy
and difference of opinion.

In treating of the food of plants, it has been already observed that the
fixed or mineral constituents which are contained in their ash, are
necessarily derived exclusively from the soil, but that the carbon,
hydrogen, nitrogen, and oxygen, of which their organic part is composed,
may be obtained either from that source or from the air. The important
distinction which thus exists between these two classes of substances,
has given rise to two different views regarding the theory of manures.
Basing his views on the presence of the organic elements in the air,
Liebig has maintained that it is unnecessary to supply them in the
manure, while others, among whom Messrs. Lawes and Gilbert have taken a
prominent position, hold that, as a rule, fertile soils, cultivated in
the ordinary manner, contain a sufficient supply of mineral matters for
the production of the largest possible crops, but that the quantity of
ammonia and nitric acid which the plants are capable of extracting from
the air is insufficient, and must be supplemented by manures containing
them. A large number of experiments have been made in support of these
views, but the inferences which can be drawn from them are not
absolutely conclusive on either side, and it is necessary to consider
the matter in a general point of view.

Setting out from the proposition already so frequently referred to, that
the plant cannot grow unless it receives a supply of all its elements,
it must be obvious that if, to a soil containing a sufficiency of
mineral matters to raise a given number of crops, a supply of ammonia be
added, its total productive capacity cannot be thus increased; and
though it may yield larger crops than it would have done without that
substance, this can only be accomplished by a proportionate diminution
of their number. In either case, the same quantity of vegetable matter
will be produced, but the time within which it is obtained will be
regulated by the supply of ammonia. That substance differs in no respect
from any other element of plant-food, and used in this way is to all
intents and purposes a special manure, and acts merely by bringing into
play those substances which the soil already contains. Its effect may
not be apparent until after the lapse of a very long period of time, but
it ultimately leads to the exhaustion of the soil. If, on the other
hand, a soil be continuously cropped until it ceases to yield any
produce, it is manifest that the exhaustion must in this instance be
entirely due to the removal of its available mineral nutriment, because
the superincumbent air constantly changed by the winds must continue to
afford the same unvarying supply of the organic elements, and the power
of supporting vegetation would be restored to it, by adding the
necessary inorganic matters. Hence when a soil, which in its natural
condition is capable of yielding a certain amount of vegetable matter,
is rendered barren by the removal of the crop, it may be laid down as an
incontrovertible position, that its infertility is due to the loss of
mineral matters, and that it may be restored to its pristine condition
by the use of them, and of them only.

But the case is materially altered when we come to consider the course
of events in a cultivated soil. The object of agriculture is to cause
the soil, by appropriate treatment, to yield much more than its normal
produce, and the question is, how this can be best and most economically
effected in practice. According to Liebig, it is attained by adding to
the soil a liberal supply of those mineral substances required by the
plant, and that it is unnecessary to use any of the organic elements,
because they are supplied by the air in sufficient quantity to meet the
requirements of the most abundant crops. Other chemists and vegetable
physiologists again hold that though a certain increase may be obtained
in this way, a point is soon reached beyond which mineral matters will
not cause the plant to absorb more ammonia from the air, although a
further increase may be obtained by the addition of nitrogen in that or
some other available form.

It is admitted on both sides, that all the elements of plant food are
equally essential, and the controversy really lies in determining what
practically limits the crop producible on any soil. The point at issue
may be put in a clear point of view by considering the course of events
on a soil altogether devoid of the elements of plants. If a small
quantity of mineral matters be added to such a soil, it immediately
becomes capable of supporting a certain amount of vegetation, deriving
from the air the organic elements necessary for this purpose, and with
every increase of the former, the air will be laid under a larger
contribution of the latter, to support the increased growth, and this
must proceed until the limit of supply from the atmosphere is reached.
At this point a further supply of mineral matters alone must obviously
be incapable of again increasing the crop, and it would thus be
absolutely necessary to conjoin them with a proportionate quantity of
organic substances. Liebig maintains that this limit is never attained
in practice, but that the air affords ammonia and the other organic
elements in excess of the requirements of the largest crop, while
mineral matters are generally though not invariably present in the soil
in insufficient quantity. Messrs. Lawes and Gilbert, on the other hand,
believe that the soil generally contains an excess of mineral matters,
and that a manure which is to bring out their full effect must contain
ammonia, or some other nitrogenous substance fitted to supplement the
deficient supply afforded by the atmosphere. In short, the question at
issue is, whether there is or is not a sufficiency of atmospheric food
to meet the demands of the largest crop which can practically be
produced.

An absolutely conclusive reply to this question is by no means easy. The
experiments by which it is to be resolved are complicated by the fact,
that all soils capable of supporting anything like a crop, contain not
only the mineral, but the organic elements of its food in large and
generally in greatly superabundant quantity, and it is impossible
satisfactorily to ascertain how much is derived from this source, and
how much from the atmosphere. There are in fact no experiments in which
the effects of a purely mineral soil have been ascertained. The
important and carefully performed researches of Messrs. Lawes and
Gilbert were made upon a soil which had been long under cultivation, and
contained decaying vegetable matters in sufficient abundance to supply
nitrogen to many successive crops, and it would be most unreasonable to
assert that the produce they did obtain by means of mineral manures,
drew the whole of its nitrogen from the air. On the contrary, it may be
fairly assumed that the soil did yield a certain quantity of its
nitrogenous compounds, but to what extent this occurs, it is impossible
to determine. This difficulty is encountered more or less in all the
other experiments, and precludes absolute conclusions. The same fallacy
also besets the arguments of Liebig when he holds that the crop,
increased by means of mineral manures alone, must derive the whole of
the additional quantity of nitrogen which it contains from the air, as
appears to be tacitly assumed throughout the whole discussion. So far
from this being the case, it is just as likely that the mineral matters
should cause the plants to take it from the soil, if it is there, as
from the atmosphere.

Taking a general view of the whole question, it is evident that a
certain amount of vegetation may always be produced by means of mineral
manures, and the quantity obtained is generally much beyond the normal
produce of the soil. But it is still open to doubt whether the largest
possible crop can be thus obtained, although the balance of evidence is
against it, and in favour of the addition of ammonia, and other
nitrogenous and organic substances, to the soil. In actual practice
manures containing nitrogen are more important, and more extensively
applied than any others, and the quantity of that element thus used is
very much larger than is generally supposed. Twenty tons of farm-yard
manure, a quantity commonly applied, and often exceeded on well
cultivated land, contain a sufficiency of organic matters to yield about
2-1/2 cwt. of nitrogen. A complete rotation, according to the six-course
shift, contains almost exactly the same quantity of nitrogen, when we
assume average crops throughout the whole, and it is thus made up.[K]

                                     Lbs. of Nitrogen.
1. Turnips (13-1/2 tons)                     60
2. { Wheat (28 bushels at 60 lbs.)           29
   { Straw                                   16
3. Hay (2-1/2 tons)                          56
4. { Oats (34 bushels at 40 lbs.)            27
   { Straw                                   14
5. Potatoes (3 tons)                         27
6. Wheat and straw as before                 45
                                           ----
                                    Total   274

The supply is therefore quite sufficient for the requirements of the
crop; and when it is borne in mind that a considerable quantity of
ammonia and nitric acid is annually carried down by the rain, and that
during a long rotation other substances are very generally used in
addition to farm-yard manure, it is obvious that the crop need not
depend to any extent upon what it derives from the air. What is true of
the nitrogenous matters applies with still greater force to the mineral
constituents of the manure. Twenty tons of farm-yard manure contain 32
cwt. of mineral matters, while the average crops of a six course-shift
contain only 1088 lbs., or less than one-third of this quantity. It is
obvious, therefore, that in well manured land there must be a gradual
increase of all the constituents of plants, but that of the mineral
matters is relatively much greater than that of the nitrogenous. If
therefore from any cause the crop produced on a soil to which farm-yard
manure had been applied were greatly to exceed the average, the amount
of produce, so far as the soil is concerned, would be limited not by
deficiency of mineral, but of nitrogenous food. Hence also when
farm-yard manure is liberally applied, there is a gradual accumulation
of valuable matters, and a progressive improvement of the productive
capacity of the soil.

It is far otherwise, however, if a special manure is employed, because
in that case the crop is thrown upon the resources of the soil itself
for all its constituents except those contained in the substance
employed, and by persisting in its exclusive use exhaustion is the
inevitable result. It would be wrong, however, to infer from this, that
special manures are to be avoided. On the contrary, great benefits are
derived from their judicious employment, and the circumstances under
which they are admissible may be readily gathered from what has already
been said. They are agents which bring into useful activity the dormant
resources of the soil, they restore the proper balance between its
different constituents, and supply the excessive demand of some
particular elements. Thus, for instance, in a soil containing an
abundant supply of mineral matters, a salt of ammonia or nitric acid
increases the crop, by promoting the absorption of the substances
already present. So likewise a soil on which young cattle and milch cows
have been long pastured has its fertility restored by phosphate of lime,
because that substance is removed in the bones and milk in relatively
much larger proportion than any others.

The choice of a special manure is necessarily dependent on a great
variety of circumstances, and is governed partly by the nature of the
soil, and partly by that of the crop. It is obvious that cases may occur
in which any individual element of the plant may be deficient, and ought
to be supplied, but experience has shown that, as a rule, nitrogen and
phosphoric acid are the substances which it is most necessary to furnish
in this way, and which in all but exceptional cases produce a marked
effect on the crop. The other substances, such as potash, soda,
magnesia, etc., occasionally act beneficially, but the results obtained
from them are very uncertain, and frequently entirely negative.

It has been commonly asserted that phosphates are specially adapted to
root crops, and ammonia or nitrates to the cereals, and this statement
is so far true, that the former are used with advantage on the turnip,
while the latter act with great benefit on grain crops and more
especially on oats and barley. The effect of the latter, however, is
more or less apparent in all crops and on all soils, because it promotes
the assimilation of the mineral matters already present. But its
peculiar importance lies in the power which it has of promoting the
rapid development of the young plant, causing it to send its roots out
into the soil, and to spread its leaves into the air, thus enabling it
to take from those two sources, abundance of the useful substances
existing in them. But it ought to be distinctly understood, that the
statement that particular manures are specially suited to particular
crops must be assumed with some reservation, because everything depends
upon the nature of the food contained in the soil. It is well known that
there are many soils in which ammonia acts more favourably on the turnip
than phosphates, and _vice versa_, and the difference is often due to
the previous treatment. In many cases in which ammonia when first used
proved most beneficial, it now begins to lose its effect, and the reason
no doubt is, that by its means the phosphates existing in these soils
have been reduced in amount, while the ammonia has accumulated, so that
a change in the system of manuring becomes necessary. A general manure
may be used year after year in a perfectly routine manner, but where a
special manure is employed, the importance of watching its effects, and
altering it as circumstances indicate, cannot be over-estimated. The
length of time during which special manures have been extensively used
has not been sufficient to bring this prominently before the
agriculturist, but its importance must sooner or later force itself upon
him, and he will then see the necessity for studying the succession of
manures as well as that of crops.

Hitherto we have considered a manure merely as a source from which
plants derive their food, but it exercises a scarcely less important
action on the chemical and physical properties of the soil. Farm-yard
manure, which, as we shall afterwards see, contains a large amount of
decomposing vegetable and animal matters, yields a supply of carbonic
acid, which operates on the mineral constituents, promotes their further
disintegration, and thus liberates their useful elements. It affects
also their physical properties, for it diminishes the tenacity of heavy
clays; each straw as it decomposes forming a channel through which the
roots of plants, air, and moisture can penetrate more readily than
through the stiff clay itself. On the other hand, it diminishes the
porosity of light sandy soils, causes them to retain moisture, and
generally makes their texture more suitable to the plant. Special
manures probably act to some extent chemically on the soil, but the
nature of the changes they produce is as yet imperfectly understood.
Superphosphates which are highly acid in all probability act powerfully
on the mineral substances, and common salt, which, though of little
importance to the plant, occasionally produces very striking effects,
appears to exercise some decomposing action on the soil. It is
difficult, however, to trace the mode in which they operate on a
substance of such complexity as the soil. Lime, as we shall afterwards
see, acts by promoting the decomposition of the vegetable matters on
the soil, and possibly some other substances may have a similar effect.

In the application of manures to the soil there are several
circumstances which must be taken into consideration. It is generally
stated that they ought to be distributed as uniformly as possible, but
this is not always necessary nor even advisable, and certainly is not
acted on in practice. Much must depend upon the nature both of crop and
soil. When the former throws out long and widely penetrating roots, the
more uniformly the manure is distributed the better; but if the rootlets
are short, it is clearly more advisable that it should be deposited at
no great distance from the seed. Practically this is observed in the
case of the potato and turnip, which are short rooted, and where the
manure is generally deposited close to the seed. But this course is
never adopted with the long rooted cereals, the manure being usually
applied to the previous crop, so that the repeated ploughings to which
the soil is subjected in the interval may distribute what remains as
widely and uniformly as possible. In soils which are either excessively
tenacious or light, the accumulation of the manure close to the plants
has also the effect of producing an artificial soil in their immediate
neighbourhood, containing abundance of plant-food, and having physical
properties better fitted for the support of the plant. On the other
hand, when a special manure is used alone, and with the view of
promoting the assimilation of substances already existing in the soil,
the more uniform its distribution the better, because it is essential
that the roots which penetrate through it should find at every point
they reach not only the original soil constituents, but also the
substances used to supplement their deficiencies.

FOOTNOTES:

[Footnote K: The quantities here taken are the averages deduced from the
agricultural statistics taken in Scotland some years since, with the
exception of hay and straw, which are not included in them. I have
therefore assumed a reasonable quantity in these cases.]




CHAPTER VIII.

THE COMPOSITION AND PROPERTIES OF FARM-YARD AND LIQUID MANURES.


In the preceding chapter, a general manure has been defined as one
containing all the constituents of the crop to which it is to be
applied, in a state fitted for assimilation. This condition is fulfilled
only by substances derived from the vegetable and animal kingdoms, and
most effectually by a mixture of both. On this account, and also because
its properties are such as enable it to act powerfully on the soil,
farm-yard manure must always be of the highest importance. It is, in
fact, the typical manure, and in proportion as other substances approach
it in properties and composition, is their value for general purposes on
the farm.

Farm-yard manure is a mixture of the dung and urine of domestic animals,
with the straw used as litter; and its value and composition must
necessarily depend upon that of these substances, as well as on the
proportion in which they are mixed. The dung of animals consists of that
part of their food which passes through the intestinal canal without
undergoing assimilation; the urine containing the portion which has been
assimilated and is again excreted, in consequence of the changes which
are proceeding in the tissues of the animal. Their composition is
naturally very different, and must be separately considered.

_Urine._--Urine consists of a variety of earthy and alkaline salts, and
of certain organic substances, generally rich in nitrogen, dissolved in
a large quantity of water. That of the different domestic animals has
been frequently examined, but the analyses of Fromberg give the most
complete view of their manurial value:--

                    Horse.   Swine.    Ox.     Goat.    Sheep.

Extractive matter } 2·132    0·142    2·248    0·100    0·340
 soluble in water }
Extractive matter } 2·550    0·387    1·421    0·454    3·330
 soluble in spirit}
Salts soluble in  } 2·340    0·909    2·442    0·850    1·957
 water            }
Salts insoluble in} 1·880    0·088    0·155    0·080    0·052
 water            }
Urea                1·244    0·273    1·976    0·378    1·262
Hippuric acid       1·260     ...     0·550    0·125     ...
Mucus               0·005    0·005    0·007    0·006    0·025
Water              88·589   98·196   91·201   98·007   92·897
                   ------  -------  -------  -------  -------
                  100·000  100·000  100·000  100·000   99·863

_Composition of the Ash of these Urines._

                       Horse.  Swine.   Ox.     Goat.   Sheep.
Carbonate of lime      12·50    ...     1·07   trace     0·82
Carbonate of magnesia   9·46    ...     6·93     7·3     0·46
Carbonate of potash    46·09   12·10   77·28   trace     ...
Carbonate of soda      10·33    ...     ...     53·0    42·25
Sulphate of potash      ...     ...    13·30     ...     2·98
Sulphate of soda       13·04    7·00    ...     25·0     7·72
Phosphate of soda       ...    19·00    ...      ...     ...
Phosphate of lime     }
Phosphate of magnesia } ...     8·80    ...      ...     0·70
Chloride of sodium      6·94   53·10    0·30    14·7    32·01
Chloride of potassium   ...    trace    ...     ...     12·00
Silica                  0·55    ...     0·35    ...      1·06
Oxide of iron and loss  1·09    ...     0·77    ...      ...
                      ------  ------  ------  ------   ------
                      100·00  100·00  100·00  100·00   100·00

Human urine has been accurately examined by Berzelius, although his
estimate of the proportion of urea is generally admitted to be above the
average. His analysis gives the following numbers:--

                                    Natural.  Dry Residue.
Urea                                  3·010     44·70
Lactic acid, lactate of ammonia,}     1·714     25·58
    and extractive matter       }
Uric acid                             0·100      1·49
Mucus                                 0·032      0·48
Sulphate of potash                    0·371      5·54
Sulphate of soda                      0·316      4·72
Phosphate of soda                     0·294      4·39
Biphosphate of ammonia                0·165      2·46
Chloride of sodium                    0·445      6·64
Muriate of ammonia                    0·150      2·46
Phosphates of magnesia and lime       0·100      1·49
Silica                                0·003      0·05
Water                                93·300
                                    -------    ------
                                    100·000    100·00

Among the special organic constituents of the urine are three
substances, urea, uric acid, and hippuric acid, which are of much
importance in a manurial point of view. The first of these is found in
considerable quantity in the urine of all animals, but is especially
abundant in the carnivora. Uric acid is found only in these animals, and
is the most remarkable constituent of the excrement of birds, serpents,
and many of the lower animals. Hippuric acid is most abundant in the
herbivora. These substances are all highly nitrogenous. They contain--

                Urea.    Uric Acid.   Hippuric Acid.
Carbon          20·00        36·0        60·7
Hydrogen         6·60         2·4         5·0
Nitrogen        46·70        33·4         8·0
Oxygen          26·70        28·2        26·3
               ------       -----       -----
               100·00       100·0       100·0

They are extremely prone to change, and in presence of animal matters
readily ferment, and are converted into salts of ammonia. Thus human
urine, which, at the time of emission is free from smell of ammonia, and
has a slightly acid reaction, becomes highly ammoniacal if it be kept
for a few days. This is due to the conversion of urea into carbonate of
ammonia; and the same change takes place, though more slowly, with uric
and hippuric acids.

It is obvious, from the foregoing analyses, that great differences must
exist in the manurial value of the urine of different animals. Not only
do they vary greatly in the proportion of solid matters which they
contain, but also in the kind and quantity of their nitrogenous
constituents. They differ also in regard to their saline ingredients;
and while salts of potash and soda form the principal part of the ash of
the urine of the ox, sheep, goat, and horse, and phosphoric acid and
phosphates are entirely absent, that of the pig contains a considerable
quantity of the latter substances, and in this respect more nearly
resembles the urine of man. Human urine is also much richer in urea and
nitrogenous constituents generally, and has a higher value than any of
the others.

It is especially worthy of notice that the urine of the purely
herbivorous animals (with the exception of the sheep, which contains a
small quantity), are devoid of phosphates and urea; and consequently,
when employed alone, they are not general manures--a matter of some
importance in relation to the subject of liquid manuring, which will be
afterwards discussed.

_Dung._--The solid excrement of animals is equally variable in
composition. That of the domestic animals which had the ordinary winter
food was found to have the following composition:--

                                Horse.  Cow.  Sheep.  Swine.
Per-centage of water in the }    77·25  82·45  56·47  77·13
  fresh excrement           }
Ash in the dry excrement         13·36  15·23  13·49  37·17

100 parts of ash contained--

                                 Horse.  Cow.  Sheep.  Swine.
Silica                           62·40  62·54  50·11  13·19
Potash                           11·30   2·91   8·32   3·60
Soda                              1·98   0·98   3·28   3·44
Chloride of sodium                0·03   0·23   0·14   0·89
Phosphate of iron                 2·73   8·93   3·98  10·55
Lime                              4·63   5·71  18·15   2·63
Magnesia                          3·84  11·47   5·45   2·24
Phosphoric Acid                   8·93   4·75   7·52   0·41
Sulphuric acid                    1·83   1·77   2·69   0·90
Carbonic acid                      ...  trace  trace   0·60
Oxide of manganese                2·13    ...    ...    ...
Sand                               ...    ...    ...  61·37
                                  ----   ----   ----   ----
                                 99·80  99·29  99·64  99·82

Human fæces contain about 75 per cent of water; and their dry residue
was found by Way to have the following composition:--

Organic matter                            88·52
Insoluble siliceous matters                1·48
Oxide of iron                              0·54
Lime                                       1·72
Magnesia                                   1·55
Phosphoric acid                            4·27
Sulphuric acid                             0·24
Potash                                     1·19
Soda                                       0·31
Chloride of sodium                         0·18
                                         ------
                                         100·00

In a sample analyzed by myself there were found--

Organic matter                                 86·75
Phosphates                                      8·19
Alkaline salts, containing 1·18 of phosphoric } 2·53
     acid                                     }
Insoluble matters                               2·53
                                              ------
                                              100·00

Nitrogen                                        4·59
Equal to ammonia                                5·57

It is to be observed that the urine and dung of animals differ
conspicuously in the composition of their ash, the former being
characterized by the abundance of alkaline salts, while the latter
contains these substances in small proportion, but is rich in earthy
matters, and especially in phosphoric acid. Salts of potash, for
example, form nine-tenths of the inorganic part of the urine of the ox,
while less than three per cent of that alkali is found in its dung.
Phosphoric acid, on the other hand, is not met with in the urine, but
forms about ten per cent of the dung. Silica is the most abundant
constituent of the dung, but a large proportion of that found on
analysis has been swallowed in the shape of grains of sand and particles
of soil mechanically mixed with the food, although part is also derived
from the straw and grains, which contain that substance in great
abundance. The difference in the quantity of nitrogen they contain is
also very marked, and is distinctly shown by the following analyses by
Boussingault, which give the quantity of carbon, hydrogen, nitrogen,
oxygen, and ash in the dung and urine of the horse and the cow in their
natural state, and after drying at 212°.

+---------+--------------------------+--------------------------+
|         |           HORSE.         |            COW.          |
|         +-------------+------------+-------------+------------+
|         |   Natural.  |     Dry.   |   Natural.  |     Dry.   |
|         +-------------+------------+-------------+------------+
|         |Urine.| Dung.|Urine.|Dung.|Urine.| Dung.|Urine.|Dung.|
|         +------+------+------+-----+------+------+------+-----+
|Carbon   |  4·46|  9·56|  36·0| 38·7|  3·18|  4·02|  27·2| 42·8|
|Hydrogen |  0·47|  1·26|   3·8|  5·1|  0·30|  0·49|   2·6|  5·2|
|Nitrogen |  1·55|  0·54|  12·5|  2·2|  0·44|  0·22|   3·8|  2·3|
|Oxygen   |  1·40|  9·31|  11·3| 37·7|  3·09|  3·54|  26·4| 37·7|
|Ash      |  4·51|  4·02|  36·4| 16·3|  4·68|  1·13|  40·0| 12·0|
|Water    | 87·61| 75·31|   0·0|  0·0| 88·31| 90·60|   0·0|  0·0|
|         +------+------+------+-----+------+------+------+-----+
|         |100·00|100·00| 100·0|100·0|100·00|100·00| 100·0|100·0|
+---------+------+------+------+-----+------+------+------+-----+

Hence, weight for weight, the urine of the horse, in its natural state,
contains three times as much nitrogen as its dung; that of the cow twice
as much; and the difference, especially in the horse, is still more
conspicuous when they are dry.

It is obvious that the quality of farm-yard manure must depend--1. On
the kind of animal from which it is produced; 2. On the quantity of
straw which has been used as litter; 3. On the nature of the food with
which the animals have been supplied; 4. On the extent to which its
valuable constituents have been rendered available by the treatment to
which it has been subjected; and 5. On the care which has been taken to
prevent the escape of the urine, or of the ammonia produced by its
decomposition.

The composition of farm-yard manure has engaged the attention of several
chemists; but there are still many points on which our information
regarding it is less complete than might be desired. Its investigation
is surrounded with peculiar difficulties, not merely on account of its
complexity, but because its properties render it exceedingly difficult
to obtain a sample which fairly represents its average composition. In
the case of long dung, these difficulties are so great that it is
scarcely possible to overcome them; and hence, discrepancies are
occasionally to be met with in the analyses of the most careful
experimenters. The most minute and careful analyses yet made are those
of Voelcker, who has compared the composition of fresh and rotten dung,
and studied the changes which the former undergoes when preserved in
different ways. He employed in his experiments both fresh and rotten
dung, and subjected them to different methods of treatment. His analyses
are given in the accompanying table, in which column 1 gives the
composition of fresh long dung, composed of cow and pig dung. 2. Is dung
of the same kind, after having lain in a heap against a wall, but
otherwise unprotected from the weather for three months and eleven days
in winter, during which time little rain fell. 3. The same manure, kept
for the same time under a shed. 4. Well rotten dung, which had been kept
in the manure heap upwards of six months. 5. The same, after having lain
against a wall for two months and nine days longer.

+-------------------------------+-------------+-------------+--------------+
|                               |    1        |    2        |   3          |
+-------------------------------+-------------+-------------+--------------+
| Water                         |       66·17 |       69·86 |        67·32 |
| Soluble organic matters       |        2·48 |        3·86 |         2·63 |
| Soluble inorganic matters·    |             |             |              |
| Silica                        | 0·237       | 0·279       | 0·239        |
| Phosphate of lime             | 0·299       | 0·300       | 0·331        |
| Lime                          | 0·066       | 0·048       | 0·056        |
| Magnesia                      | 0·011       | 0·019       | 0·004        |
| Potash                        | 0·573       | 1·096       | 0·676        |
| Soda                          | 0·051       | 0·187       | 0·192        |
| Chloride of sodium            | 0·030       | 0·106       | 0·058        |
| Sulphuric acid                | 0·055       | 0·160       | 0·119        |
| Carbonic acid and loss        | 0·218       | 0·775       | 0·445        |
|                               | -----  1·54 |  ...   2·97 | -----   2·12 |
|                               |             |             |              |
| Insoluble organic matters     |       25·76 |       18·44 |        20·46 |
| Insoluble inorganic matters-- |             |             |              |
|   Soluble silica              | 0·967       | 0·712       | 1·893        |
|   Insoluble silica            | 0·561       | 0·857       | 1·075        |
|   Oxide of iron, alumina, and |             |             |              |
|     phosphates                | 0·596       | 0·810       | 1·135        |
+-------------------------------+-------------+-------------+--------------+

+-------------------------------+-------------+-------------+
|                               |   4         |  5          |
+-------------------------------+-------------+-------------+
| Water                         |       75·42 |       73·90 |
| Soluble organic matters       |        3·71 |        2·70 |
| Soluble inorganic matters     |             |             |
| Silica                        | 0·254       | 0·147       |
| Phosphate of lime             | 0·382       | 0·129       |
| Lime                          | 0·117       | 0·018       |
| Magnesia                      | 0·047       | 0·018       |
| Potash                        | 0·446       | 0·960       |
| Soda                          | 0·023       | 0·082       |
| Chloride of sodium            | 0·037       | 0·052       |
| Sulphuric acid                | 0·058       | 0·072       |
| Carbonic acid and loss        | 0·106       | 0·584       |
|                               | -----  1·47 | -----  2·06 |
|                               |             |             |
| Insoluble organic matters     |       12·82 |       14·39 |
| Insoluble inorganic matters-- |             |             |
|   Soluble silica              | 1·424       | 1·10        |
|   Insoluble silica            | 1·010       | 1·54        |
|   Oxide of iron, alumina, and |             |             |
|     phosphates                | 0·947       | 0·37        |
+-------------------------------+-------------+-------------+

+-------------------------------+-------------+-------------+--------------+
|                               |   1         |   2         |   3          |
+-------------------------------+-------------+-------------+--------------+
| Containing phosphoric acid    |(0·178)      |(0·177)      |(0·298)       |
| Equal to bone earth           |(0·386)      |(0·277)      |(0·646)       |
| Lime                          | 1·120       | 1·291       | 1·868        |
| Magnesia                      | 0·143       | 1·029       | 0·078        |
| Potash                        | 0·099       | 0·127       | 0·208        |
| Soda                          | 0·019       | 0·046       | 0·038        |
| Sulphuric acid                | 0·061       | 0·099       | 0·098        |
| Carbonic acid and loss        | 0·484       | 0·929       | 1·077        |
|                               | -----  4·05 | -----  4·90 | -----   7·47 |
|                               |       ----- |       ----- |       ------ |
|                               |      100·00 |      100·00 |       100·00 |
|                               |             |             |              |
| Containing nitrogen           | 0·149       | 0·270       | 0·170        |
| Equal to ammonia              | 0·181       | 0·320       | 0·206        |
| Containing nitrogen           | 0·494       | 0·470       | 0·580        |
| Equal to ammonia              | 0·599       | 0·570       | 0·700        |
| Total nitrogen                | 0·643       | 0·740       | 0·750        |
| Equal to ammonia              | 0·780       | 0·890       | 0·906        |
+-------------------------------+-------------+-------------+--------------+

+-------------------------------+-------------+-------------+
|                               |   4         |   5         |
+-------------------------------+-------------+-------------+
| Containing phosphoric acid    |(0·274)      |(0·06)       |
| Equal to bone earth           |(0·573)      |(0·10)       |
| Lime                          | 1·667       | 2·25        |
| Magnesia                      | 0·091       | 0·02        |
| Potash                        | 0·045       | 0·12        |
| Soda                          | 0·038       | 0·01        |
| Sulphuric acid                | 0·063       | 0·10        |
| Carbonic acid and loss        | 1·295       | 1·44        |
|                               |-----   6·58 | ----   6·95 |
|                               |       ----- |      ------ |
|                               |      100·00 |      100·00 |
|                               |             |             |
| Containing nitrogen           | 0·297       | 0·149       |
| Equal to ammonia              | 0·360       | 0·180       |
| Containing nitrogen           | 0·309       | 0·613       |
| Equal to ammonia              | 0·375       | 0·744       |
| Total nitrogen                | 0·606       | 0·762       |
| Equal to ammonia              | 0·735       | 0·924       |
+-------------------------------+-------------+-------------+

On examining and comparing these analyses, it appears that the
differences are by no means great, although, on the whole, they tend to
show that, weight for weight, well-rotten dung is superior to fresh,
provided it has been properly treated. Not only is the quantity of
valuable matters existing in the soluble state materially increased,
whereby the dung is enabled to act with greater rapidity, but, owing to
evaporation and the escape of carbonic acid, produced by the
decomposition of the organic substances, the proportion of those
constituents which are most important to the plant is increased. This is
particularly to be noticed, in regard to the nitrogen, which has
distinctly increased in all cases in which the dung has been kept for
some time; and the practical importance of this observation is very
great, because it has been commonly supposed that, during the process of
fermentation, ammonia is liable to escape into the air. It would appear,
however, that there is but little risk of loss in this way, so long as
the dung-heap is left undisturbed; and it is only when it is turned that
any appreciable quantity of ammonia volatilizes. It is different,
however, with the action of rain, which soon removes, by solution, a
considerable quantity of the nitrogen contained in farm-yard manure; and
the deterioration must necessarily be most conspicuous in rotten dung,
which sometimes contains nearly half of its nitrogen in a soluble
condition. The effect produced in this way is conspicuously seen, by the
results of weighings and analyses of small experimental dung-heaps, made
by Dr. Voelcker at different periods. The subjoined table shows the
composition of the heap, lying against a wall, and exposed to the
weather at different periods:--

+--------------------------+-------------------------------------------+
|                          |           WHEN PUT UP.                    |
|                          +---------+------------+---------+----------+
|                          | Nov 3d  | April 30th | Aug 23d | Nov 15th |
|                          | 1854.   | 1855.      | 1855.   | 1855.    |
|                          +---------+------------+---------+----------+
| Weight of manure in lbs. |  2838   |   2026     |  1994   |  1974    |
|                          +---------+------------+---------+----------+
| Water                    | 1877·9  | 1336·1     | 1505·3  | 1466·5   |
| Dry Matter               |  960·1  |  689·9     |  488·7  |  507·5   |
|                          +---------+------------+---------+----------|
|   Consisting of--        |         |            |         |          |
| Soluble organic matter   |   70·38 |   86·51    |   58·83 |   54·04  |
|   "    mineral matter    |   43·71 |   57·88    |   39·16 |   36·89  |
| Insoluble organic matter |  731·07 |  389·74    |  243·22 |  214·92  |
|   "     mineral matter   |  114·94 |  155·77    |  147·49 |  201·07  |
|                          +---------+------------+---------+----------+
| Total nitrogen           |   18·23 |   18·14    |   13·14 |   13·03  |
| Equal to ammonia         |   22·14 |   22·02    |   15·96 |   15·75  |
+--------------------------+---------+------------+---------+----------+

In this case, during the winter six months, which were very dry, the
manure lost 541·8 lbs. of water and 270·2 lbs. of dry matter, but the
nitrogen remained completely unchanged. But during the succeeding
semi-annual period, when rain fell abundantly, the quantity of nitrogen
is diminished by nearly a third, while the water has increased, and the
loss of dry matter by fermentation, notwithstanding the high temperature
of the summer months, was only 182·4 lbs. The soluble mineral matters
also, which increased during the first period, are again reduced during
the second, until they also fall to about two-thirds of their maximum
quantity. That this effect is to be attributed to the solvent action of
rain is sufficiently obvious, from a comparison of the results afforded
by the other heaps, which had been kept under cover during the same
period, as shown below.

+--------------------------+----------------------------------------------|
|                          |           WHEN PUT UP.                       |
|                          +----------+------------+----------+-----------+
|                          | Nov. 3d, | April 30th | Aug. 23d | Nov. 15th |
|                          | 1854.    | 1855.      | 1855.    | 1855.     |
|                          +----------+------------+----------+-----------+
| Weight of manure in lbs. |  3258    |  1613      |  1297    |  1235     |
|                          +----------+------------+----------+-----------|
| Water                    | 2156·0   |   917·6    |  563·2   |  514·5    |
| Dry Matter               | 1102·0   |   695·4    |  733·8   |  720·5    |
|                          +----------+------------+----------+-----------|
|   Consisting of--        |          |            |          |           |
| Soluble organic matter   |   80·77  |    74·68   |   53·56  |   66·28   |
|   "    mineral matter    |   50·14  |    54·51   |   39·55  |   54·68   |
| Insoluble organic matter |  839·17  |   410·24   |  337·32  |  341·97   |
|   "     mineral matter   |  131·92  |   155·97   |  303·37  |  257·57   |
|                          +----------+------------+----------+-----------+
| Total nitrogen           |   20·93  |    19·26   |   16·54  |    1·79   |
| Equal to ammonia         |   25·40  |    23·33   |   20·08  |    2·81   |
+--------------------------+----------+------------+----------+-----------+

The loss of nitrogen is here comparatively trifling, and during the
whole year, but little exceeds two pounds, of which the greater part
escapes during the first six months, and the soluble inorganic matters
are almost unchanged. The total weight of the manure, however, undergoes
a very great reduction, due chiefly to evaporation of water, but in part
also to the loss of organic matters evolved in the form of carbonic acid
during fermentation.

When the manure is spread out, as it is usually found under cattle in
open yards, the deterioration is very great, a quantity thus treated
having lost, in the course of a year, nearly two-thirds of its nitrogen,
and four-fifths of its soluble inorganic matters.

The general conclusion deducible from these analyses is that, provided
it be carefully prepared, farm-yard manure does not differ very largely
in value, although the balance is in favour of the well-rotten dung.
This result is in accordance with that obtained by other experimenters,
who have generally found from 0·5 to 0·6 per cent of nitrogen, and 1 or
2 per cent of phosphates. But when carelessly managed, it may fall
greatly short of this standard, as is particularly seen in a sample
examined by Cameron, which had been so effectually washed out by the
rain, as to retain only 0·15 per cent of ammonia. These cases, however,
are exceptional, and well made and well preserved farm-yard manure will
generally be found to differ comparatively little in value; and when
bought at the ordinary price, the purchaser, as we shall afterwards more
particularly see, is pretty sure to get full value for his money, and
the specialities of its management are of comparatively little moment to
him. But the case is very different when the person who uses the manure
has also to manufacture it. The experiments already quoted have shown
that, though the manure made in the ordinary manner may, weight for
weight, be as valuable as at first, the loss during the period of its
preservation is usually very large, and it becomes extremely important
to determine the mode in which it may be reduced to the minimum.

In the production of farm-yard manure of the highest quality, the object
to be held in view is to retain, as effectually as possible, all the
valuable constituents of the dung and urine. But in considering the
question here, it will be sufficient to refer exclusively to its
nitrogen, both because it is the most important, and also because the
circumstances which favour its preservation are most advantageous to the
other constituents. In the management of the dung-heap, there are three
things to be kept in view:--First, To obtain a manure containing the
largest possible amount of nitrogen; secondly, To convert that nitrogen
more or less completely into ammonia; and thirdly, To retain it
effectually.

As far as the first of these points is concerned, it must be obvious
that much will depend on the nature and quantity of the food with which
the animals yielding the dung are supplied, and the period of the
fattening process at which it is collected. When lean beasts are put up
to feed, they at first exhaust the food much more completely than they
do when they are nearly fattened, and the manure produced is very
inferior at first, and goes on gradually improving in quality as the
animal becomes fat.

When the food is rich in nitrogenous compounds, the value of the manure
is considerably increased. It has been ascertained, for instance, that
when oil-cake has been used, not less than seven-eighths of the valuable
matters contained in it reappear in the excrements; and as that
substance is highly nitrogenous, the dung ought, weight for weight, to
contain a larger amount of that element. That it actually does so, I
satisfied myself by experiments, made some years since, when the dung
and urine of animals fed on turnips, with and without oil-cake, were
examined; but unfortunately, no determination of the total quantity of
the excretions could be made, so that it was impossible to estimate the
increased value. It has been commonly supposed that when cattle are fed
with oil-cake, the increased value of the manure is equal to from
one-half to two-thirds the price of the oil-cake; but this is a rather
exaggerated estimate as regards linseed-cake, although it falls short of
the truth in the case of rape, as we shall afterwards more particularly
see.

Although it may be possible, in this way, to increase the quantity of
nitrogen as a manure, there is a limit to its accumulation, due to the
fact, that it is contained most abundantly in the urine, which can only
be retained by the use of a sufficient supply of litter. Where that is
deficient, the dung-heap becomes too moist, and the fluid and most
valuable part drains off, either to be lost, or to be collected in the
liquid manure-tank. In the well managed manure-heap, the quantity of
litter should be sufficient to retain the greater part of the liquid
manure, and to admit of only a small quantity draining from it, which
should be pumped up at intervals, so as to keep the whole in a proper
state of moisture. Attention to this point is of great moment, and
materially affects the fermentation. When it is too moist or too dry,
that process is equally checked; in the former case by the exclusion of
air, which is essential to it; and in the latter, by the want of water,
without which the air cannot act. The exact mode in which the manure is
to be managed must greatly depend on whether the supply of litter is
large or small. In the latter case the urine escapes, and is collected
in the liquid manure-tank, and must be used by irrigation, and in some
cases this mode of application has advantages, but in general, it is
preferable to avoid it, and have recourse to substances which increase
the bulk of the heap sufficiently to make it retain the whole of the
liquid. For this purpose, clay, or still better, the vegetable refuse of
the farm, such as weeds, ditch cleanings, leaves, and, in short, any
porous matters, may be used. But by far the best substance, when it can
be obtained, is dry peat, which not only absorbs the fluid, but fixes
the ammonia, by converting it more or less completely into humate.
Reference has been already made to the absorbent power of peat in the
section on soils, but it may be mentioned here that accurate experiment
has shown that a good peat will absorb about 2 per cent[L] of ammonia,
and when dry will still retain from 1 to 1·5 per cent, or nearly twice
as much as would be yielded by the whole nitrogen of an equal weight of
farm-yard manure. Peat charcoal has been recommended for the same
purpose, but careful experiment has shown that it _does not absorb
ammonia_, although it removes putrid odour; and though it may be
usefully employed when it is wished to deodorize the manure heap, it
must not be trusted to for fixing the ammonia.

Much stress has frequently been laid on the advantage to be derived from
the use of substances capable of combining chemically with the ammonia
produced during the fermentation of dung and gypsum, sulphate of iron,
chloride of manganese, sulphate of magnesia, and sulphuric acid, have
been proposed for this purpose, and have been used occasionally, though
not extensively. They all answer the purpose of _fixing_ the ammonia,
that is, of preventing its escaping into the air; but the risk of loss
in this way appears to have been much exaggerated, for a delicate
test-paper, held over a manure-heap, is not affected; and during
fermentation, humic acid is produced in such abundance, as to combine
with the greater part of the ammonia. The real source of deterioration
is the escape of the soluble matters in the drainings from the
manure-heap, which is not prevented by any of these substances; and
where no means are taken to preserve or retain this portion, the loss is
extremely large, and amounts, under ordinary circumstances, to from a
third to a half of the whole value of the manure. Manure, therefore,
cannot be exposed to the weather without losing a proportion of its
valuable matters, depending upon the quantity of rain which falls upon
it. Hence it is obvious that great advantage must be derived, especially
in rainy districts, from covered manure-pits. This plan has been
introduced on some farms with good effect; but the expense and doubts
as to the benefits derived from it, have hitherto prevented the practice
becoming general. The principal difficulty experienced in the use of the
covered dung-pit is, that, where the litter is abundant, the urine does
not supply a sufficiency of moisture to promote the active fermentation
of the dung, and it becomes necessary to pump water over it at
intervals; but when this is properly done, the quality of the manure is
excellent, and its valuable matters are most thoroughly economized.

Although covered dung-pits have been but little used, their benefits
have been indirectly obtained by the method of box-feeding, one of the
great advantages of which is held to be the production of a manure of
superior quality to that obtained in the old way. In box-feeding none of
the dung or urine is removed from under the animals, but is trampled
down by their feet, and new quantities of litter being constantly added,
the whole is consolidated into a compact mass, by which the urine is
entirely retained. Whatever objection may be taken to this system, so
far as the health of the animals is concerned, there is no doubt as to
the complete economy of the manure, provided the quantity of litter used
be sufficient to retain the whole of the liquid. But its advantage is
entirely dependent on the possibility of fulfilling this condition.

Whether box manure is really superior to that which can be prepared by
the ordinary method is very questionable, but it undoubtedly surpasses a
large proportion of that actually produced. It is more than probable,
however, that the careful management of the manure-heap would yield an
equally good product. It is manifest that the same number of cattle, fed
in the same way, on the same food, and supplied with the same quantity
of litter, must always excrete the same quantities of valuable matters;
and the only question to be solved is, whether they are more effectually
preserved in the one way than the other? It will be readily seen that
this cannot be done by analysis alone, but that it is necessary to
conjoin with it a determination of the total weight of manure produced;
for though, weight for weight, box manure may be better than ordinary
farm-yard manure, the total quantity obtained by the latter method, from
a given number of cattle, may be so much greater, that the deficiency in
quality may be compensated for. At the present time our knowledge is too
limited to admit of a definite opinion on this subject, but it is highly
deserving of the combined investigation of the farmer and the chemist.

Supposing the conditions which produce the manure containing the largest
quantity of nitrogen to have been fulfilled, we have now to consider
those which affect its evolution in the form of ammonia. This change is
effected by _fermentation_. When a quantity of manure is left to itself
it becomes hot, and gradually diminishes in bulk, and if it be turned
over after some time, the smell of ammonia may be more or less
distinctly observed. This ammonia is produced, in the first instance,
from the urine, the nitrogenous constituents of which are rapidly
decomposed, and the fermentation thus set up in the mass of manure
extends first to the solid dung, and then to the straw of the litter,
and gradually proceeds until a large quantity of ammonia is produced.

When fresh manure is deposited in the soil, the same changes occur, but
they then proceed more slowly, and experience has shown that a much
smaller effect is produced on the crop to which it has been applied than
when it has been well fermented in the heap. This effect is consistent
with theory, which would further indicate that well-fermented dung must
be especially advantageous when applied to quick-growing crops, and less
necessary to those which come slowly to maturity. As a rule, well
fermented manure is to be preferred, provided it has been well managed
and carefully prepared; but when this has not been done, and the manure
has been exposed to the weather, or made in open courts or hammels, the
economic advantages are all on the side of the fresh dung. It may be
questioned also whether, now that there are so many other available
sources of ammonia, it may not in many instances be advantageous to use
the dung fresh, conjoined with a sufficient quantity of some salt of
ammonia, or other substance fitted to supply the quantity of that
element necessary for the requirements of the crop.

After the farm-yard manure has been prepared at the homestead, it is
often necessary to cart it out to the field some time before it is to be
applied, and it is a question of some importance to determine how it may
be best preserved there. The general practice is to store it in heaps in
the corners of the fields, but some difference of opinion exists as to
whether it should be lightly thrown up so as to leave it in a porous
state, and so promote its further fermentation, or whether it should be
consolidated as much as possible by driving the carts on to the top of
the heap during its construction. Considering the risks to which the
manure is exposed on the field, the latter plan would appear to be the
best. It is advisable also to interstratify the dung with dry soil, so
as to absorb any liquid which may tend to escape from it, and it should
also be covered with a well-beaten layer of earth, in order to exclude
the rain. Although these precautions must not be omitted if the manure
is to be stored in heaps, it will probably be often found quite as
advantageous to spread it at once, and leave it lying on the surface
until it is convenient to plough it. The loss of ammonia by
volatilization will, under such circumstances, especially in the cold
season of the year, be very trifling, and the rain which falls will only
serve to incorporate the soluble matters with the soil, where they will
be retained by its absorptive power.

In the actual application of the manure to the crop, several points
require consideration. It is especially important to determine whether
it ought to be uniformly distributed through the soil, or be kept near
the roots of the plants. Both systems have their advocates, and each has
advantages in particular cases. The choice between the two must greatly
depend upon the nature of the crop and the soil. When the former is of a
kind which spreads its roots wide and deep through the soil, the more
uniformly the manure can be distributed the better; but when it is used
with plants whose roots do not travel far, it is more advantageous to
accumulate it near the seeds. Obvious advantages also attend this
practice in soils which are either too heavy or too light. When, for
example, it is necessary to cultivate turnips in a heavy clay, the
manure put into the drills produces a kind of artificial soil in the
neighbourhood of the plants, in which the bulbs expand more readily than
in the clay itself. On the other hand, when a large quantity of dung, in
a state of active fermentation, comes into immediate contact with the
roots, its effect is not unfrequently injurious. These and many other
points, which will readily suggest themselves to any one who studies the
composition and properties of farm-yard manure, belong more strictly to
the subject of practical agriculture, and need not be enlarged on here.

In the present state of agriculture, a proper estimate of the money
value of farm-yard manure is of much importance in an economic point of
view, and many matters connected with the profitable management of a
farm must hinge upon it. If an estimate be made upon the principle which
will be explained when we come to treat of artificial manures, it
appears that fresh farm-yard manure of good quality is worth from 12s.
to 15s. per ton, and well-rotted dung rather more. It is questionable,
however, whether the system of valuation which is accurate in the case
of a guano or other rapidly acting substance, is applicable to farm-yard
manure, the effects of which extend over some years. A deduction must be
made for the years during which the manure remains unproductive, and
also for the additional expense incurred in carting and distributing a
substance so much more bulky than the so-called portable manures, and it
would not be safe to estimate its value at more than 7s. or 8s. per ton.

_Liquid Manure._--This term is applied to the urine of the animals fed
on the farm, and to the drainings from the manure-heap, which, in place
of being returned to it, are allowed to flow away, and collected in
tanks, from which they are distributed by a watering-cart, or according
to the method recently introduced in Ayrshire, and since adopted in
other places, by pipes laid under-ground in the fields, and through
which the manure is either pumped by steam-power, or, where the
necessary inclination can be obtained, is distributed by gravitation.
That liquid manure must necessarily be valuable, is an inference which
maybe at once drawn from the analyses of the urine of different animals
already given, and of which it chiefly consists. In addition to the
urine, however, it contains also the soluble organic and mineral matters
of the dung, as well as a quantity of solid matters in suspension, among
which phosphates are found, and thus it possesses a supply of an
element which would be almost entirely deficient if it were composed of
urine alone. In the following analyses by Professor Johnston, No. 1 is
the drainings of the manure-heap when exposed to rain; and No. 2 the
same, when moistened with cows' urine pumped over it, the results being
expressed in grains per gallon:--

                                   No. 1.   No. 2.
Ammonia                              9·6    21·5
Organic matter                     200·8    77·6
Ash                                268·8   518·4
                                   -----   -----
Total solids in a gallon           479·2   617·5

The ash contained--

Alkaline salts                     207·8   420·4
Phosphates                          25·1    44·5
Carbonate of lime                   18·2    31·1
Carbonate of magnesia, and loss      4·3     3·4
Silica and alumina                  13·4    19·0
                                   -----   -----
                                   268·8   518·4

More elaborate analyses of the same fluid have since been made by Dr.
Voelcker, with the subjoined results per gallon:--

                                          1.      2.      3.
Organic matters and ammoniacal }       263·80   250·63   70·121
    salts                      }
Silica                                   2·49     9·98    1·154
Oxide of iron                            0·70     0·68      ...
Lime                                     5·34    25·18   13·011
Magnesia                                 2·96    15·33    1·660
Potash                                 103·23   112·26   13·411
Chloride of potassium                   72·00    77·38    7·712
Chloride of sodium                      17·18    46·03   17·258
Phosphoric acid                          2·70     9·51    2·304
Sulphuric acid                          22·31    37·60    3·408
Carbonic acid, and loss                 33·90    27·95   14·025
                                       ------   ------  -------
    Total solids                       526·61   612·53  144·064
Ammonia                                114·16    22·31   26·647

The differences are here very remarkable, especially in the quantity of
ammonia, which is exceedingly large in the first sample. All of them are
particularly rich in potash, and contain but a small proportion of
phosphoric acid. The general inference to be deduced from them is, that
liquid manure is a most important source of the alkalis and ammonia, and
must be peculiarly valuable on soils in which these substances are
deficient.

The system of liquid manuring, originally introduced by Mr. Kennedy of
Myremill, Ayrshire, and which has since been adopted in some other
places, differs from liquid manuring in its _strict_ sense, for not only
are the drainings of the manure-heap employed, but the whole solid
excrements are mixed with water in a tank, and rape-dust and other
substances occasionally added, and distributed through the pipes.

It has been abandoned on Mr. Kennedy's farm, but is in use at Tiptree
Hall, and on the farm of Mr. Ralston, Lagg, where the fluid is
distributed by gravitation.

The arrangements employed by Mr. Mechi are identical with those formerly
in use at Myremill. The greater part of the stock is kept on boards, and
the liquid and solid excrements are collected together in the tank, and
largely diluted before distribution. The liquid from the tanks has been
recently examined by Dr. Voelcker, who found it to contain per gallon--

Organic matter and ammoniacal salts       53·03
Soluble silica                             6·47
Insoluble siliceous matter (clay)         15·17
Oxide of iron and alumina                  2·36
Lime                                       6·60
Magnesia                                   1·73
                                          -----
Potash                                     0·35
Chloride of potassium                      1·95
Chloride of sodium                         4·81
Phosphoric acid                            3·72
Sulphuric acid                             1·94
Carbonic acid, and loss                    0·47
                                          -----
     Total solids                         96·60
Ammonia                                    8·10

The quantity of this liquid distributed per acre is about 50,000
gallons, at a cost of 2d. per gallon. As this quantity contains about 39
lbs. of ammonia, it must be nearly equivalent to 2 cwt. of Peruvian
guano, which costs, with the expense of spreading, from 28s. to 30s. per
acre, while the cost of distributing the liquid exceeds £1: 17s. per
acre. On the other hand, the rapidity with which liquid manure produces
its effect must be taken into account. It is on this that its chief
value depends, and especially when applied to grass land in early
spring, it produces an abundant crop just when turnips and other winter
food are exhausted. Mr. Telfer, Cunning Park, who has used this system
for a good many years, has come to the conclusion that it is only in
this way that it can be made profitable; and though pipes are laid all
over his farm, he has latterly restricted the use of the liquid manure
entirely to Italian ryegrass. Its effect on the cereals is much less
marked, and it can scarcely be considered as capable of advantageous
application to the general operations of the farm. Neither can liquid
manure be applied to all soils. It fails entirely on heavy clays, but is
peculiarly adapted to light sandy soils; and even barren sand may by its
repeated application, be made to yield luxuriant crops. It is not likely
that the system of liquid manuring will extend, except in localities
where it is possible to distribute it by gravitation; and even then, it
will probably be found most economical to restrict its use to one
portion of the farm; and for that purpose, the poorest and most sandy
soil ought to be selected.

_Sewage Manure._--The use of the sewage of towns as a manure is closely
connected with that of the liquid manure produced on the farm. Its
application must take place in a similar manner, and be governed by the
same principles. Although numerous attempts have been made to convert it
into a solid form, or to precipitate its valuable matter, none of them
have succeeded; nor can it be expected that any plan can be devised for
the purpose, because the most important manurial constituents are
chiefly soluble, and cannot be converted into an insoluble state, or
precipitated from their solution. In its liquid form, however, sewage
manure has been employed with the best possible effect in the
cultivation of meadows. The most important instance of its application
is in the neighbourhood of Edinburgh, where 325 acres receive the sewage
of nearly half the town, and have been converted from barren sand into
land which yields from £20 to £30 per acre. The contents of the sewer,
taken just before it flows into the first irrigated meadow, near
Lochend, were found to contain per gallon--

Soluble organic matter              21·90
Insoluble organic matter            21·70
Peroxide of iron and alumina         2·01
Lime                                10·50
Magnesia                             2·00
Sulphuric acid                       6·09
Phosphoric acid                      6·14
                                    -----
Chlorine                            12·20
Potash                               2·89
Soda                                13·27
Silica                               6·50
                                   ------
                                   105·20
Ammonia                             14·90

It is interesting to notice that this sewage is superior in every
respect to the liquid manure used at Tiptree Hall; and the good effects
obtained from its application, in the large quantities in which it is
used in the Craigentinny meadows, may be well imagined. It operates, not
merely by the substances which it holds in solution, but also by
depositing a large quantity of matters carried along in suspension, and
is in reality warping with a substance greatly superior to river-mud. A
deposit collected in a tank, where the sewage passes through a farm, is
used as a manure, and contains--

Peroxide of iron and alumina             4·45
Lime                                     1·74
Magnesia                                 0·39
Potash                                   0·10
Soda                                     0·06
Phosphoric acid                          1·08
Sulphuric acid                           0·16
Organic matter                          17·95
Sand                                    20·51
Water                                   53·56
                                       ------
                                       100·00
Ammonia                                  0·93

And even, though containing more than half its weight of water and 20
per cent of sand, this substance has considerable value as a manure.

The growing evils of the existing system of sewage, and the enormous
waste of a manurial matter, which the experience of the Craigentinny
meadows has shewn to be productive of the most important effects, has
recently directed much attention to the conversion of the contents of
our sewers into a useful manure. Numerous plans for its precipitation
and conversion into a solid manure have been proposed, but most of these
have shewn an entire ignorance of the fundamental principles of
chemistry, and the best only succeed in precipitating a very small
proportion of its valuable matters, and leave almost the whole of the
ammonia, as well as the greater part of the fixed alkalies, in solution.
Nor is it to be expected that any process will be discovered by which
these substances can be precipitated, because solubility is the special
characteristic of their compounds, and no means is known by which it is
possible to convert them into an insoluble form. If sewage is to be used
at all, there seems little doubt that it must be by applying it entire,
and in the liquid state. But here again, the expense of conveying it on
to the land becomes an obstacle which it must frequently be impossible
to overcome. When it can be conveyed by gravitation, as is the case in
the neighbourhood of Edinburgh, it may undoubtedly be used with the
utmost advantage, and with the very best economic results. But when it
requires to be carried to a great distance through pipes, and raised to
a high level by pumping, all these advantages disappear. If the cost of
application amounts to 2d. a gallon, as in Mr. Mechi's case, or even to
half that sum, it may be fairly concluded that it cannot be used with
any great prospect of large economic results, and that, unless under
very exceptional cases, it must be unprofitable.

The chances of success must also greatly depend upon the kind of soil on
which it is used. Experience has shewn that its effects are most
beneficial on light and deep sandy soils, but that on heavy retentive
clays it is without effect, or even absolutely injurious. In clay soils
it is important to use every means of getting rid of moisture, and any
plan which adds 200 or 300 tons of water to them, only aggravates their
natural defects to an extent which more than counterbalances the
benefits derived from the manurial matter it contains. Whatever the
ultimate result of the use of town sewage in the liquid form may be, it
is unlikely that it will be employed in general agricultural practice.
It is more probable that it will be found necessary to set apart a
certain breadth of land to be treated by it exclusively. Many plans have
been proposed for conveying it through considerable districts, and
selling to the surrounding farmers the quantities which they require,
but wherever large sewage-works are established, it will be impossible
to depend on a precarious demand, and the promoters of such schemes will
be compelled, as part of their speculation, to supply not only the
manure, but the land on which it is to be used. Indeed, the difficulties
attending the whole question are so formidable, that even those who are
most anxious to see a stop put to the waste of manurial matter must
admit that the prospect of a successful economic result is not
encouraging. Nor is it likely that anything will be done until the whole
system of managing town refuse is changed, and in place of deluging it
with water, some plan can be contrived which, while fulfilling sanatory
requirements, shall preserve it in a concentrated form, or convert it
into a dry and inodorous substance.

FOOTNOTES:

[Footnote L: Report on the economic uses of peat. Highland Society's
Transactions, N.S., vol. iv. p. 549.]




CHAPTER IX.

COMPOSITION AND PROPERTIES OF VEGETABLE MANURES.


Many vegetable substances have been employed as manures, either alone or
as auxiliaries to farm-yard manure. Like that substance, they are
general manures, and contain all the constituents of ordinary crops;
but, owing to the absence of animal matter, they in general undergo
decomposition and fermentation much more slowly, although some of them
contain a so largely preponderating proportion of nitrogen, that they
may in some respects be compared to the strictly nitrogenous manures.

_Rape-dust, Mustard, Cotton and Castor Cake._--Rape-dust has long been
employed as a manure, and the success which has attended its use has led
to the introduction of the refuse cake from some other oil seeds, such
as those of mustard and castor-oil, which cannot be employed for
feeding. Like the seeds of all plants, these substances are rich in
nitrogen, and their ash, containing of course all the constituents of
the plant, supplies the necessary inorganic elements. The following are
analyses of these substances, which, in addition to the amount of
nitrogen and phosphates, shew also that of water and oil, to which
reference will be made in a future chapter, in relation to the feeding
value of some of them. The detailed composition of their ash may be
judged of from that of the seeds from which they are made, and which
have been given under that head.

+--------------------+------------+-------------+-------------+---------+
|                    | Rape-Cake. | Poppy-Cake. | Cotton-seed | Castor- |
|                    |            |             |   Cake.     | Cake.   |
|                    +------------+-------------+-------------+---------+
| Water              |  10·68     |   11·63     |   11·19     |   12·31 |
| Oil                |  11·10     |    5·95     |    9·08     |   24·32 |
| Albuminious      } |  29·53     |   31·16     |   25·16     |   21·91 |
|    compounds     } |            |             |             |         |
| Ash                |   7·79     |   12·98     |    5·64     |    6·08 |
| Other constituents |  40·90     |   38·18     |   48·93     |   35·38 |
|                    +------------+-------------+-------------+---------+
|                    | 100·00     |  100·00     |  100·00     |  100·00 |
| Nitrogen           |   4·38     |    4·94     |    3·95     |    3·20 |
| Silica             |   1·18     |    3·36     |    1·32     |    1·96 |
| Phosphates         |   3·87     |   69·3      |    2·19     |    2·81 |
| Phosphoric acid  } |            |             |             |         |
|  in combination  } |   0·39     |    3·27     |    0·15     |    0·64 |
|  with alkalies   } |            |             |             |         |
+--------------------+------------+-------------+-------------+---------+

A general similarity may be observed in the composition of all these
substances; they are rich in nitrogen, and contain as much of that
element as is found in six or seven times their weight of farm-yard
manure, and a somewhat similar proportion exists in the amount of
phosphates, and probably of their other constituents. They have all been
employed with success, but the most accurate observations have been made
with rape-dust, which has been longer and more extensively used than any
of the others. It has been employed alone for turnips, or mixed with
farm-yard manure, and also as a top-dressing to cereals. But the most
marked advantage is derived from it when applied in the latter way on
land which has been much exhausted, and its effects are then very
striking. An adequate supply of moisture is essential to the production
of its full effects, and hence it often proves a failure in very dry
seasons, and on dry soils. It must not be applied in too great
abundance, experience having shewn that after a certain point has been
reached, an increase in the quantity produces no benefit, and even
sometimes positively diminishes the crop. The other substances of the
same class, in all probability, act in the same way, but as their
introduction is recent, and their use limited, less is known regarding
their effects.

_Malt-Dust, Bran, Chaff, etc._--The value of these substances as manures
is chiefly dependent on the nitrogen they contain, though to some extent
also on their inorganic constituents. Malt-dust contains about 4·5 per
cent, and bran 3·2 per cent of nitrogen. But they are little used as
manures, as they can generally be more advantageously employed for
feeding. The value of chaff more nearly resembles that of straw.

_Straw_ is occasionally employed as a manure, and sometimes even as a
top-dressing for grass land. It is generally admitted, however, that its
application in the dry state, and especially as a top-dressing, is a
practice not to be recommended, as it decomposes too slowly in the soil;
and it is always desirable to ferment it in the manure heap, so as to
facilitate the production of ammonia from its nitrogen. Still
circumstances may occur in which it becomes necessary to employ it in
the dry state, and it will generally prove most valuable on heavy soils,
which it serves to keep open, and so promotes the access of air, and
enables it to act on the soil. On light sandy soils it generally proves
less advantageous, as its tendency of course is to increase the openness
of the soil, and render it less able to retain the essential
constituents of the plant.

The quantity of nitrogen in straw does not exceed 0·2 per cent, and its
value is mainly due to its inorganic constituents and to its mechanical
effect on the soil.

_Saw-dust_ has little value as a manure, as it undergoes decomposition
with extreme slowness. It is a good _mechanical_ addition to heavy
soils, and diminishes their tenacity; and though its manurial effects
are small, it sooner or later undergoes decomposition, and yields what
valuable matters it contains. The saw-dust of hard wood is to be
preferred, both because it contains more valuable matters than that of
soft wood, and because the absence of resinous matters permits its more
rapid decomposition. It is a useful absorbent of liquid manure, and may
be advantageously added to the dung-heap for that purpose.

_Manuring with Fresh Vegetable Matter--Green Manuring._--The term green
manuring is applied to the system of sowing some rapidly growing plant,
and ploughing it in when it has attained a certain size, and the success
attending it, especially on soils poor in organic matters, is very
marked. It is obvious that this mode of manuring can add nothing to the
mineral matters contained in the soil, and its utility must therefore be
due to the plant gathering organic matters from the air, which, by their
decomposition, yield nitrogen and carbonic acid--the former to be
directly made use of by subsequent crops, the latter, in all
probability, acting also on the soil, and setting free its useful
constituents. Hence those plants which obtain the largest quantity of
their organic elements from the air ought to be most advantageous for
green manuring. The plants used for this purpose act also as a means of
bringing up from the lower parts of the soil the valuable matters which
exist in it out of reach of ordinary crops, and mixing them again with
the surface part. Many of the plants found most useful for green
manuring send down their roots to a considerable depth; and when they
are ploughed in, all the substances which they have brought up are of
course deposited in the upper few inches of the soil. Vegetable matter
when ploughed in in the fresh state, also decomposes rapidly, and is
therefore able immediately to improve the subsequent crop; and as this
decomposition takes place in the soil without the loss of ammonia and
other valuable matters, which is liable to occur to a greater or less
extent when they are fermented on the dung-heap, it will be obvious that
in no other mode can equally good results be obtained by its use.

Many plants have been employed as green manure, and different opinions
have been expressed as to their relative values. In the selection of any
one for the purpose, that should of course be taken which grows most
rapidly, and produces within a given time the largest quantity of
valuable matters, but no general rule can be given for the selection, as
the plant which fulfils those conditions best will differ in different
soils and climates. The plants most commonly employed in this country
are spurry, white mustard, and turnips. Rye, clover, buckwheat, white
lupins, rape, borage, and some others, have been largely employed
abroad. Some of these are obviously unfitted for the climate of the
British Islands; and the others, although they have been tried
occasionally, do not appear to have been very extensively employed. The
turnip is sown broadcast at the end of harvest, and ploughed in after
two months. White mustard and spurry are employed in the same way as a
preparation for winter wheat, and with the best results. The latter is
sometimes sown as a spring crop in March, ploughed in in May, and
another crop sown which is ploughed in in June, and immediately
followed by a third. The effect of this treatment is such that the worst
sands may be made to bear a remunerative crop of rye.

It is not easy to estimate the addition made by green manuring to the
valuable matters contained in the soil, but it is probably far from
inconsiderable. A crop of turnips, cultivated on the ordinary
agricultural system, after two months' growth, weighs between five and
seven tons per acre, and contains nitrogen equivalent to about 48 lbs.
of ammonia, and half a ton of organic matters; but nothing is known as
to the quantity produced when it is sown broadcast, and is not thinned,
although it must materially exceed this. Neither is it possible to
determine the relative proportions derived from the soil and the air,
although it is, in all probability, dependent on the resources of the
soil itself,--plants grown on a rich soil obtaining their chief supplies
from it, while, on poorer soils, a larger proportion is drawn from the
atmosphere. Hence light and sandy soils are most benefited by green
manuring, partly on this account, and partly also, no doubt, because the
valuable inorganic matters, which are so liable to be washed out of
these soils, are accumulated by the plants and retained in them in a
state in which they are readily available for the subsequent crop.

_Sea-Weed._--Sea-weeds have been employed from time immemorial as a
manure on the coasts of Scotland and England, in quantities varying from
10 to 20 tons per acre. Their action is necessarily similar to that of
green manure ploughed in, as they contain all the ordinary constituents
of land plants.

The subjoined analyses of three of the most abundant species will
sufficiently indicate their general composition.

+----------------+--------+----------+----------------------------+-----------+
|                |        |          |LAMINARIA DIGITATA.         |           |
|                |        |          |                            |Mixed Weeds|
|                |Fucus   | Fucus    +----------------+-----------+in the     |
|                |nodosus.| vesicu.- |                | Stem and  |state in   |
|                |        |   losus  | Collected      | Frond     |which they |
|                |        |          |  in Autumn.    | collected |actually   |
|                |        |          |                | in Spring.|are used   |
+----------------+--------+----------+----------------+-----------+-----------+
|Water           | 74·31  |   70·57  |       88·69    |    77·31  |   80·44   |
|Albuminous      |        |          |                |           |           |
|  compounds     |  1·76  |    2·01  |        0·93    |     3·32  |    2·85   |
|Fibre, etc.     | 19·04  |   22·05  |        4·92    |    10·39  |    6·40   |
|Ash             |  4·89  |    5·37  |        5·46    |     8·98  |   10·31   |
|                +--------+----------+----------------+-----------+-----------+
|                |100·00  |  100·00  |      100·00    |   100·00  |  100·00   |
|Nitrogen        |  0·28  |    0·32  |        0·15    |     0·53  |    0·45   |
|                |        |          |                |           |           |
|The ash         |        |          |                |           |           |
| consisted of   |        |          |                |           |           |
|                |        |          |  Stem.  Frond. |           |           |
|Peroxide of iron|  0·25  |    0·35  |   0·20    0·50 |     0·45  |    2·35   |
|Lime            |  9·60  |    8·92  |   7·21    7·29 |     4·62  |   18·15   |
|Magnesia        |  6·65  |    5·83  |   2·73    5·91 |    10·94  |    6·48   |
|Potash          | 20·03  |   20·75  |   5·55   11·91 |    12·16  |   12·77   |
|Chloride of     |        |          |                |           |           |
|      potassium |   ...  |     ...  |  58·42   26·59 |    25·83  |    9·10   |
|Iodide of       |        |          |                |           |           |
|    potassium   |  0·44  |    0·23  |   1·51    2·09 |     1·22  |    1·68   |
|Soda            |  4·58  |    6·09  |    ...     ... |      ...  |     ...   |
|Sulphuret       |        |          |                |           |           |
|    of sodium[M]|  3·66  |    ...   |    ...     ... |      ...  |     ...   |
|Chloride        |        |          |                |           |           |
|   of sodium    | 24·33  |   24·81  |  15·29   30·77 |    19·34  |   22·08   |
|Phosphoric acid |  1·71  |    2·14  |   2·42    2·66 |     1·75  |    4·59   |
|Sulphuric acid  | 21·97  |   28·01  |   2·23    8·80 |     7·26  |    6·22   |
|Carbonic acid   |  6·39  |    2·20  |   4·11    2·49 |    15·23  |   13·58   |
|Silicic acid    |  0·38  |    0·67  |   0·33    0·99 |     1·20  |    3·00   |
|                | -----  |  ------  | ------  ------ |   ------  |  ------   |
|                |100·00  |  100·00  | 100·00  100·00 |   100·00  |  100·00   |
+----------------+--------+----------+----------------+-----------+-----------+

The first four analyses give the composition of the weeds after they
have been separated from all foreign substances; the last, that of the
mixture taken from the heap just as it is used in Orkney; and its value
is then enhanced by small shells and marine animals adhering to the
plants, which increase the amount of phosphoric acid and nitrogen.

The ease with which all sea-weeds pass into a state of putrefaction,
adapts them in a peculiar manner to the manurial requirements of a cold
and damp climate. The rapidity of their decomposition is such, that when
spread on the land they are seen to soften and disappear in a short
time. They form therefore a rapid manure, and their effects are said to
be confined to the crop to which they are applied; but this is probably
due to the fact, that they are chiefly used in inferior sandy soils, in
which any manure is rapidly exhausted. In good soils there is no reason
why their effect should not be as lasting as that of farm-yard manure,
which, in many particulars, they considerably resemble. The method of
applying sea-weeds most generally in use, is to spread them on the soil,
and plough them in after putrefaction has commenced, and it is on the
whole the most advantageous. But they are sometimes composted with lime
and earth, or mixed with farm-yard manure, and occasionally, also, they
are used as a top-dressing to grass land.

On some parts of the western coast of Scotland and in the Hebrides,
sea-weed is the chief manure. It gives excellent crops of potatoes, but
they are said to be of inferior quality, unless marl or shell-sand is
employed at the same time.

_Leaves_ may be used as a manure, simply by ploughing them in, by
composting them with lime, or by adding them to the manure heap.

_Peat._--As a source of organic matter, peat may be used with advantage,
especially on soils in which it is naturally deficient. Dry peat of good
quality contains about one per cent of nitrogen, and a quantity of ash
varying from five to twenty per cent. These substances, however, become
available very slowly, owing to the tardy decay of peat in its natural
state; and in order to make it useful, it is necessary to compost it
with lime, or to mix it with farm-yard manure, or some readily
putrescible substance, so that its decomposition may be accelerated. It
may be most advantageously used as an absorbent of liquid manure, and on
this account, forms a useful addition to the manure heap.

The observations which have been made regarding the use of these
substances, lead directly to the inference that all vegetable matters
possess a certain manurial value, and that they ought to be carefully
collected and preserved. In fact, the careful farmer adds everything of
the sort to his manure heap, where, by undergoing fermentation along
with the manure, their nitrogen becomes immediately available to the
plant; while the seeds of weeds are destroyed during the fermentation,
and the risk of the land being rendered dirty by their springing up when
the manure comes to be used is prevented.

FOOTNOTES:

[Footnote M: The presence of sulphuret of sodium in this case is due to
the difficulty of completely burning the ash. It exists in the plant as
sulphate of soda.]




CHAPTER X.

COMPOSITION AND PROPERTIES OF ANIMAL MANURES.


Manures of animal origin are generally characterized by the large
quantity of nitrogen they contain, which causes them to undergo
decomposition with great rapidity, and to yield the greater part of
their valuable matters to the crop to which they are applied.

_Guano._--By far the most important animal manure is guano, which is
composed of the solid excrements of carnivorous birds in a more or less
completely decomposed state, and is accumulated in immense quantities on
the coasts of South America and other tropical countries. It has been
used as a manure in Peru from time immemorial, but the accounts given by
the older travellers of its marvellous effects were considered to be
fabulous, until Humboldt, from personal observation, confirmed their
statements. It was first imported into this country in 1840, in which
year a few barrels of it were brought home; and from that time its
importation rapidly increased. Soon after large deposits of it were
found in Ichaboe; and it has since been brought from many other
localities. The quantity of guanos of all kinds imported into this
country and retained for home consumption now exceeds 240,000 tons a
year.

The value of guano differs greatly according to the extent to which its
decomposition has gone, and this is chiefly dependent on the climate of
the locality from which it is obtained. When deposited in the rainless
districts of Peru it still retains some of the uric acid and the greater
part of the ammonia naturally existing in it, and the quantity which has
escaped by decomposition is unimportant. But that obtained from other
districts has suffered a more or less complete decomposition according
to the humidity of the climate, which reduces the quantity of organic
matters and ammonia, until, in some varieties, they are so small as to
be of little importance. The following are minute analyses of three
specimens of Peruvian guano, shewing all the different constituents it
contains, and the amount of difference which may exist:--

                                   I.      II.      III.
Urate of ammonia                  10·70     9·0      3·24
Oxalate of ammonia                12·38    10·6     13·35
Oxalate of lime                    5·44     7·0     16·36
Phosphate of ammonia              19·25     6·0      6·45
Phosphate of magnesia and ammonia  ...      2·6      4·20
Sulphate of potash                 4·50     5·5      4·23
Sulphate of soda                   1·95     3·8      1·12
Sulphate of ammonia                3·36     ...      ...
Muriate of ammonia                 4·81     4·2      6·50
Phosphate of soda                  ...      ...      5·29
Chloride of sodium                 ...      ...      0·10
Phosphate of lime                 15·56    14·3      9·94
Carbonate of lime                  1·80     ...      ...
Sand and alumina                   1·59     4·7      5·80
Water                              9·14 }
                                        }  32·3     23·42
Undetermined humus-like organic}  10·00 }
matters                        }        }
                                 ------   -----    ------
                                 100·48   100·0    100·00

These analyses illustrate two points--_first_, that in some samples the
decomposition has advanced to a greater extent than in others; for we
observe that the quantity of uric acid, or rather of urate of ammonia,
is greatly less in the last analysis than in the other two, and much
smaller than in the fresh dung, which contains from 50 to 70 per cent of
uric acid; and _secondly_, that guano is rich in all the constituents of
the plant, but especially in ammonia, the best form in which nitrogen
can be supplied, in uric acid which by decomposition yields ammonia, and
in phosphoric acid. But such analyses are too elaborate for ordinary
purposes, and much less convenient for comparison and for estimating the
value of the guano than the shorter analysis commonly in use, which
gives the water, the loss by ignition (that is, the sum of the organic
matters and ammoniacal salts), the phosphates, the alkaline salts, and
the quantity of phosphoric acid contained in them, and existing there in
a state similar to that in which it is found in the soluble phosphates
of a superphosphate. In addition to these, the quantities of sand and
other less valuable ingredients are also stated.

In the subjoined tables the composition of a great variety of different
kinds of guano is given. Most of these are averages deduced from a
considerable number of analyses of good samples. Those of some kinds of
guano, such as Peruvian, which present a considerable amount of
uniformity, afford a sufficiently accurate idea of the general
composition of the variety, but in other cases they are of less value,
because the imports of different seasons, and even of different cargoes,
differ so greatly in composition that no proper average can be made.
Several of these varieties are already exhausted, the importation of
others has ceased, and new varieties are constantly being introduced.

_Table showing the Average Composition of different varieties of Guano._

-------------------+--------+-------+-------------+---------------------------+
                   |Angamos.|Peru-  |  ICHABOE.   |Bolivian or Upper          |
                   |        | vian. |             |Peruvian.                  |
                   |        |       +-------------+------+-----------+--------+
                   |        |       |Old.  |New.  |Old.  |Government.|Inferior|
-------------------+--------+-------+------+------+------+-----------+--------+
Water              | 12·36  | 13·73 |24·21 |18·89 | 12·55| 16·44     | 14·15  |
Organic matter    }|        |       |      |      |      |           |        |
  and ammoniacal  }| 59·92  | 53·16 |39·30 |32·49 | 35·89| 12·28     | 26·14  |
  salts           }|        |       |      |      |      |           |        |
Phosphates         | 17·01  | 23·48 |30·00 |19·63 | 27·63| 56·09     | 23·13  |
Sulphate of lime   |  ...   |  ...  | ...  | ...  | ...  | ...       |  9·65  |
Carbonate of lime  |  ...   |  ...  | ...  | ...  | ...  | ...       | 12·87  |
Alkaline salts     |  7·20  |  7·97 | 4·19 | 8·82 | 15·29| 11·33     |  5·97  |
Sand               |  3·51  |  1·66 | 2·30 | 6·72 |  8·64|  2·81     |  8·09  |
                   +--------+-------+------+------+------+-----------+--------+
                   | 100·00 |100·00 |100·00|100·00|100·00|100·00     |100·00  |
Ammonia            |  21·10 | 17·00 |  8·50| 10·42|  8·99|  2·57     |  3·26  |
Phosphoric        }|        |       |      |      |      |           |        |
  acid in alkaline}|   1·20 |  2·50 |  ... |  ... |  ... |  3·11     |  ...   |
  salts           }|        |       |      |      |      |           |        |
-------------------+--------+-------+------+------+------+-----------+--------+


-------------------+---------+-------+--------+-----------+-------------+
                   |         |       |        |           |             |
                   |Pacquico.|Latham |Saldanha|Australian.|Kooriamooria.|
                   |         |Island.|Bay.    |           |             |
                   |         |       |        |           |             |
-------------------+---------+-------+--------+-----------+-------------+
Water              |  8·38   | 24·96 | 21·03  | 13·20     |  8·91       |
Organic matter    }|         |       |        |           |             |
  and ammoniacal  }| 23·10   | 10·96 | 14·93  | 13·77     |  7·72       |
  salts           }|         |       |        |           |             |
Phosphates         | 32·36   | 54·47 | 56·40  | 44·47     | 44·15       |
Sulphate of lime   |  2·92   |  2·82 | ...    |  4·55     |  3·19       |
Carbonate of lime  | ...     |  2·20 | ...    |  8·82     |  3·37       |
Alkaline salts     | 25·43   |  4·06 |  6·10  |  7·34     | 11·23       |
Sand               |  7·81   |  0·51 |  1·54  |  7·85     | 21·43       |
                   +---------+-------+--------+-----------+-------------+
                   |100·00   |100·00 |100·00  |100·00     |100·00       |
Ammonia            |  6·58   |  1·26 |  1·62  |  1·01     |  0·42       |
Phosphoric        }|         |       |        |           |             |
  acid in alkaline}|  3·50   |  ...  |  ...   |  ...      |  ...        |
  salts           }|         |       |        |           |             |
-------------------+---------+-------+--------+-----------+-------------+

-------------------+-----------+--------+--------+
                   |           |        |        |
                   |Patagonian.|Chilian.|Mexican.|
                   |           |        |        |
                   |           |        |        |
-------------------+-----------+--------+--------+
Water              | 20·61     | 14·89  | 18·80  |
Organic matter    }|           |        |        |
  and ammoniacal  }| 19·72     | 16·81  | 12·88  |
  salts           }|           |        |        |
Phosphates         | 30·66     | 36·90  | 18·38  |
Sulphate of lime   |  1·30     |  ...   | 27·79  |
Carbonate of lime  |  3·06     | 10·28  | ...    |
Alkaline salts     |  7·01     |  6·84  | 16·95  |
Sand               | 17·04     | 14·26  |  5·20  |
                   +-----------+--------+--------+
                   |100·00     |100·00  |100·00  |
Ammonia            |  2·69     |  1·42  |  0·42  |
Phosphoric        }|           |        |        |
  acid in alkaline}|  3·00     |  ...   |  ...   |
  salts           }|           |        |        |
-------------------+-----------+--------+--------+


_Table shewing the Composition of some of the less common varieties of
Guano._

NOTE.--The numbers in this Table are mostly derived only from a single
analysis and have no value as determining the average composition of
these Guanos, but they serve to give a general idea of their value.

-------------------+--------+---------+---------+-----------+------------+
                   | Sea    | Indian. | Holme's | Ascension | Possession |
                   | Bear   |         | Bird    | Island.   | Island.    |
                   | Bay.   |         | Island. |           |            |
-------------------+--------+---------+---------+-----------+------------+
Water              | 30·82  |  23·62  |  25·00  |  15·97    |  10·92     |
Organic matter    }|        |         |         |           |            |
  and ammoniacal  }| 31·78  |  60·05  |  32·10  |  23·15    |  15·42     |
  salts           }|        |         |         |           |            |
Phosphates         | 24·33  |   7·18  |  27·36  |  32·54    |  46·41     |
Sulphate of lime   |  3·84  |   ...   |   ...   |   ...     |   7·46     |
Carbonate of lime  |  0·58  |   2·79  |   ...   |   ...     |   ...      |
Alkaline salts     |  7·38  |   5·58  |   8·82  |  15·92    |   6·15     |
Sand               |  1·27  |   0·78  |   6·72  |  12·42    |  13·64     |
                   +--------+---------+---------+-----------+------------+
                   |100·00  | 100·00  | 100·00  | 100·00    | 100·00     |
                   |        |         |         |           |            |
Ammonia            | 10·45  |  10·27  |   7·75  |   6·06    |   1·34     |
Phosphoric        }|        |         |         |           |            |
  acid in alkaline}|   ...  |   ...   |   ...   |   1·82    |   ...      |
  salts           }|        |         |         |           |            |
-------------------+--------+---------+---------+-----------+------------+

-------------------+-------+---------+---------+---------+
                   | Algoa | New     | Bird's  | Leone   |
                   | Bay.  | Island. | Island. | Island. |
-------------------+-------+---------+---------+---------+
Water              | 30·55 |  28·78  |  16·52  |  23·65  |
Organic matter    }|       |         |         |         |
  and ammoniacal  }|  6·85 |  13·78  |  14·84  |   4·27  |
  salts           }|       |         |         |         |
Phosphates         | 21·24 |  22·46  |  25·21  |  13·58  |
Sulphate of lime   | 36·42 |   ...   |  40·47  |  29·95  |
Carbonate of lime  |  ...  |  13·78  |   ...   |   ...   |
Alkaline salts     |  3·32 |  12·62  |   1·16  |   5·40  |
Sand               |  1·62 |  11·58  |   1·80  |  23·15  |
                   +-------+---------+---------+---------+
                   |100·00 | 100·00  | 100·00  | 100·00  |
                   |       |         |         |         |
Ammonia            |  0·54 |   0·84  |   1·26  |   0·67  |
Phosphoric        }|       |         |         |         |
  acid in alkaline}|  ...  |   ...   |   ...   |   ...   |
  salts           }|       |         |         |         |
-------------------+-------+---------+---------+---------+

On examining the tables given above, it is obvious that guanos may be
divided into two classes, the one characterized by the abundance of
ammonia, the other by that of phosphates; and which, for convenience
sake, may be called ammoniacal and phosphatic guanos. Peruvian and
Angamos are characteristic of the former, and Saldanha Bay and Bolivian
of the latter class. The value of these two classes of guano differs
materially, and they are also applicable under different circumstances,
but to these points reference will afterwards be made.

Very special precautions are necessary on the part of the farmer in
order to insure his obtaining a guano which is not adulterated, and of
good quality if genuine. In the case of Peruvian guano, which is
tolerably uniform in its qualities, it is possible to form some opinion
by careful examination, and the following points ought to be attended
to:

1st, The guano should be light coloured. If it is dark, the chances are
that it has been damaged by water.

2d, It should be dry, and when a handful is well squeezed together it
should cohere very slightly.

3d, It should not have too powerful an ammoniacal odour.

4th, It should contain lumps, which, when broken, appear of a paler
colour than the powdery part of the sample.

5th, When rubbed between the fingers it should not be gritty.

6th, A bushel of the guano should not weigh more than from 56 to 60 lbs.

These characters must not, however, be too implicitly relied on, for
they are all imitated with wonderful ingenuity by the skilful
adulterator, and they are applicable only to Peruvian guano; the others
being so variable that no general rules can be given for determining
whether they are genuine. Neither are they so precise as to enable us to
give any opinion regarding the relative values of several samples where
all are genuine. The only way in which adulteration can with certainty
be detected, and the value of different guanos be determined, is by
analysis, and the importance of this can easily be illustrated.

In the table above, the _average_ composition of the different guanos is
given; but in order to shew how much individual cargos may differ from
the mean, we give here analyses of samples of the highest and lowest
quality of the genuine guanos of most importance:

+-------------------+-------------------+------------------+------------------+
|                   |    Angamos.       |    Peruvian.     |    Bolivian.     |
|                   +---------+---------+---------+--------+---------+--------+
|                   |Highest. |Lowest.  |Highest. |Lowest. |Highest. |Lowest. |
|                   +---------+---------+---------+--------+---------+--------+
| Water             |  12·60  |   7·09  |  10·37  | 21·49  |  11·53  |  16·20 |
| Organic matter  } |         |         |         |        |         |        |
|  and ammoniacal } |  65·62  |  50·83  |  55·73  | 46·26  |  11·17  |  12·86 |
|  salts          } |         |         |         |        |         |        |
| Phosphates        |  10·83  |   8·70  |  25·20  | 18·93  |  62·99  |  52·95 |
| Alkaline salts    |   7·50  |  16·30  |   7·50  | 10·64  |   9·93  |  13·83 |
| Sand              |   3·45  |  17·08  |   1·20  |  2·68  |   4·38  |   4·16 |
|                   +---------+---------+---------+--------+---------+--------+
|                   | 100·00  | 100·00  | 100·00  |100·00  | 100·00  | 100·00 |
|                   |         |         |         |        |         |        |
| Ammonia           |  25·33  |  17·15  |  18·95  | 14·65  |   1·89  |   2·23 |
+-------------------+---------+---------+---------+--------+---------+--------+

The differences are here exceedingly large; and when the values of the
two Peruvian guanos are calculated according to the method to be
afterwards described, it appears that the highest exceeds the lowest in
value by nearly £3 per ton. Of course, this is an extreme case, but it
is no uncommon occurrence to find a difference of £1 or even £2 per ton
between the values of cargos of Peruvian guano, which are sold at the
same price.

The adulteration of guano is carried on to a very large extent; and
though perhaps not quite so extensively now as it was some years since,
it is only kept in check by the utmost vigilance on the part of the
purchaser. The chief adulterations are a sort of yellow loam very
similar in appearance to guano, sand, gypsum, common salt, and
occasionally also ground coprolites and inferior guano. These substances
are rarely used singly, but are commonly mixed in such proportions as
most closely to imitate the colour and general appearance of the genuine
article. The extent to which the adulteration is carried may be judged
of from the following analyses taken at random from those of a large
number of guanos, all of which were sold as first-class Peruvian.

Water                  12·85   15·19   12·06   27·86   6·32
Organic matter and }
  ammoniacal salts }   26·84   44·31   34·14   30·41  27·42
Phosphates             15·54   20·95   22·08   22·17  33·61
Sulphate of lime        ...     ...    11·08    ...   22·11
Alkaline salts          6·07    9·40   12·81    7·92  22·50
Sand                   38·70   10·15    7·83    1·64  10·15
                      ------  ------  ------  ------ ------
                      100·00  100·00  100·00  100·00 100·00

Ammonia                 9·34   13·90    9·77    8·64   9·76

In all those cases a very large depreciation in the value has taken
place, and several of them are worth considerably less than half the
price of the genuine guano, while they are generally offered for sale at
about £1 under the usual price. The adulteration is chiefly practised in
London, and cases occasionally occur which can be traced to Liverpool
and other places; but it always takes place in the large towns, because
it is only there that facilities exist for obtaining the necessary
materials and carrying it out without exciting suspicion. The
sophisticated article then passes into the hands of the small country
dealers, to whom it is sold with the assurance that it is genuine, and
analysis quite unnecessary. In other instances, adulterated and inferior
guanos are sold by the analysis of a genuine sample, and sometimes an
analysis is made to do duty for many successive cargos of a guano which,
though all obtained from one deposit, may differ excessively in
composition. In order to insure obtaining a genuine guano, it is above
all things important to deal only with a person of established
character, who will generally, for his own sake, satisfy himself that
the article he vends is genuine and of good quality; and it is always
important that the buyer should examine the analysis, and in all cases
where there is the slightest doubt, should ascertain that the bulk sent
corresponds with it. In the case of a Peruvian guano, a complete
analysis is not necessary for this purpose; but an experienced chemist,
by the application of a few tests, can readily ascertain whether the
sample is genuine. Where the difference in value between different
samples is required, a complete analysis is necessary, and this is
indispensable in the case of the inferior guanos. Many of these are
obtained from deposits of limited extent, and in loading it considerable
quantities of the subjacent soil are taken up, so that very great
differences may exist even in different parts of the same cargo. Nor
must it be forgotten that, except in the case of Peruvian, the name is
no guarantee for the quality of the guano, even if genuine. Peruvian
guano is all obtained from the same deposits, those of the Chincha
Islands, but the guanos which are brought into the market under the name
of Patagonian, Chilian, etc., are obtained from a great variety of
deposits scattered along the coasts of these countries, sometimes at a
distance of several hundred miles from each other, and which have been
accumulated under totally different circumstances. In illustration of
this, it is only necessary to refer to the subjoined analysis of
samples, all of which I believe to be genuine as imported, and which
were sold under the name of Upper Peruvian Guano.

                                         I.    II.   III.
Water                                   7·80   6·65   8·85
Organic matter and ammoniacal salts    10·85  19·16  10·20
Phosphates                             67·00  20·41  17·10
Carbonate of lime                       ...   21·15   ...
Alkaline salts                         11·10   5·31  61·30
Sand                                    3·25  27·32   2·55
                                      ------ ------ ------
                                      100·00 100·00 100·00

Ammonia                                 2·29   5·73   1·48
Phosphoric acid in the alkaline salts   2·24   ...    1·70
Equal to phosphate of lime              4·89   ...    3·70

With the exception of Peruvian, the supply of _good_ guanos of uniform
composition is by no means large, and phosphatic guanos of good quality
are now especially rare. The Saldanha Bay, and other similar deposits,
have been exhausted, and few guanos of equally good quality have been
lately discovered. There is no doubt, however, that such guanos are very
useful, and if obtained in large quantity, and of uniform composition,
would be used to a much larger extent than they at present are.

The value and use of guano are now so well understood, that it is
scarcely necessary to enlarge on the mode of its application. Peruvian
guano owes its chief value to its ammonia and phosphates, but it also
contains potash, soda, and all the other constituents of plants in small
quantity, although in a readily available condition, as is seen in the
detailed analysis given in page 205.

In other guanos which have undergone more complete decomposition, and
from which the soluble matters have been more or less completely
exhausted by rain, the alkaline salts, or at least the potash they
originally contained, have almost entirely disappeared. Hence an
important difference between Peruvian guano and most other varieties.
The former can be used as a complete substitute for farm-yard manure,
and excellent crops of turnips and potatoes can be raised by means of it
alone, and at a less cost than with ordinary dung. But though this may
be done, and in many cases is attended with great economic advantages,
it is a practice that cannot be recommended for general use, because the
quantity of valuable matters contained in the usual application of guano
is much smaller than in farm-yard manure, and the probability is that it
would not, if used alone during a succession of years, be sufficient to
maintain the soil permanently in a high state of fertility. Five cwt. of
Peruvian guano, which is a liberal application per acre, contains about
95 lbs. of ammonia, and 130 of phosphates, while 20 tons of good
farm-yard manure contain 312 of ammonia, and about the same quantity of
phosphates, and when the other constituents, such as potash and soda,
are compared with those in guano, the difference is still more striking.
On the other hand, guano is a rapidly acting manure; its constituents
are in a condition in which they are more immediately accessible by the
plant, and its immediate effect is far more marked, as it is chiefly
expended on the crop to which it is applied. It has indeed been alleged
that it produces no effects on the subsequent crops, but this opinion
can scarcely be considered as well founded. In no case does the crop
raised by means of it contain the whole of the ammonia or phosphates
present in the manure, and the unappropriated quantity, though it may,
and probably does, escape from the lighter soils, must be retained and
preserved for the use of subsequent crops by heavy and retentive clay
soils. The general inference is, that though guano may at an emergency
be used as an entire substitute for farm-yard manure, the practice is
one to be generally avoided. When, however, as occasionally happens
after a long continued use of farm-yard manure, organic matters have
accumulated in the soil, and passed into an inert condition, then
Peruvian guano may be used alone with very great advantage. In all cases
the rapidity of the action of guano makes it an important auxiliary of
farm-yard manure, and it is in this way that it may be most
advantageously employed. Experience has shewn that one-half the
farm-yard manure may be replaced by guano with the production of a
larger crop than by the former alone in its full quantity. The
proportion of guano usually employed is from three to five cwt., and it
is alleged that a much larger quantity produces prejudicial effects on
the subsequent crops, although it is not very easy to see on what this
depends.

The variety of guano to be selected must depend to a great extent on the
use to which it is to be put. Peruvian guano is most advantageously
applied as a top-dressing to young corn and particularly to oats. For
the turnip, the ammoniacal guanos were formerly preferred, and on strong
soils, under good cultivation, their effects are excellent, but on
light soils they are less applicable, their soluble salts being more
rapidly washed out, and their effects lost, and in these cases they are
surpassed by the phosphatic guanos.

No definite rules can be given for determining the soils on which these
different varieties are most applicable, but each individual must
determine by experiment that which best suits his own farm; and the
inquiry is of much importance to him, as, of course, if the phosphatic
guanos will answer as well as the ammoniacal, there is a large saving in
the cost of the manure. A very excellent practice is to employ a mixture
of equal parts of the two sorts of guano.

_Pigeons' Dung._--The dung of all birds, which more or less closely
resembles guano, may be employed with much advantage as a manure, but
that of the pigeon and the common fowl are the only ones which can be
got in quantity. Pigeons' dung, according to Boussingault, contains 8·3
per cent of nitrogen, equivalent to 10·0 of ammonia. Its value,
therefore, will be more than half that of guano, but it varies greatly,
and a sample imported from Egypt into this country, and analysed by
Professor Johnston, contained only 5·4 per cent of ammonia. Hens' dung
has not been accurately analysed, but its value must be about the same
as pigeons'.

_Urate and Sulphated Urine._--We have already discussed the urine of
animals, in reference to farm-yard manure. But human urine, the
composition of which was then stated, is of much higher value than that
of the lower animals, and many attempts have been made to preserve and
convert it into a dry manure. Urate is prepared by adding gypsum to
urine, and collecting and drying the precipitate produced. It contains a
considerable quantity of the phosphoric acid of the urine, but very
little of its ammonia; and as the principal value of urine depends on
the latter, it is necessarily a very inefficient method of turning it to
account. A better method has been proposed by Dr. Stenhouse, who adds
lime-water to the urine, and collects the precipitate, which, when dried
in the air, contains 1·91 per cent of nitrogen, and about 41 per cent of
phosphates. This method is subject to the same objection as that by
which urate is made, namely, that the greater part of the ammonia is not
precipitated. This might probably be got over to some extent by the
addition of sulphate of magnesia, or, still better, of chloride of
magnesium, which would throw down the phosphate of magnesia and ammonia.
By much the best mode of employing urine is in the form of sulphated
urine, which is made by adding to it a sufficient quantity of sulphuric
acid to neutralize its ammonia, and evaporating to dryness. In this form
all the valuable constituents are retained, and excellent results are
obtained from it. Its effects, though mainly attributable to its
ammonia, are also in part dependent on the phosphates and alkaline salts
which it contains; and it is therefore capable of supplying to the plant
a larger number of its constituents than the animal matters already
mentioned.

_Night-Soil and Poudrette._--The value of night-soil, which is well
known, depends partly on the urine, and partly on the fæces of which it
is formed. Its disagreeable odour has prevented its general use, and
various methods have been contrived both for deodorising and converting
it into a solid and portable form. The same difficulties which beset the
conversion of urine into the solid form occur here, and in most of the
methods employed the loss of ammonia is great. It is sometimes mixed
with lime or gypsum, and dried with heat, and sometimes with animal
charcoal or peat charcoal. The manufacture of a manure from night-soil,
called "poudrette," has long been practised in the neighbourhood of
Paris and other continental towns. The process employed at Montfauçon
and at Bondy is very simple. The contents of the cesspools are conveyed
to the work in large barrels, which are then emptied into tanks capable
of containing the accumulation of several months. When filled they are
allowed to stand for some time, during which the smell diminishes and
the contents become nearly dry. The residue is then dug out and mixed
with ashes, dry loam, charcoal powder, peat, peat-charcoal, saw-dust,
and other matters, so as to deodorize it, and render it sufficiently dry
for transport. Its general composition may be judged of from the
subjoined analyses of samples from different places:--

                       Montfauçon.   Bondy.  Dresden.  American.
Water                   28·00        13·60    19·50     39·97
Organic matters         29·00        24·10    20·80     20·57
Phosphates               7·65         4·96     5·40      1·88
Carbonates of lime and }
Magnesia, alkaline     } 7·35        14·14    11·30      7·63
salts, etc.            }
Sand                    28·00        43·20    43·00     29·95
                       ------       ------   ------    ------
                       100·00       100·00   100·00    100·00

Ammonia                  1·54         1·98     2·60      1·23

These analyses shew sufficiently the extent to which the animal matters
have been mixed with valueless driers, the second and third samples
containing considerably more than half their weight of worthless
matters.

_Hair, Skin, and Horn._--The refuse of manufactories in which these
substances are employed, are frequently used as manures. They are highly
nitrogenous substances, and owe their entire value to the nitrogen they
contain, their inorganic constituents being in too small quantity to be
of any importance, wool and hair having only 2 per cent, and horn 0·7
per cent of ash. In the pure and dry state, and after subtraction of the
ash, their composition is,--

                   Skin.  Human hair.       Wool.     Horn.
Carbon             50·99    50·65           50·65     51·99
Hydrogen            7·07     6·36            7·03      6·72
Nitrogen           18·72    17·14           17·71     17·28
Oxygen             23·22    20·85   }       24·61     24·01
Sulphur             ...      5·00   }
                  ------   ------          ------    ------
                  100·00   100·00          100·00    100·00

It rarely if ever happens, however, that the refuse offered for sale as
a manure is pure. It always contains water, sand, and other foreign
matters. Woollen rags are mixed with cotton which has no manurial value,
and the skin refuse from tan-works contains much lime. Due allowance
must therefore be made for such impurities which are sometimes present
in very large quantity.

Refuse horse hair generally contains 11 or 12 per cent of nitrogen.
Woollen rags of good quality contain 12·7 per cent of nitrogen; woollen
cuttings about 14; and what is called shoddy only 5·5 per cent. Horn
shavings are extremely variable in their amount of nitrogen; when pure,
they sometimes contain as much as 12·5 per cent, but a great deal of the
horn shavings from comb manufactories, etc., contain much sand and bone
dust, by which their percentage of nitrogen is greatly diminished, and
it sometimes does not exceed 5 or 6 per cent.

All these substances are highly valuable as manures, but it must be
borne in mind that they undergo decomposition very slowly in the soil,
and hence are chiefly applicable to slow growing crops, and to those
which require a strong soil. Woollen rags have been largely employed as
a manure for hops, and are believed to surpass every other substance for
that crop. As a manure applicable to the ordinary purposes of the farm
they have scarcely met with that attention which they deserve, probably
because their first action is slow and the farmer is more accustomed to
look to immediate than to future results; but they possess the important
qualification of adding permanently to the fertility of the soil.

_Blood_ is a most valuable manure, but it is not much employed in this
country, at least in the neighbourhood of large towns, as there is a
demand for it for other purposes, and it can rarely be obtained by the
farmer in large quantity, and at a sufficiently low price. In its
natural state it contains about 3 per cent of nitrogen, and after being
dried up, the residue contains about 15 per cent. It is best used in the
form of a compost with peat or mould, and this forms an excellent manure
for turnips, and is also advantageously applied as a top-dressing to
wheat.

_Flesh._--The flesh of all animals is useful as a manure, and is
especially distinguished by the rapidity with which it undergoes
decomposition, and yields up its valuable matters to the plant. It is
rarely employed in its natural state, but horse flesh was at one time
converted into a dry and portable manure, although, I understand, this
manufacture is not now prosecuted. The dead animal after being skinned
is cut up and boiled in large cauldrons until the flesh is separated
from the bones. The latter are removed, and the flesh dried upon a flat
stove. The flesh as sold has the following composition:--

Water                                12·17
Organic matter                       78·44
Phosphate of lime, etc.               3·82
Alkaline salts                        3·64
Sand                                  1·93
                                    ------
                                    100·00
Nitrogen                              9·22
Ammonia to which the nitrogen is }
equivalent                       }   11·20

The dried flesh and small bones of cattle, from the great slaughtering
establishments of South America, was at one time imported into this
country under the name of flesh manure. Its composition was--

Water                                9·05
Fat                                 11·13
Animal matter                       39·52
Phosphate of lime                   28·74
Carbonate of lime                    3·81
Alkaline salts                       0·57
Sand                                 7·18
                                   ------
                                   100·00
Nitrogen                             5·56
Ammonia to which the nitrogen is }
equivalent                       }   6·67

But owing to the large proportion of phosphates contained in it, it may
be most fairly compared with bones. It is not now imported, the results
obtained from its use being said not to have proved satisfactory,
although this statement appears very paradoxical.

_Fish_ have been employed in considerable quantity as a manure. That
most extensively employed in this country is the sprat, which is
occasionally caught in enormous quantities on the Norfolk coast, and
used as an application for turnips. They are sold at 8d. per bushel,
and their composition is--

Water             64·6
Organic matter    33·3
Ash                2·1
                 -----
                 100·0
Nitrogen          1·90
Phosphoric acid   0·91

The refuse of herring and other fish-curing establishments, whales'
blubber, and similar fish refuse, are all useful as manure, and are
employed whenever they can be obtained. They are not usually employed
alone, but are more advantageously made into composts with their own
weight of soil, and allowed to ferment thoroughly before being applied.

Many attempts have been made to convert the offal of the great
fish-curing establishments, and the inedible fish, of which large
quantities are often caught, into a dry manure, which has received the
name of "fish guano." The processes employed have consisted in boiling
with sulphuric acid and other agents, and then evaporating, or sometimes
by simply drying up the refuse by steam heat. A manure made in this way
proved to have the following composition:--

Water                                   8·00
Fatty matters                           7·20
Nitrogeneous organic matters           71·46
Phosphate of lime                       8·70
Alkaline salts                          3·80
Sand                                    0·84
                                      ------
                                      100·00

Nitrogen                                11·25
Equal to ammonia                        13·68
Phosphoric acid in the alkaline salts, } 0·65
   equal to 1·41 phosphate of lime     }

The expense of manufacturing manures of this description has hitherto
acted as a barrier to their introduction. In this country several
manufactories have been established, but either owing to this cause, or
to the difficulty of obtaining sufficiently large and uniform supplies
of the raw material, some of them have not proved successful, but a
manufactory is now in operation in Norway, which exports the manure to
Germany. It is probable that most of the processes used in this country
failed because they were too costly, and it is much to be desired that
the subject should be actively taken up. It is said that the refuse from
the Newfoundland fisheries is capable of yielding about 10,000 tons of
fish guano annually; and the quantity obtainable on our own coasts is
also very considerable.

_Bones._--Bones have been used as a manure for a long period, but they
first attracted the particular attention of agriculturists from the
remarkable effects produced by their application on the exhausted
pasture lands of Cheshire. During the present century they came into
general use on arable land, and especially as a manure for turnips; and
they are now imported in large quantities from the continent of Europe.
The bones used in agriculture are chiefly those of cattle, but sheep and
horse bones are also employed. They do not differ much in quality when
genuine. The subjoined analysis is that of a good sample.

Water                                   6·20
Organic matter                         39·13
Phosphate of lime                      48·95
Lime                                    2·57
Magnesia                                0·30
Sulphuric acid                          2·55
Silica                                  0·30
                                      ------
                                      100·00
Ammonia which the organic matter }
   is capable of yielding        }      4·80

In general, bones may be said to contain about half their weight of
phosphate of lime, and 10 or 12 per cent of water. But, in addition to
their natural state, they are met with in other forms in commerce, in
which their organic matter has been extracted either by boiling or
burning. The latter is especially common in the form of the spent animal
charcoal of the sugar refiners, which usually contains from 70 to 80 per
cent of phosphate of lime, but when deprived of their organic matter,
they may be more correctly considered under the head of mineral manures.

From the analysis given above, it is obvious that the manurial value of
bones is dependent partly on their phosphates and partly on the ammonia
they yield. It has been common to attribute their entire effects to the
former, but this is manifestly erroneous; and although there are no
doubt cases in which the former act most powerfully, the benefit derived
from the ammonia yielded by the organic matter is unequivocal. When the
phosphates only are of use, burnt bones or the spent animal charcoal of
the sugar refiners are to be preferred.

At their first introduction, bones were applied in large fragments, and
in quantities of from 20 to 30 cwt., or even more, per acre, but as
their use became more general they were gradually employed in smaller
pieces, until at last they were reduced to dust, and it was found that,
in a fine state of division, a few hundredweights produced as great an
effect as the larger quantity of the unground bones. Even the most
complete grinding which can be attained, however, leaves the bones in a
much less minute state of division than guano, and they necessarily act
more slowly than it does, the more especially as they contain no
ready-formed ammonia. They may be still further reduced by fermentation,
which acts by decomposing the organic matter, and causing the production
of ammonia; but not as is frequently, though erroneously supposed, by
converting the phosphates into a soluble condition, for this does not
occur to any extent, and their more rapid action is solely due to the
partial decomposition of the organic matter, by which it is brought into
a condition capable of undergoing a more rapid change in the soil. The
rapidity of action of bones is still more promoted by solution in
sulphuric acid, by which they are converted into the form of dissolved
bones or superphosphate. At the present moment, however, very little of
the superphosphates sold in the market are made exclusively from bones
in their natural state, by far the larger portion being manufactured
from mineral phosphates, or from bones after destruction of their
organic matter, sometimes with the addition of small quantities of
unburnt bones, but more frequently of sulphate of ammonia, to yield the
requisite quantity of ammonia. These substances may therefore be best
considered under the head of mineral manures.




CHAPTER XI.

COMPOSITION AND PROPERTIES OF MINERAL MANURES.


Mineral manure is a term which is now used with great laxity. In its
strict sense, it means manures which contain only, and owe their
exclusive value to the presence of, those substances which go to make up
the inorganic part or ash of plants. It has, however, been usually taken
to include all saline matters, and especially the compounds of ammonia
and nitric acid, which are indebted for their manurial effects to the
nitrogen they contain; and thus is so far incorrect. It would, however,
be manifestly impossible to arrange these compounds with any degree of
accuracy among either animal or vegetable manures, and hence the
necessity of including them amongst those which are strictly mineral.
The most important practical distinction between them and the substances
discussed in the two preceding chapters is, that the latter generally
contain the whole or the greater part of the constituents of plants.
Even bones yield a certain quantity of alkalies, magnesia, sulphuric
acid, and chlorine, and may in some sense be considered as a general
manure. But those to which the term mineral manure is applied for the
most part contain only one or two of the essential elements of plants,
and hence cannot be applied as substitutes for the substances already
discussed, although they are frequently most important additions to
them.

_Sulphate and Muriate of Ammonia._--These and other salts of ammonia
have been tried experimentally as manures, and it has been ascertained
that they may all be used with equal success; but as the sulphate is by
much cheaper, it is that which probably will always be employed to the
exclusion of every other. It contains, when pure, 25·7 per cent ammonia.

It is now manufactured of excellent quality for agricultural use, and
when good, contains from 95 to 97 per cent of actual sulphate, the
remainder consisting chiefly of moisture and a small quantity of fixed
residue; but specimens are occasionally met with containing as much as
10 per cent of impurities, which, as its price is high, makes a material
difference in its value. Inferior descriptions are also occasionally
sold, among which is a variety distinguished by containing a large
quantity of water and fixed salts, although it appears to the eye a good
article. Its composition is--

                                  I.              II.
Water                            9·05            5·77
Sulphate of ammonia             79·63           85·21
Fixed salts                     11·17            9·02
                               ------          ------
                               100·00          100·00
Ammonia                         20·55           21·94

An article called sulphomuriate of ammonia is also sold for agricultural
use. It is obtained as a refuse product in the manufacture of magnesia,
and is a mixture of sulphate and muriate of ammonia, with various
alkaline salts. It differs somewhat in quality, and is sold by analysis
at a price dependent on the ammonia it contains.

                          I.          II.
Water                    14·49       25·39
Sulphate of ammonia      62·55       47·79
Muriate of ammonia       15·3         ...
Sulphate of soda          ...         9·12
Sulphate of magnesia      ...        18·38
Chloride of potassium     4·75        2·94
Chloride of sodium       17·35        0·35
                        ------      ------
                        100·00      100·00
Ammonia                  16·50       11·28

The quality of sulphate of ammonia may generally be judged of from its
dry and uniformly crystalline appearance, and it may be tested by
heating a small quantity on a shovel over a clear fire, when it ought to
volatilize completely, or leave only a trifling residue. Some care,
however, is necessary in applying this test, as in the hands of
inexperienced persons it is sometimes fallacious. The salts of ammonia
may be applied in the same way as guano; but they are most
advantageously employed as a top-dressing, and principally to grass
lands. In this way very remarkable effects are produced, and within a
week after the application, the difference between the dressed and
undressed portions of a field is already conspicuous. Experience has
shewn that success is best insured when the salt is applied during or
immediately before rain, so that it may be at once incorporated with the
soil; as when used in dry weather little or no benefit is derived from
it. It seems also to exert a peculiarly beneficial effect upon clover;
and hence it ought to be employed only on clover-hay, as where ryegrass
or other grasses form the whole of the crop we have better manures.

_Ammoniacal Liquor of the Gas-Works, and of the Animal Charcoal
Manufacturers._--Both of these are excellent forms in which to apply
ammonia, when they can be obtained. The ammoniacal liquor of the
gas-works is very variable in quality, but contains generally from 4 to
8 ounces of dry ammonia per gallon, which corresponds in round numbers
to from 1 to 2 lb. of sulphate of ammonia. It is best applied with the
watering-cart, but must be diluted before use with three or four times
its bulk of water, as if concentrated it burns up the grass, and it is
also advisable to use it during wet weather. The ammoniacal liquor of
the ivory-black works contains about 12 per cent of ammonia, or about
four or five times as much as gas liquor. It has been used in some parts
of England, made into a compost, and applied to the turnip and other
crops, and, it is said, with good effect. _Bone oil_, which distils over
along with it, has also been used in the form of a compost; it contains
a large quantity of ammonia and of nitrogen in other forms of
combination; the total quantity of nitrogen it contains being 9·04 per
cent, which is equivalent to 10·98 of ammonia. Only part of this
nitrogen is actually in the state of ammonia; and some circumstances
connected with the chemical relations of the other nitrogenous compounds
in this substance render it probable that they may pass very slowly into
ammonia, and may therefore be of inferior value; but the substance
deserves a trial, as it is very cheap. It must be carefully composted
with peat, and turned over several times before being used.

_Nitrates of Potash and Soda._--Nitrate of potash has long been used as
a manure, but its high price has prevented its general application, and
its place has now been almost entirely taken by nitrate of soda, which
is much cheaper and contains weight for weight a larger quantity of
nitrogen. Both these salts are employed as sources of nitrogen; but
nitrate of potash owes also a certain proportion of its value to the
potash it contains. Nitrate of soda, on the other hand, must be
considered to owe its entire value to its nitric acid, as soda is of
little value to the plant; and, moreover, can be obtained in common salt
at a price so low, as to make it a matter of no moment in the valuation
of the nitrate. In its ordinary state, as imported from Peru, nitrate of
soda contains from 5 to 10 per cent of impurities, and it bears a price
proportionate to the quantity of the pure salt present in it. When of
good quality it contains about 15 per cent of nitrogen, equivalent to 18
of ammonia, and is, therefore, richer in that constituent of plants than
Peruvian guano. It is essentially a rapidly acting manure, and produces
a marked effect within a very few days after its application; but owing
to the fact that nitric acid cannot be absorbed and retained by the soil
in the same manner as ammonia, it is liable to be lost unless it can be
at once assimilated by the plant. For this reason it acts best when
applied in small quantity as a top-dressing to grass-land, and to young
corn. A large application has no advantages, and there can be no doubt
that the best effect would be produced by several very small quantities,
applied at intervals. In one experiment, Mr. Pusey found 42 lb. per acre
to increase the produce of barley by 7 bushels, and very favourable
results have been obtained by other experimenters. The beneficial
effects of nitrate of soda appear to be almost entirely confined to the
grasses and cereals. At least experience here has shewn that it produces
little or no effect on clover; and one farmer has stated, that having
recently adopted the practice of sowing clover with a very small
proportion of ryegrass only, he has been led to abandon the use of
nitrate of soda, which he formerly employed abundantly, when ryegrass
formed a principal part of his crop. The action of nitrate of soda is
very remarkable, not only in this respect, but also because a given
quantity of nitrogen in it _appears_ to produce a greater effect than
the same quantity in sulphate of ammonia or guano. At the same time this
statement must be taken as very general, definite experiments being
still too few to admit of its being stated as an absolute fact. The
probability is, that the same quantity of nitrogen, in the form either
of ammonia or nitrate of soda, will produce the same effect, although
the conditions necessary for its successful action may not be the same
with the two manures. It is alleged that nitrate of soda is
advantageously conjoined with common salt, which is said to check its
tendency to make the grain crops run to straw, and to prevent their
lodging, as they are apt to do, when it is employed alone. But
considerable difference of opinion exists in this point, many farmers
believing that salt produces no effect. When employed for hay,
especially when mixed with clover, it is advisable to use it along with
an equal quantity of sulphate of ammonia, which gives a better result
than either separately.

_Salts of Potash and Soda._--The substances just mentioned must be
considered to owe their chief manurial value to nitric acid; but other
salts have been used as manures in which the effect is undoubtedly due
to the alkalies themselves. With the exception of common salt, most of
the alkaline salts have only been used to a limited extent; and it is
remarkable that, so far as our present experience goes, there is no
class of substances from which more uncertain results are obtained.

_Muriate and Sulphate of Potash_ have both been used, and the former
has in some cases, and in particular seasons, produced a very remarkable
effect in the potato; but in other instances it has proved quite
useless. The cause of this difference has not been ascertained. Sulphate
of soda has also been used to some extent, but apparently without much
benefit; and there is no reason to expect that it should act better than
common salt, which can be obtained at a much lower price.

_Chloride of Sodium, or Common Salt_, has at different times been
employed as a manure, but its effects are so variable and uncertain,
that its use, in place of increasing, has of late years rather
diminished, it having frequently been found that on soils in all
respects similar, or even on the same soil, in different years, it
sometimes proves advantageous, at others positively injurious. Its use
as an addition to nitrate of soda has been already alluded to, and it is
said that it produces the same effect when mixed with guano and salts of
ammonia. The accuracy of this statement is doubted by many persons, and
the explanation which has been given of the cause of its action is more
than dubious. It is supposed to enable the plant to absorb more silica
from the soil; but this is a speculative explanation of its action, and
has not been supported by definite experiment. Although little effect
has been observed from salt, it deserves a more accurate investigation,
as not withstanding the extent to which it has been employed, we are
singularly deficient in definite experiments with it.

_Carbonates of Potash and Soda_ have only been tried experimentally, and
that to a small extent, nor is it likely that they will ever come into
use, owing to their high price. The remarks we have made in the section
on the ashes of plants regarding the subordinate value of soda, will
enable the reader to see that greater effects are to be anticipated
from the former than from the latter of these salts. They _may_,
however, exert a chemical action on the soil, altogether independent of
their absorption by the plant, but its nature and amount are still to
determine.

_Silicates of Potash and Soda_ have been employed with the view of
supplying silica to the plant, but the results have been far from
satisfactory. This may perhaps have been due to the doubtful nature of
the commercial article, but now that silicate of soda can be obtained of
good quality, it is desirable that the experiments should be repeated.
It is said to have produced good effects on the potato.

_Sulphate of Magnesia_ can be obtained at a low cost, and has been used
as a manure in some instances with very marked success. It has been
chiefly applied as a top-dressing to clover hay, but it seems probable
that it might prove a useful application to the cereals, the ash of
which is peculiarly rich in magnesia.

Many other saline substances have been tried as manures; but in most
instances on too limited a scale to permit any definite conclusions as
to their value. The experiments have also been too frequently performed
without the precautions necessary to exclude fallacy, so that the
results already arrived at must not be accepted as established facts,
but rather as indications of the direction in which further
investigation would be valuable. There is little doubt that many of
these substances might be usefully employed, if the conditions necessary
for their successful application were eliminated; and no subject is at
present more deserving of elucidation by careful and well-devised field
experiments.

_Phosphate of Lime._--The use of bones in their natural state as a
manure has been already adverted to, and it was stated, that though
their value depended mainly on the phosphates, the animal matters and
other substances contained in them were not without effect. The action
of phosphates is greatly promoted by solution in sulphuric acid, and the
application of the acid has brought into use many varieties of
phosphates of purely mineral origin, or which have been deprived of
their organic matters by artificial processes. Of these, the spent
animal charcoal of the sugar-refiners, usually containing about 70 per
cent of phosphates, and South American bone ash, are the most important.
The latter is now imported in very large quantity, and has the
composition shewn in the following analyses:--

                       I.      II.       III.

Water                 6·10     6·28      3·03
Charcoal              5·05     2·19      2·02
Phosphates           79·20    71·10     88·55
Carbonate of lime     4·05     3·55      5·60
Alkaline salts        0·15    traces     ...
Sand                  5·45    16·90      0·80
                    ------   ------    ------
                    100·00   100·00    100·00

Bone ash has hitherto been almost entirely consumed as a raw material
for the manufacture of superphosphates; but as it is sold at from £4:
10s. to £5: 10s. per ton when containing 70 per cent of phosphates, it
is, in reality, a very cheap source of these substances, and merits the
attention of the farmer as an application in its ordinary state.

Of strictly mineral phosphates, a considerable variety is now in use,
but they are employed exclusively in the manufacture of superphosphates,
as in their natural state they are so hard and insoluble, that the plant
is incapable of availing itself of them.

_Coprolites._--This name was originally applied by Dr. Buckland to
substances found in many geological strata, and which he believed to be
the dung of fossil animals. It has since been given to phosphatic
concretions found chiefly in the greensand in Suffolk and
Cambridgeshire, which are certainly not the same as those described by
Dr. Buckland, but consist of fragments of bones, ammonites, and other
fossils. Coprolites are now collected in very large quantities, and
about 43,000 tons are annually employed. They are extremely hard, and
require powerful machinery to reduce them to powder, and hence their
price is considerable, being about £2: 10s. per ton. Their composition
varies somewhat according to the care taken in selecting them, and the
locality from which they have been obtained. A general idea of their
composition may be derived from the subjoined analyses:--

Water                     1·95        1·90
Organic matter            2·59        6·85
Phosphate of lime        55·21}      61·15
Phosphate of iron         3·84}
Carbonate of lime        26·70       16·20
Sulphate of lime          1·97         "
Alkaline salts            1·85        3·21
Sand                      5·89       11·65
                        ------      ------
                        100·00      100·00

Within the last two or three years, coprolites have been found in great
abundance in France, but they are of inferior quality, and rarely
contain more than 40 per cent of phosphates.

_Apatite_, or mineral phosphate of lime, is found in large deposits in
different places. It is particularly abundant in Spain, and occurs also
in America and Norway. From the latter country it has been imported to
some extent; and during the last year considerable quantities have been
brought from Spain, and the importations will undoubtedly increase very
largely as the means of transport improve in that country. Spanish
apatite contains--

Water                 0·80
Phosphate of lime    93·30
Carbonate of lime     0·50
Chlorine, etc.       traces
Sand                  4·70
                     -----
                     99·30

Several other varieties of mineral phosphates have been imported under
the name of guano. The most important is Sombrero Island guano, which is
found on a small island in the Gulf of Mexico, where it occurs in a
layer said to be forty feet thick. It contains--

Water                            8·96
Phosphate of lime               37·71
Phosphates of alumina and iron  44·21
Phosphate of magnesia            4·20
Sulphate of lime                 0·86
Carbonate of lime                3·36
Sand                             0·70
                               ------
                               100·00

A somewhat similar substance, but in hard crusts, has been imported,
under the names of Maracaybo guano, Pyroguanite, etc., which contains--

Water                                   1·03
Organic matter                          6·78
Phosphates                             75·69
Alkaline salts                          4·91
Sand                                   11·64
                                      ------
                                      100·00
Phosphoric acid in the alkaline   }     0·78
   salts = 1·68 phosphate of lime }

These substances are all excellent sources of phosphates, but they are
so hard that the plants cannot extract phosphoric acid from them, and
they are only useful when made soluble by chemical processes.

_Superphosphate; Dissolved Bones._--These names were at first applied to
bones which had been treated with sulphuric acid; but superphosphates
are now rarely made from bones alone, but bone ash and some of the
mineral phosphates just described are employed, either along with them,
or very frequently alone. The manufacture of superphosphates depends on
the existence of two different compounds of phosphoric acid and lime,
one of which contains three times as much lime as the other. That which
contains the larger quantity of lime is found in the bones and all other
natural phosphates, and is quite insoluble in water; but when two-thirds
of its lime are removed, it is converted into the other compound, which
is exceedingly soluble. This change is effected by the use of sulphuric
acid, which combines with two-thirds of the lime of the ordinary
insoluble phosphate of lime, and converts it into the _biphosphate of
lime_, which is soluble. When, therefore, we add to 100 lbs. of common
phosphate of lime the necessary quantity of sulphuric acid, it yields 64
lbs. of biphosphate, containing the whole of the phosphoric acid, which
is the valuable constituent, the diminution in weight being due to the
removal of the valueless lime. Hence it follows, also, that as the lime
so removed is converted into sulphate, there must, for every 100 lbs. of
phosphate of lime converted into biphosphate, be produced 87 lbs. of dry
sulphate of lime, or 110 of the ordinary sulphate called gypsum. This is
the minimum quantity which can be present, but in actual practice it is
liable to be greatly exceeded, more especially where coprolites are
used, owing to the large amount of carbonate of lime they contain, which
is also converted into sulphate by the action of the acid, so that it is
far from uncommon to find the gypsum twice as great as it would be if
materials free from carbonates could be obtained. By employing a
sufficiency of sulphuric acid, the whole quantity of phosphoric acid in
the bones may be thus brought into a soluble state, but in actual
practice it is found preferable to leave part of it in the insoluble
condition; as where it is entirely soluble, its effect is too great
during the early part of the season, and deficient at its end. In order
to dissolve bones, bone ash, or mineral phosphates, they are mixed with
from a third to half their weight of sulphuric acid, of specific gravity
1·70 or 140° Twaddell. When mineral phosphates, and particularly
coprolites, are used, the quantity of sulphuric acid must be increased
so as to compensate for the loss of that which is consumed in
decomposing the carbonate of lime they contain. When operating on the
small scale, the materials are put into a vessel of wood, stone, or lead
(iron is to be avoided, as it is rapidly corroded by the acid), and
mixed with from a sixth to a fourth of their weight of water, which may
with advantage be used hot. The sulphuric acid is then added, and mixed
as uniformly as possible with the bones. Considerable effervescence
takes place, and the mass becomes extremely hot. At the end of two or
three days it is turned over with the spade, and after standing for some
days longer, generally becomes pretty dry. Should it still be too moist
to be sown, it must be again turned over, and mixed with some dry
substance to absorb the moisture. For this purpose everything containing
lime or its carbonate must be carefully avoided, as they bring back the
phosphates into the insoluble state, and undo what the sulphuric acid
has done. Peat, saw-dust, sand, decaying leaves, or similar substances,
will answer the purpose, and they should all be made thoroughly dry
before being used. An excellent plan is to sift the bones before
dissolving, to apply the acid to the coarser part, and afterwards to mix
in the fine dust which has passed through the sieve, to dry up the mass;
or a small quantity of bone ash, of good quality, or Peruvian guano, may
be used. On the large scale, mechanical arrangements are employed for
mixing the materials, so as to economise labour, and mineral phosphates,
such as apatite, can then be used with advantage. In such cases, blood,
sulphate of ammonia, soot, and other refuse matters, are occasionally
used to supply the requisite quantity of nitrogenous substances, but
large quantities are also made from bone ash, etc., without these
additions.

The composition of superphosphates must necessarily vary to a great
extent, and depends not only on the materials, but on the proportion of
acid used for solution. The following analysis illustrates the
composition of good samples made from different substances--

+-----------------------------------+------------------+------------------+
|                                   |                  |                  |
|                                   |    Bones alone.  |   Bone-Ash.      |
|                                   |                  |                  |
+-----------------------------------+------------------+------------------+
| Water,                            |  7·74 ...  7·79  |  5·33 ... 10·40  |
| Organic matters and ammoniacal    |                  |                  |
|    salts,                         | 17·83 ... 21·69  |  6·94 ...  4·92  |
| Biphosphate of lime               | 13·18 ...  9·87  | 21·35 ... 23·09  |
| Equivalent to soluble phosphates, |(20·57)...(15·39) |(33·33)...(36·02) |
| Insoluble phosphates              | 10·31 ... 21·17  |  5·92 ...  6·08  |
| Sulphate of lime,                 | 46·00 ... 35·30  | 56·16 ... 47·78  |
| Alkaline salts,                   |  1·46 ...  0·94  |     trace.       |
| Sand,                             |  3·48 ...  3·00  |  4·23 ...  4·30  |
|                                   +------------------+------------------+
|                                   |100·00 ...100·00  |100·00 ...100·00  |
| Ammonia,                          |  2·11 ...  3·01  |  0·23 ...  0·31  |
+-----------------------------------+------------------+------------------+

+-----------------------------------+-------------------+-------------------+
|                                   |                   |   Mixtures        |
|                                   |Chiefly Coprolites.|containing Salts of|
|                                   |                   |   Ammonia, etc.   |
+-----------------------------------+-------------------+-------------------+
| Water,                            |  5·90 ... 10·17   |  7·07 ... 15·82   |
| Organic matters and ammoniacal    |                   |                   |
|    salts,                         |  5·10 ...  4·13   |  9·87 ... 13·96   |
| Biphosphate of lime               | 12·24 ... 13·75   | 17·63 ... 12·67   |
| Equivalent to soluble phosphates, |(19·10)...(21·43)  |(27·50)...(19·77)  |
| Insoluble phosphates              | 16·90 ...  0·17   | 12·60 ...  8·40   |
| Sulphate of lime,                 | 52·39 ... 62·62   | 49·77 ... 45·14   |
| Alkaline salts,                   |  2·47 ...  0·96   |  0·06 ...  1·07   |
| Sand,                             |  6·00 ...  8·20   |  3·00 ...  2·94   |
|                                   +-------------------+-------------------+
|                                   |100·00 ...100·00   |100·00 ...100·00   |
| Ammonia,                          |  0·11 ...  0·57   |  1·28 ...  1·55   |
+-----------------------------------+-------------------+-------------------+

Superphosphates made from bones alone are generally distinguished by a
large quantity of ammonia, and a rather low per centage of biphosphate
of lime. This is owing to the difficulty experienced in making the acid
react in a satisfactory manner on bones, the phosphates being protected
from its action by the large quantity of animal matter which, when
moistened, swells up, fills the pores, and prevents the ready access of
the acid to the interior of the fragments. Superphosphates from
bone-ash, on the other hand, contain a mere trifle of ammonia, and when
well made a very large quantity of biphosphate of lime. Their quality
differs very greatly, and depends, of course, on that of the bone-ash
employed, which can rarely be obtained of quality sufficient to yield
more than 30 or 35 per cent of soluble phosphates. Coprolites are seldom
used alone for the manufacture of superphosphates, but are generally
mixed with bone-ash and bone dust. Mixtures containing salts of ammonia,
flesh, blood, etc., are also largely manufactured, and some are now
produced containing as much as four or five per cent of ammonia, and the
consumption of such articles is largely increasing.

The analyses above given are all those of good superphosphates, in which
abundance of acid has been used so as to convert a large proportion of
insoluble into soluble phosphates; but there are many samples of very
inferior quality to be met with in the market, in which the proportion
of acid has been reduced, and the quantity of phosphates made soluble is
consequently much lower than it ought to be. The following analyses
illustrate the composition of such manures, which are all very inferior
and generally worth much less than the price asked for them.

Water                                   21·60           5·37          7·19
Organic matter and ammoniacal
    salts,                              11·62          13·91          8·80
Biphosphate of lime                      2·98           2·02          6·42
Equivalent to soluble
    phosphates                          (4·65)         (3·15)       (10·02)
Insoluble phosphates                    25·70          15·80         14·03
Sulphate of lime                        23·66          47·52         51·93
Alkaline salts                          10·70           3·73          3·43
Sand                                     3·80          11·65          8·20
                                       ------         ------        ------
                                       100·00         100·00        100·00
Ammonia,                                 1·32           0·59          0·33

The deliberate adulteration of superphosphate, that is, the addition to
it of sand or similar worthless materials, I believe to be but little
practised. The most common fraud consists in selling as pure dissolved
bones, articles made in part, and sometimes almost entirely, from
coprolites. Occasionally refuse matters are used, but less with the
intention of actually diminishing the value of the manure as for the
purpose of acting as driers. It is said that sulphate of lime is
sometimes employed for this purpose, but this is rarely done, because
that substance is always a necessary constituent of superphosphate in
very large quantities; and as farmers look upon it with great suspicion,
all the efforts of the manufacturers are directed towards reducing its
quantity as much as possible. It is very commonly supposed by farmers
that the sulphate of lime found in so large quantity in all
superphosphates, and often amounting to as much as fifty per cent, has
been added to the materials in the process of manufacture, but this is a
mistake; it is a necessary and inevitable product of the chemical action
by which the phosphates are rendered soluble, although its quantity
depends on the materials from which the manure is made. When pure bones
are used its quantity is small, and it does not greatly exceed twice
that of the biphosphate of lime; but in a manure made from coprolites,
or other substances containing a large proportion of carbonate of lime,
which must in the process of manufacture be converted into sulphate, it
may be four or five times as much.

Although there is no manure which varies more in quality, or requires
greater vigilance on the part of the purchaser, in order to obtain a
good article, there is no doubt that superphosphates, owing to the
process of manufacture being better understood, and to increased
competition, have considerably improved in quality. Six or eight years
since a manure containing thirty per cent of phosphates, of which twelve
or fifteen had been converted into biphosphate, was considered a fair
sample, but now the proportion rendered soluble is greatly increased;
and where bone ash alone is employed, as much as thirty and even forty
per cent of soluble phosphates is occasionally found. This, of course,
is an exceptional case, and great attention and care in the selection of
materials are necessary to obtain so large a proportion. The analyses
already given will shew the farmer what he has to expect in good
superphosphates, but it is very necessary that he should take care to
obtain from the manufacturer a manure equal to the guarantee; and he
ought to bear in mind that, owing to the difficulty of getting materials
of constant composition, variations often take place to a considerable
extent in manures which are supposed to be made in exactly the same
manner.

_Phospho-Peruvian Guano._--Under this name a kind of superphosphate,
which is understood to be made by dissolving a native "rock guano," has
recently attracted considerable attention, and is used to a large
extent. Its composition is--

Water                                                   9·54
Organic matter                                         21·38
Biphosphate of lime, equivalent to 25·22 soluble
   phosphates                                          16·81
Insoluble phosphates                                   10·88
Sulphate of lime                                       37·21
Alkaline salts, containing 1·32 of phosphoric acid,
   and equivalent to 2·86 soluble phosphates            2·22
Sand                                                    1·81
                                                      ------
                                                      100·00
Ammonia,                                                3·50

It is chiefly distinguished by the large proportion of valuable
ingredients it contains, and the care taken to secure uniformity of
composition.

A variety of substances are sold under the name of nitrophosphate,
potato manure, cereal manure, etc. etc., which are all superphosphates,
differing only in the proportion of their ingredients, and in the
addition of small quantities of alkaline salts, sulphate of magnesia,
and other substances, but they present little difference from ordinary
superphosphates in their effects.

The use of superphosphate has greatly extended of late years, and its
consumption has increased in a greatly more rapid ratio than that of
guano or any other manure. Ten or twelve years since it was
comparatively little known, but it has now come to be used in many cases
in which Peruvian guano was formerly employed. It produces a better
effect than that manure on light soils, although in general a mixture of
the two answers better than either separately. When Peruvian guano is
to be applied along with it, the farmer will naturally select a
superphosphate made from bone ash, and containing the largest obtainable
quantity of soluble phosphates; but when it is to be used alone, it is
advisable to take one made from bones, or at all events one containing a
considerable quantity of nitrogenous matter or ammonia. The kind to be
selected must, however, be greatly dependent on the particular soil, and
the situation in which it is to be used.

_Lime._--Lime is by far the most important of the mineral manures, and
an almost indispensable agent of agricultural improvement. It has been
used as chalk, marl, shell and coral sand, ground limestone, and as
quick and slaked lime, and its action varies according as it is applied
in any of its natural forms, or after being burnt. In all of its native
forms the lime is combined with carbonic acid in the proportion of
fifty-six parts of lime to forty-four of carbonic acid, and the
carbonate is generally mixed with variable quantities of earthy
ingredients, which in some instances are important additions to it, and
affect its utility as a manure.

_Chalk_ is a very pure form of carbonate of lime, and where it abounds
has been largely employed as an application on the soil. It is dug out
of pits and exposed to the action of the winter's frost, by which it is
thoroughly disintegrated, and in spring it is applied in quantities,
which, in many instances, are only limited by the question of cost.

_Marl_ is a name given to a mixture of finely-divided carbonate of lime,
with variable proportions of clay and siliceous matters, which is found
at the bottom of valleys and in hollow places in beds often of
considerable extent and thickness, where it is deposited from the waters
of lakes holding lime in solution, fed by streams passing over
limestone, or rocks rich in lime. The composition of marls differs
greatly in different districts, and they have been divided into true
marls, and clay marls, according as the carbonate of lime or clay is the
preponderating ingredient. The following table illustrates the
composition of different varieties:--

+---------------------------+------------+-----------+-----------+-------------
|                           | Barbadoes. | Luneburg. | Ayrshire. | Wesermarsh.
+---------------------------+------------+-----------+-----------+-------------
| Carbonate of lime         |   93·2     |   85·4    |   8·4     |    8·2
| Carbonate of magnesia     |    ...     |    1·3    |   ...     |    3·0
| Sulphate of lime          |    ...     |    0·1    |   ...     |    0·5
| Phosphate of lime         |    0·1     |    2·3    |   ...     |    1·2
| Alumina and oxide of iron |    1·6     |    4·6    |   2·2     |    7·2
| Alkaline salts            |    ...     |    0·1    |   ...     |    1·0
| Silica and clay           |    4·6     |    5·6    |  84·9     |   78·9
| Organic matter            |    0·5     |    0·6    |   2·8     |    ...
| Water                     |    ...     |    ...    |   1·4     |    ...
+---------------------------+------------+-----------+-----------+-------------
|                           | 100·00     | 100·00    |  99·7     | 100·00
+---------------------------+------------+-----------+-----------+-------------

The true marls, that is those in which carbonate of lime abounds, are
greatly preferable to clay marls, the latter, indeed, operate chiefly
mechanically, by altering the texture of the soil--the lime they contain
being frequently too small to exercise much appreciable effect.

Shell and coral sands consist chiefly of fragments of shells and coral
disintegrated by the action of the waves, and mixed with more or less
siliceous sand, and containing small quantities of phosphate of lime.
They occur to a considerable extent both on our own coasts and those of
France, and have been used with good effect on some descriptions of
soil.

The general composition of limestones has been already adverted to,
when treating of the origin of soils, and a distinction drawn between
the common limestones and dolomite or magnesium limestone. Few
limestones can be considered as even approaching to purity, and they
almost all contain a small quantity of carbonate of magnesia as well as
earthy matters, and occasionally a little phosphate of lime. In good
specimens the quantities of these substances are generally small, and
they usually contain about half their weight of lime. When limestone is
burnt in the kiln, the change which ensues consists in the expulsion of
the carbonic acid, and the consequent conversion of the lime into the
uncombined or quick state. If water be thrown upon it when in this
condition, it becomes hot, swells up, and falls to a fine soft powder,
and has then entered into combination with water. If it be exposed to
the air, the same action takes place, although, of course, more slowly;
and if it be left for a sufficient time, it at length absorbs carbonic
acid, and reverts to its original form of carbonate of lime, although
now in a state of very fine division.

While lime may be applied in the state of carbonate, either as chalk,
marl, or pounded limestone, and with a certain amount of advantage, much
greater effects are obtained from the use of lime itself in the quick or
slaked state. These advantages are dependent partly on the mechanical
effect of the burning and slaking, which enable us to reduce the lime to
a much more minute state of division, and consequently to incorporate it
more uniformly and thoroughly with the soil, and partly on the more
powerful chemical action which it exists when in the quick or caustic
state. Other minor advantages are also secured, such as the production
of a certain quantity of sulphate of lime, produced by the oxidation of
the sulphur of the coal used in burning, etc., which, though
comparatively trifling, may, under particular circumstances and in some
soils, be of considerable importance.

The action of lime is of a complicated character. Where the soil is
deficient in lime, it must necessarily act by supplying that substance
to the plants growing in it. But this is manifestly a very subordinate
part of its action,--_1st_, Because no soil exists which does not
contain lime in sufficient quantity to supply that element to the
plants. _2d_, Because its effects are not restricted to those soils in
which it exists naturally in small quantity; and, _3d_, Because it is
found that a small application, such as would suffice for the wants of
the crops, is not sufficient to produce its best effects.

It is a familiar fact that the quantity of lime applied to the soil for
agricultural purposes is very large, as much as ten, and even twenty
tons per acre having been used, while the smallest application is
exceedingly large when compared with the mere requirements of the crops.
Of late years the very large applications once in use have become less
common, as it has been found preferable to employ smaller doses more
frequently repeated. The quantity used depends, however, to a great
extent, on the nature and condition of the soil, heavy clays, especially
if undrained, and soils of a peaty nature, requiring a large
application; while on well drained and light soils a smaller quantity
suffices. Thin soils also require only a small application. The
geological origin of the soil is also not without its influence, and its
beneficial effect is peculiarly seen on granite, porphyry, and gneiss
soils, both because these are naturally deficient in lime, and because
the decompositions by which their valuable constituents are liberated
take place with extreme slowness.

The greater part of the action of lime is unquestionably dependent on
its exerting a chemical decomposition on the soil; and it acts equally
on both the great divisions of its constituents, the inorganic and the
organic. On the former, it operates by decomposing the silicates, which
form the main part of the soil, and the alkalies they contain being thus
set free, a larger supply becomes available to the plant. On the organic
constituents its effects are principally expended in promoting the
decomposition which converts their nitrogen into ammonia; and thus a
supply of food, which might remain for a long period locked up, is set
free in a state in which the plant can at once absorb it. But these
chemical decompositions are attended by a corresponding change in the
mechanical characters of the soil. Heavy clays are observed to become
lighter and more open in their texture; and those which are too rich in
organic matter have it rapidly reduced in quantity, and the excessive
lightness which it occasions diminished.

The effects of an application of lime are not generally observed
immediately, but become apparent in the course of one or two years, when
it has had time to exert its chemical influence on the soil; but from
that time its effects are seen gradually to diminish and finally to
cease entirely. The period within which this occurs necessarily varies
with the amount of the application and the nature of the soil, but it
may be said generally that lime will last from ten to fifteen years. The
cessation of its effects is due to several circumstances, partly of
course to the absorption of lime by the plants, partly to its being
washed out of the soil by the rains, and partly to its tendency to sink
to a lower level, a tendency which most practical men have had
opportunities of observing. In the latter case, deep-ploughing often
produces a marked effect, and sometimes makes it possible to postpone
for a year or two the reapplication of lime. All these circumstances
have their influence in bringing its action to an end, but the most
important is, that after a time it has exhausted its decomposing effect
on the soil, having destroyed all the organic matter, or liberated all
the insoluble mineral substances which the quantity added is competent
to do, and so the soil passes back to its old state. It does even more,
for unless active measures are taken to sustain it by other means, it is
found that the fertility of the soil is apt to become less than it was
before the use of lime. And that it should be so is manifest, if we
consider that the lime added has liberated a quantity of inorganic
matter, which, in the natural state of the soil, would have become
slowly available to the plant, and that it must have acted chiefly in
those very portions which, from having already undergone a partial
decomposition, were ready to pass into a state fitted for absorption,
and thus as it were must have anticipated the supplies of future years.
This effect has been frequently observed by farmers, and is indeed so
common, that it has passed into a proverbial saying, that "lime enriches
the fathers and impoverishes the sons." But this is true only when the
soil is stinted of other manures, for when it is well manured the
exhausting effect of lime is not observed; and it must be laid down as a
practical rule, that its use necessitates a liberal treatment of the
soil in all other respects. But when lime has been once employed it
becomes almost necessary to resort to it again; and generally so soon as
its effects are exhausted a new quantity is applied, not so large as
that which is used when the soil is first limed, but still considerable.
When this is done very frequently, however, bad effects ensue; the soil
gets into a particular state, in which it is so open that the grain
crops become uncertain, and such land is said, in practical language, to
be overlimed. The explanation of this state of matters commonly assumed
by those unacquainted with chemistry is, that the land has become too
full of lime; but a moment's consideration of the very small fraction of
the soil which even the largest application of lime forms, will serve to
shew that this cannot be the cause. Ten tons of lime per acre amounts to
only one per cent of the soil, and as a considerable part of the lime is
carried off by drainage in the course of years, it is obvious that even
very large and frequently repeated doses are not likely to produce any
great accumulation of that substance. In point of fact, analyses of
overlimed soils have proved that the lime does not exceed the ordinary
quantity found in fertile land. The explanation of the phenomenon is
probably to be found in the rapid decomposition of organic matter by the
lime, and its escape as carbonic acid, by which the soil is left in that
curious porous condition so well known in practice. The cure for
overliming is found to be the employment of such means as consolidate
the soil, such as eating off with sheep, rolling, or laying down to
permanent pasture.

The immediate effect of lime on the vegetation of the land to which it
is applied is very striking. It immediately destroys all sorts of moss,
makes a tender herbage spring up, and eradicates a number of weeds. It
improves the quantity and quality of most crops, and causes them to
arrive more rapidly at maturity. The extent to which it produces these
effects is dependent on the form in which it is applied. When the lime
is used hot, that is, immediately after it has been slaked, they are
produced most rapidly and effectually; but if it has been so long
exposed to the air as to absorb much of the carbonic acid it lost in
burning, and has got into what is commonly called the mild state, it
operates more slowly; and when it is applied as chalk, marl, or pounded
limestone, its action is still more tardy. Various circumstances, which
must depend upon very different considerations, must necessarily
influence the farmer in the selection of one or other of these different
forms of lime; but on the whole, it will be found that the greatest
advantages are on the side of the well-burned and freshly slaked lime.
The consideration of all the minutiæ to be attended to, however, would
carry us far beyond the limits of this work, and trench to some extent
on the subject of practical agriculture.

Various kinds of refuse matters containing lime have been used in
agriculture, but they are generally inferior to good lime, and not
generally more economical. The most important of these is gas lime, or
lime which has been used for purifying coal gas. In going through this
process it absorbs carbonic acid from the gas, and consequently passes
back, more or less, completely into the form of carbonate of lime. But
it also takes up sulphur, which remains in it in the form of sulphuret
of calcium. It is well known that all sulphurets are prejudicial to
vegetable life, and hence, when fresh gas lime is used, its effects are
often injurious rather than beneficial. But if it be exposed for some
time to the air, oxygen is absorbed, the sulphur is converted into
sulphuric acid, gypsum is produced to the extent of some per cent, and
the lime then becomes innocuous. When composted with dry soil, the
admission of air into the interior of the lime is facilitated, and this
change takes place with greater rapidity. The waste lime from
bleach-works, tanneries, and other manufactories, is occasionally used
by farmers; but unless obtained at a nominal price, it cannot compete
with good quick lime, owing to the large amount of water it contains,
and the consequent increase in the cost of carriage.

_Sulphate of Lime or Gypsum._--Gypsum has been extensively used as a
manure, and is found to exert a very remarkable influence upon clover,
and leguminous crops generally. It is employed in quantities varying
from two cwt. per acre up to a very large quantity, and almost
invariably with good results, in some instances even with the production
of double crops. Much speculation has taken place as to the cause of
this action which is so specific in its character, and from Sir Humphrey
Davy down to the present time, many chemists and agriculturists have
considered the matter. Sir Humphrey Davy attributed its action to its
supplying sulphur to those plants which, according to him, contain an
unusually large quantity of that element. That opinion has been since
entertained by others, but it can scarcely be considered as well
founded, for the more accurate experiments recently made do not point to
any conspicuous differences between the quantities of sulphur contained
in these and other plants. It is, moreover, to gypsum alone that these
effects are due, and if it were merely as a source of sulphur that it
was employed, there are other salts which could be equally, perhaps more
advantageously, used; such, for instance, as sulphate of soda. Others
have attributed its action to its power of fixing ammonia, but this
explanation is certainly untenable, for the soil itself possesses this
property very powerfully, and it is inconceivable that the addition of a
few hundred weights of gypsum should have any effect in promoting this
action. The experiments which have been made with gypsum leave no doubt
as to its effect, more especially on leguminous plants, but they do not
afford an explanation of its mode of action, for which further
inquiries, directed especially to that object, are required.

The application of gypsum to the soil appears to have diminished of late
years, and this is probably due to the large consumption of
superphosphates, and other manufactured manures, which contain it in
abundance. In an ordinary application of these substances, there are
contained from one to two hundredweight of gypsum; and it is not likely
that when they have been extensively used, much benefit will be derived
from a further application of it by itself.




CHAPTER XII.

THE VALUATION OF MANURES.


The determination of the value of a manure is in many respects a
commercial rather than a chemical question, but as it must be founded on
the analysis, and presents some peculiarities dependent on the
complicated nature of the substances to be valued, it has fallen to some
extent into the hands of the chemist. The principle on which the value
of any commercial sample is estimated is very simple. It is only
necessary to know the price of the pure article, and that of the
particular sample to be valued is obtained by making a deduction from
this price proportionate to the per centage of impurities shewn by the
analysis. Thus, for example, if pure sulphate of ammonia sells at £16
per ton, a sample containing 10 per cent of impurities ought to be
purchased for £14: 8s., and so on for any other quantity. This system
which answers perfectly with sulphate of ammonia, nitrate of soda, or
any other substance whose value depends on one individual element, is
inapplicable in the case of complex manures, such as guano and the like,
in which several factors combine to make up the value. In such cases,
manures of very different composition may have the same value, the
deficiency in one particular element being counterbalanced by the excess
of another. Hence it becomes necessary to obtain an estimate of the
value of each factor, from which that not only of one particular
substance, but of every possible mixture may be determined.

When we come to inquire minutely into this question, it appears that the
commercial value of any substance is not estimated solely by
considerations of composition, but is dependent to a great extent on
questions of demand and supply, and applicability to particular
purposes. Thus coprolites containing from 55 to 60 per cent of
phosphates sell at about £2: 12s. per ton, while bone-ash containing the
same quantity of that ingredient brings about twice as much; in other
words, phosphates are nearly twice as valuable in bone-ash as in
coprolites, and as a phosphatic guano their price is generally still
higher; and the reason for this is obvious, in bones and guano the
phosphates are in a high state of division, in which they are easily
attacked and disintegrated by the carbonic acid of the soil, and
rendered available to plants; while in coprolites they are in a hard and
compact form, and are of little use unless they have previously
undergone an expensive preparation. In the same way, if the market price
of different kinds of guano be inquired into, very great differences are
found to exist in the rate at which phosphates are sold, and this is
attributable in part to the fact that the price at which any article is
charged commercially, is such as to cover the prime cost, expense of
freight, and other charges, and to leave a profit to the importer; and
partly, also, no doubt, to the carelessness with which manures are often
purchased, and to the want of careful field experiments in which the
effects produced by them are properly compared. It will be readily
understood that the state of division of any substance, the readiness
with which its constituents can be rendered available to the plants,
care of application, and many other circumstances must influence its
price; but making due allowance for these, differences are met with
which appear to some extent to be merely the result of caprice. It is
easy to understand why bone-ash should sell at double the price of
coprolites, but no good reason can be shewn why the phosphates in one
kind of guano should be sold at a much higher price than another, and
the difference would probably disappear if greater attention were paid
to the results of field experiments.

However great and inexplicable these differences may be, it is not the
business of the valuator of a manure to discuss them. On the contrary,
he is bound to accept them as the basis of his calculation, and to
endeavour to deduce from them a proper system of estimation for each
substance. Strictly speaking, each individual manure ought to be valued
according to a plan special to itself, and deduced from its own standard
market price; but it is obvious that this would lead to innumerable
complications and defeat its own ends, and hence an attempt has been
made to contrive a general system suited to all manures, and which,
though not absolutely correct, is a sufficient approximation for all
practical purposes, and a tolerably accurate guide to the determination
of their relative values.

The constituents of a manure which are of actual value are ammonia,
insoluble phosphates, biphosphate of lime (soluble phosphates), sulphate
of lime, nitric acid (as nitrate of soda), potash, soda, and organic
matter. These substances differ greatly in value. Ammonia and
phosphates, soluble and insoluble, are costly; and by far the larger
part of the value of all guanos, and the common manufactured manures,
depends on them. Nitric acid and potash are also very valuable
substances, but as they are rarely found in manufactured manures, and
never in sufficient quantity to exert any material influence in their
price, it is not usual to take them into consideration except in
particular cases. The alkali which commonly exists in artificial manures
is soda, and when alkaline salts appear in any analysis, they must be
assumed to consist almost entirely of that substance generally in the
form of common salt, and be valued accordingly. Sulphate of lime and
organic matter though abundant constituents of most manures, add but
little to their value, and it is a moot point whether they ought to be
taken into consideration, although most persons allow a small value for
them. Carbonate of lime, sand, or siliceous matter, and water, of
course, are altogether worthless.

In order to obtain the value of a manure containing several of these
substances, it is necessary to ascertain the average commercial price of
each individually. This is easily done when they are met with in
commerce separately, or at least mixed only with worthless substances,
but some of them are only found in complex mixtures, and in these cases
it is necessary to arrive at a result by an indirect process, according
to methods which will be immediately explained. The question to be
solved is the price actually paid for a ton of each substance in a pure
state, and we shall proceed to consider them in succession.

_Insoluble Phosphates._--These are purchased alone, chiefly in the form
of coprolites and bone-ash, or the spent animal charcoal of the sugar
refiners. Ground coprolites, containing about 58 per cent of phosphates,
sell at £2: 12s. per ton, which is at the rate of £4: 8s. for pure
phosphates. Bone-ash varies considerably in price, but of late samples
containing 70 per cent of phosphates have sold as low as £4: 10s. per
ton, and consequently pure phosphates in this form are worth £6: 8s. per
ton. Although these are the only forms in which phosphates are
purchased alone, it is possible to determine the price at which they are
sold in bones and phosphatic guanos, by first deducting the value of the
ammonia they contain, and assuming the remainder to represent the price
paid for the phosphates. In this way we find the following values for
insoluble phosphates:--

In Coprolites          £4 10  0
   Bone-ash             6  8  0
   Bones                7  5  0
   Phosphatic guanos   10  0  0

It is to be observed that these are actual prices, and they are liable
to fluctuate with the state of the market, although they are pretty fair
averages. It is important to notice how much they vary in the different
forms; the farmer who buys a phosphatic guano paying for phosphates a
much higher price than he could have obtained those for in other
substances--a difference which must be attributed to the high state of
division in which they exist in the guano. We do not here enter upon the
question how far this difference in price is justified; we are content
with the fact that it exists, and we are compelled to estimate the value
of phosphates in a phosphatic guano at the price given above, although
in Peruvian guano they are sold at a lower rate. For all other manures,
of which bones and bone-ash form the basis, £7 may be taken as a fair
price, and it is that usually adopted, though £8 and £10 have sometimes
been assumed as the average.

_Ammonia_ is met with in commerce as muriate and sulphate of ammonia.
The former, owing to its high price, is practically excluded from use as
a manure; the latter sells at present at from £15 to £15: 10s. per ton,
and, making allowance for the usual amount of impurity (5 or 6 per
cent), the actual ammonia is worth about £63 per ton. Calculating from
other substances it appears that ammonia is worth, per ton, in--

Sulphate of ammonia    £63 0 0
Bones                   61 0 0
Peruvian guano          57 0 0

the average being £60, which is the price usually adopted.

_Sulphate of Lime_ and _Alkaline Salts_ (consisting chiefly of soda) are
generally estimated at £l per ton; and potash in those cases, in which
it is necessary to take it into account, is usually valued at from £20
to £30 per ton, the former being its value in kelp, the form in which it
can be most cheaply purchased.

_Nitrate of Soda_ is usually sold at from £15 to £15: 10s. per ton, and,
making allowance for impurities, £16 may be taken as the value of the
pure salt.

_Biphosphate of Lime, Soluble Phosphates._--Considerable difficulty is
experienced in estimating the value of these substances, because they
are not met with in commerce alone, or in any form except that of
superphosphate, and the prices at which they are sold in different
samples of that manure differ excessively. The only course by which any
result can be obtained, is to determine the average price of a good
superphosphate, and putting the values already ascertained on all the
other constituents to reckon the difference between that sum and the
market price as the value of soluble phosphates. Throwing out, as
inferior, all samples containing less than 10 per cent of soluble
phosphates, and taking the good only, I find that the average
composition of the phosphates in the market during the present year has
been--

Water                                    10·71
Organic matter                            9·33
Biphosphate of lime equivalent to 19·43
    "soluble phosphates"                 12·45
Insoluble phosphates                     14·78
Sulphate of lime                         45·24
Alkaline salts                            2·11
Sand                                      5·38
                                        ------
                                        100·00
Ammonia                                   1·71

It is more difficult to fix the average price of superphosphate, as in
many cases no information could be obtained on this point; but among
those analyzed were samples at all prices, from £7 up to £10: 10s. per
ton, so that on the whole, £8 may be assumed as an average, and in that
case soluble phosphates are worth £27: 19s. per ton. Had the inferior
samples been included, the price would have been higher, and in fact the
rate at which soluble phosphates have been commonly estimated is £30 per
ton, or £46: 16s. for biphosphate of lime, although sometimes the former
have been reckoned as low as £25, with a corresponding rate for the
latter. It is important that biphosphate of lime and soluble phosphates
should not be confounded with one another in valuing a manure, the
latter having one and a half times the value of the former.

As manures are liable to considerable fluctuations in price, the value
attached to each of their constituents ought to be varied with the state
of the market; but it is obviously impossible for the farmer to watch
the changes in price with such minuteness as to enable him to do this,
and it is much more convenient, as well as safer, to adopt a fixed
average, which can be used with reasonable accuracy at all times. The
fact is, that this system of valuation is only an approximation to the
truth; and if absolute accuracy were aimed at, it would be necessary to
vary the estimates, not only at different times, but at different
localities at the same time, and to some extent also according to the
kind of manure. The price of soluble phosphates more especially,
fluctuates to a great extent, being practically fixed by each
manufacturer according to the facilities which his position or command
of raw material offer for producing them at a low rate. We thus find
that when made from bones alone, the cost of that substance is not
unfrequently as high as £40 per ton, and when bone-ash alone is used it
is sometimes as low as £20. Such extreme differences, of course, cannot
be taken into account in the system of valuation adopted, where all that
can be done is to take average values, which, when applied to average
samples, ought to bring out their value.

The data which have already been given regarding the price of the
individual constituents of manures can be applied to the determination
of the value of any mixture in two different ways by means of the
subjoined table:--

+----------------------------+-----------------+-------------------+
|                            | Price per Ton.  | Per cent per Ton. |
+----------------------------+-----------------+-------------------+
| Ammonia                    | £60  0  0       | £0 12  0          |
| Insoluble phosphates       |   7  0  0       |  0  1  5          |
|   Do. in phosphatic guanos |  10  0  0       |  0  2  0          |
| Soluble phosphates         |  30  0  0       |  0  6  0          |
| Biphosphate of lime        |  46 16  0       |  0  9  4-1/2      |
| Alkaline salts             |   1  0  0       |  0  0  2-4/10     |
| Sulphate of lime           |   1  0  0       |  0  0  2-4/10     |
| Potash                     |  20  0  0       |  0  4  0          |
| Nitrate of soda            |  16  0  0       |  0  3  2-1/2      |
| Organic matter             |   0 10  0       |  0  0  1-1/4      |
+----------------------------+-----------------+-------------------+

Supposing it be desired to calculate the value of a manure by the first
column, it is obvious that if we suppose 100 tons to be purchased, the
per centages of the different constituents shewn in the analysis will
give the number of tons of each contained in 100 tons of the mixture,
and, selecting the analysis of the superphosphate given in a previous
page, we proceed in the calculation as follows:--

14·11 tons of organic matter at 10s.   £7  0  0
14·86 " soluble phosphates at £30     446  0  0
15·13 " insoluble phosphates at £7    105  0  0
39·43 " sulphate of lime at £1         39  0  0
 3·82 " alkaline salts at £1            4  0  0
 2·10 " ammonia at £60                126  0  0
                                     ----------
          Value of 100 tons          £727  0  0
or £7 : 5s. per ton.

According to the second column, the numbers give the sum by which the
per centages of each ingredient must be multiplied, to give its value
in a ton of manure, and it is used for the same manure in the following
manner:--

14·11 organic matter, multiplied by      1-1/4d.       £0  1  5
14·88 soluble phosphates  "          6s.                4  9  2
15·13 insoluble phosphates "         1s. 5d.            1  1  4
39·43 sulphate of lime    "              2-4/10d.       O  8 10
 3·82 alkaline salts     "               2-4/10d.       O  O  9
 2·10 ammonia       "               12s.                1  5  3
                                                       --------
                    Value per ton                      £7  6  9

The difference is due to the less minute calculation of fractional
quantities in the latter case.

The calculation of the value of any other manure is effected in exactly
the same manner, taking care, however, to use the higher value for
phosphates in the case of a phosphatic guano. It will be obvious to
every one who tries the two methods that the first greatly exceeds the
second in convenience and simplicity in the calculations, and it is that
most commonly in use, although some persons prefer the second.

Although the data just given must always form the basis of the valuation
of any manure, there are a variety of other circumstances which must be
taken into account, and which give great scope for the judgment and
experience of the valuator. Of these the most important is the proper
admixture of the ingredients, and the condition of the manure as regards
dryness, complete reduction to the pulverulent state, and the like. A
certain allowance ought always to be made for careful manufacture; and,
on the other hand, where the manure is damp or ill reduced, a small
deduction (the amount of which must be decided by the experience of the
valuator) ought to be made on account of the risk which the farmer runs
of loss from unequal distribution, and the extra cost of carriage of an
unnecessary quantity of water.

It is also necessary to take into account the particular element
required by the soil. Thus, a farmer who finds his soil wants
phosphates, will look to the manure containing the largest quantity of
that substance, and possibly not requiring ammonia, will not care to
estimate at its full value any quantity of that substance which he may
be compelled to take along with the former, but will look only to the
source from which he can obtain it most cheaply. It may be well,
therefore, to point out that ammonia is most cheaply purchased in
Peruvian guano; insoluble phosphates in coprolites; and soluble
phosphates in superphosphates, made from bone-ash alone. In general,
however, it will be found most advantageous to select manures in which
the constituents are properly adjusted to one another, so that neither
ammonia, soluble nor insoluble phosphates, preponderate; but, of course,
it must frequently happen that it will prove more economical to buy the
substances separately and to make the mixture, than to take the manure
in which they are ready mixed.

In judging of the value of any manure, it is also important to make sure
that the analysis which forms the basis of the calculation is that of a
fair sample, which correctly represents the bulk actually delivered to
the purchaser, and not one which has been made to do duty for an
unlimited quantity of manure, which is supposed to be all of equal
quality, as often happens in the hands of careless manufacturers, and
too great attention cannot be devoted to the selection of the sample,
which is very often done in an exceedingly slovenly manner.




CHAPTER XIII.

THE ROTATION OF CROPS.


Reference has already been more than once made to the fact that a crop
growing in any soil must necessarily exhaust it to a greater or less
extent by withdrawing from it a certain quantity of the elements to
which its fertility is due. That this is the case has been long admitted
in practice, and it has also been established that the exhausting
effects of different species of plants are very different; that while
some rapidly impoverish the soil, others may be cultivated for a number
of years without material injury, and some even _apparently_ improve it.
Thus, it is a notorious fact that white crops exhaust, while grass
improves the soil; but the improvement in the latter case is really
dependent on the fact, that when the land is laid down in pasture,
nothing is removed from it, the cattle which feed on its produce
restoring all but a minute fraction of the mineral matters contained in
their food; and as the plants derive a part, and in some instances a
very large part, of their organic constituents from the air, the
fertility of the soil must manifestly be increased, or at all events
maintained in its previous state. When, however, the plant, or any
portion of it, is removed from the soil, there must be a reduction of
fertility dependent on the quantity of valuable matters withdrawn by it;
and thus it happens that when a plant has grown on any soil, and has
removed from it a large quantity of nutritive matters, it becomes
incapable of producing an equally large crop of the same species; and if
the attempt be made to grow it in successive years, the land becomes
incapable of producing it at all, and is then said to be thoroughly
exhausted. But if the exhausted land be allowed to lie for some time
without a crop, it regains its fertility more or less rapidly according
to circumstances, and again produces the same plant in remunerative
quantity. The observation of this fact led to the introduction of naked
fallows, which, up to a comparatively recent period, were an essential
feature in agriculture. But after a time it was observed that the land
which had been exhausted by successive crops of one species was not
absolutely barren, but was still capable of producing a luxuriant growth
of other plants. Thus peas, beans, clover, or potatoes, could be
cultivated with success on land which would no longer sustain a crop of
grain, and these plants came into use in place of the naked fallow under
the name of fallow crops. On this was founded the rotation of crops; for
it was clear that a judicious interchange of the plants grown might
enable the soil to regain its fertility for one crop at the time when it
was producing another; and when exhausted for the second, it might be
again ready to bear crops of the first.

The necessity for a rotation of crops has been explained in several
ways. The oldest view is that of Decandolle, who founded his theory on
the fact that the plants excrete certain substances from their roots. He
found that when plants are grown in water, a peculiar matter is thrown
off by the roots; and he believed that this extrementitious substance is
eliminated _because_ it is injurious to the plant, and that, remaining
in the soil, it acts as a poison to those of the same species, and so
prevents the growth of another crop. But this excretion, though
poisonous to the plants from which it is excreted, he believed to be
nutritive to those of another species which is thus enabled to grow
luxuriantly where the others failed. Nothing can be more simple than
this explanation, and it was readily embraced at the time it was
propounded and considered fully satisfactory. But when more minutely
examined, it becomes apparent that the facts on which it is founded are
of a very uncertain character. Decandolle's observations regarding the
radical excretions of plants have not been confirmed by subsequent
observers. On the contrary, it has been shewn that though some plants,
when growing in water, do excrete a particular substance in small
quantity, nothing of the sort appears when they are grown in a siliceous
sand. And hence the inference is, that the peculiar excretion of plants
growing in water is to be viewed as the result of the abnormal method of
their growth rather than as a natural product of vegetation. But even
admitting the existence of these matters, it would be impossible to
accept the explanation founded upon them, because it is a familiar fact
that, on some soils, the repeated growth of particular crops is
perfectly possible, as, for instance, on the virgin soils of America,
from which many successive crops of wheat have been taken; and in these
cases the alleged excretion must have taken place without producing any
deleterious effect on the crop. Besides, it is in the last degree
improbable that these excretions, consisting of soluble organic matters,
should remain in the soil without undergoing decomposition, as all
similar substances do; and even if they did, we cannot, with our present
knowledge of the food of plants, admit the possibility of the direct
absorption of any organic substance whatever. Indeed, the idea of
radical excretions, as an explanation of the rotation of crops, must be
considered as being entirely abandoned.

The necessity for a rotation of crops is now generally attributed to the
different quantities of valuable matters which different plants remove
from the soil, and more especially to their mineral constituents. It has
been already observed that great differences exist in the composition of
the ash of different plants in the section on that subject; and it was
stated that a distinction has been made between lime, potash, and silica
plants, according as one or other of these elements preponderate in
their ashes. The remarkable difference in the proportion of these
elements has been supposed to afford an explanation of rotation. It is
supposed that if a plant requiring a large quantity of any one element,
potash, for example, be grown during a succession of years on the same
soil, it will sooner or later exhaust all, or nearly all, the potash
that soil contains in an _available_ form, and it will consequently
cease to produce a luxuriant crop. But if this plant be replaced by
another which requires only a small quantity of potash and a large
quantity of lime, it will flourish, because it finds what is necessary
to its growth. In the meantime, the changes which are proceeding in the
soil, are liberating new quantities of the inorganic matters from those
forms of combination in which they are not immediately available, and
when after a time the plant which requires potash is again sown on the
soil, it finds a sufficient quantity to serve its purpose. We have
already, in treating of the ashes of plants, pointed out the extent of
the differences which exist; but these will be made more obvious by the
annexed table, giving the quantity of the different mineral matters
contained in the produce of an imperial acre of the different crops.

TABLE shewing the quantities of Mineral Matters and Nitrogen in average
Crops of the principal varieties of Farm Produce.

+---------------+--------------+---------+----------+---------+-------+-------+
|               | Produce per  | Total   | Total    |         |       |       |
|               | Imperial     | Weight  | Mineral  | Potash. | Soda. | Lime. |
|               | Acre.        | in lbs. | Matters. |         |       |       |
+---------------+--------------+---------+----------+---------+-------+-------+
|Wheat--Grain   | 28 bushels   |  1,680  |   34·12  |  10·11  |  1·20 |  1.04 |
|               | at 60 lbs.   |         |          |         |       |       |
|    Straw      | 1 ton 3 cwt. |  2,576  |  114·48  |  20·70  |  2·84 |  8·53 |
|    Total      |   ...        |   ...   |  148·60  |  30·81  |  4·04 |  9·57 |
|               |              |         |          |         |       |       |
|Barley--Grain  | 33 bushels   |  1,749  |   44·24  |   9·40  |  0·30 |  0·76 |
|               | at 53 lbs.   |         |          |         |       |       |
|    Straw      | 18 cwt.      |  2,106  |   99·14  |  11·24  |  1·14 |  5·81 |
|    Total      |   ...        |   ...   |  143·38  |  20·64  |  1·44 |  6·57 |
|               |              |         |          |         |       |       |
|Oats--Grain    | 34 bushels   |  1,360  |   48.89  |  11·00  |   ... |  5·31 |
|               | at 40 lbs.   |         |          |         |       |       |
|    Straw      | 1 ton.       |  2,240  |  143·53  |  30·71  |  6·10 | 10·29 |
|    Total      |    ...       |   ...   |  192·42  |  41·71  |  6·10 | 15·60 |
|               |              |         |          |         |       |       |
|Beans, Peas--  | 25 bushels   |  1,650  |   55·97  |  30·00  |  0·31 |  3·01 |
|    Grain      | at 60 lbs.   |         |          |         |       |       |
|    Straw      | 1 ton.       |  2,240  |  108·51  |  48·61  | 13·14 | 29·37 |
|    Total      |    ...       |   ...   |  164·48  |  78·61  | 13·45 | 32·38 |
|               |              |         |          |         |       |       |
|Turnips--Bulbs | 13-1/2 tons. | 30,240  |  213·75  |  57·35  | 44·71 | 28·60 |
|               |              |         |          |         |       |       |
|Potatoes       | 3 tons.      |  6,720  |   55·58  |  28·92  |  2·85 |  1·20 |
|               |              |         |          |         |       |       |
|Hay            | 2-1/2 tons.  |  5,600  |  391·31  | 129·79  |  4·80 | 35·46 |
+---------------+--------------+---------+----------+---------+-------+-------+

+---------------+-----------+--------+-----------+----------+---------+-------+
|               |           |        |           |          |         |       |
|               | Magnesia. | Chlor. | Sulphuric | Phosphor | Silica. | Nitro |
|               |           |  -ine  | Acid.     | -ic Acid.|         | -gen. |
+---------------+-----------+--------+-----------+----------+---------+-------+
|Wheat--Grain   |    4.80   |   ...  |    0.32   |  16.22   |   0.43  | 29.20 |
|               |           |        |           |          |         |       |
|    Straw      |    2·23   |   ...  |    3·55   |   3·16   |  73·47  | 16·13 |
|    Total      |    7·03   |   ...  |    3·87   |  19·38   |  73·90  | 45·33 |
|               |           |        |           |          |         |       |
|Barley--Grain  |    3·10   |  1·12  |    0·85   |  15·52   |  13·19  | 34·98 |
|               |           |        |           |          |         |       |
|    Straw      |    2·75   |  1·30  |    1·10   |   7·22   |  68·58  |  6·03 |
|    Total      |    5·85   |  2·42  |    1·95   |  22·74   |  81·77  | 41·01 |
|               |           |        |           |          |         |       |
|Oats--Grain    |    4·04   |  0·20  |    ...    |  26·07   |   2·27  | 27·54 |
|               |           |        |           |          |         |       |
|    Straw      |    5·50   |  5·55  |    5·18   |   7·35   |  72·85  | 14·10 |
|    Total      |    9·54   |  5·75  |    5·18   |  33·42   |  75·12  | 41·64 |
|               |           |        |           |          |         |       |
|Beans, Peas--  |    4·00   |   ...  |     1·76  |  16·65   |   0·24  | 46·10 |
|    Grain      |           |        |           |          |         |       |
|    Straw      |    3·74   |  7·00  |    2·07   |   0·74   |   3·84  | 26·88 |
|    Total      |    7·74   |  7·00  |    3·83   |  17·39   |   4·08  | 72·98 |
|               |           |        |           |          |         |       |
|Turnips--Bulbs |    4·65   | 10·35  |   39·02   |  22·57   |   6·50  | 60·48 |
|               |           |        |           |          |         |       |
|Potatoes       |    2·11   |  3·21  |   10·24   |   5·76   |   1·29  | 26·00 |
|               |           |        |           |          |         |       |
|Hay            |    9·62   | 39·61  |   16·57   |  21·79   | 133·67  | 56·22 |
+---------------+-----------+--------+-----------+----------+---------+-------+

The minor constituents, such as oxide of iron, manganese, etc., have
been omitted as being of little importance; and the quantity of
nitrogen, which is of great moment in estimating the exhaustive effects
of various crops, has been added.

In examining this table, it becomes apparent that while in regard to
some of the elements, the quantities removed by different crops do not
differ to any marked extent, in others the variation is very great. The
cereals and grasses are especially distinguished by the larger quantity
of silica they contain, and the exhaustive effect consequent upon the
removal of both grain and straw from soils which contain but a limited
supply of that substance in an available condition is obvious. It is
clear that under such circumstances the frequent repetition of a cereal
crop may so far diminish the amount of available silica as to render its
cultivation impossible, although the other substances may be present in
sufficient quantity to produce a plentiful crop of any plant which does
not require that element. Beans and peas, turnips and hay, on the other
hand, require a very large quantity of alkalies, and especially of
potash.

Looking more minutely, however, into this matter, certain points attract
attention which appear to be at variance with commonly received
opinions. With the exception of silica, for example, the cereals do not
withdraw from the soil so large a quantity of mineral matters as some of
the so-called fallow crops, and if their straw be returned to the soil
they are by far the least exhaustive of all cultivated plants; and we
thus recognise the justice of that practical rule, which lays it down as
an essential point of good husbandry that the straw ought, as far as
possible, to be consumed on the farm on which it is produced. As regards
the general constituents of the ash, it is also to be remarked that
though differences in their proportions exist, they are by no means so
marked as might be expected; thus there are no plants for which a large
quantity of potash, nitrogen, and phosphoric acid is not required; and
it is not very easy to see how the substitution of the one for the other
should be of much importance in this respect. Indeed, the more minutely
the subject is examined, the more do we become convinced of the
insufficiency of that view which attributes the necessity for a rotation
of crops to differences in chemical composition alone. There can be no
doubt that the nature of the plant and the particular mode in which it
gathers its nutriment, have a most important influence. Certain plants
are almost entirely dependent on the soil for their organic
constituents, while others derive a large proportion of them from the
air, and a plant of the latter class will flourish in a soil in which
one of the former is incapable of growing. In other cases, the structure
and distribution of the roots is the cause of the difference. Some
plants have roots distributed near the surface and exhaust the
superficial layer of the soil, others penetrate into the deeper layers,
and not only derive an abundant supply of food from them, but actually
promote the fertility of the surface soil by the refuse portions of them
which are left upon it. Experience has in this respect arrived at
results which tally with theory, and it is for this reason that the
broad-leafed turnip, which obtains a considerable quantity of its
nutriment from the air, alternates with grain crops which are chiefly
dependent on the soil. It is undoubtedly to some such cause that several
remarkable instances of what may be called natural rotations are to be
attributed. It is well known in Sweden that when a pine forest is
felled, a growth, not of pine but of birch, immediately springs up. Now
the difference in composition of the ash of these trees is not
sufficient to explain this fact, and it must clearly be due to some
difference in the distribution of their roots, or the mode in which they
obtain their food.

Whatever weight may be given to these different explanations of
rotation, there is no doubt about the importance of attending to it, and
there are various practical deductions of much importance to be drawn
from the facts with which we are acquainted. Thus it is to be observed
that the quantities of mineral matters withdrawn by plants of the same
class are generally similar, and hence it may be inferred that crops of
the most opposite class ought as much as possible to alternate with one
another, and each plant should be repeated as seldom as possible, so
that, even when it is necessary to return to the same class, a different
member of it should be employed. Thus, for instance, in place of
immediately repeating wheat, when another grain crop is necessary, it
would theoretically be preferable to employ oats or barley, and to
replace the turnip by mangold-wurzel or some other root. It is obvious,
however, that this system cannot be carried out in practice to its full
extent; for the superior value of individual crops causes the more
frequent repetition of those which make the largest return. But
experience has so far concurred with theory that it has taught the
farmer the advantage of long rotations; and we have seen the successive
introduction of the three, four, five, and six-course shift, and even,
in some instances, of longer periods.

Such is the theory of rotation, and while it will always be most
advantageous to adhere to it, it is by no means necessary that this
should be done in an absolutely rigid manner. In the practice of
agriculture, plants are placed in artificial circumstances, and instead
of allowing them to depend entirely on the soil, they are supplied with
a quantity of manure containing all the elements they require, and if it
be used in sufficiently large quantity, the same crop may be grown year
after year. And accordingly the order of rotation, which is
theoretically the best, may be, and every day is, violated in practice,
although this must necessarily be done at the expense of a certain
quantity of the valuable matters of the manure added, and is so far a
practice which ought theoretically to be avoided. In actual practice,
however, the matter is to be decided on other grounds. The object then
is, not to produce the largest crops, but those which make the largest
money return, and thus it may be practically economical to grow a crop
of high commercial value more frequently than is theoretically
advantageous. In such cases the farmer must seek to do away as far as
possible with the disadvantages which such a course entails, and this he
will endeavour to accomplish by careful management and a liberal
treatment of the soil.

But while this system may be adopted to some extent, it must also be
borne in mind that the frequent repetition of some crops cannot be
practised with impunity, for plants are liable to certain diseases which
manifest themselves to the greatest extent when they have been too often
cultivated in the same soil. Clover sickness, which affects the plant
when frequently repeated on light soils, and the potatoe disease and
finger and toe have been attributed to the same cause. Whether this is
the sole origin of these diseases is questionable, but there is no doubt
that they are aggravated by frequent repetition, and hence a strong
argument in favour of rotation. It has been asserted by great
authorities in high farming, that with our present command of manures,
rotations may be done away with; but this is an opinion to which science
gives no countenance, and he would be a rash man who attempted to carry
it out in practice.




CHAPTER XIV.

THE FEEDING OF FARM STOCK.


The feeding of cattle, once a subordinate part of the operations of the
farm, has now become one of its most important departments, and a large
number of minute and elaborate experiments have been made by chemists
and physiologists with the view of determining the principles on which
its successful and economical practice depends. These investigations,
while they have thrown much light on the matter, have by no means
exhausted it, and it will be readily understood that the complete
elucidation of a subject of such complexity, touching on so many of the
most abstruse and difficult problems of chemistry and physiology, and in
which the experiments are liable to be affected by disturbing causes,
dependent on peculiarities of constitution of different animals, cannot
be otherwise than a slow process.

In considering the principles of feeding, it is necessary to point out,
in the first instance, that the plant and animal are composed of the
same chemical elements, hence the food supplied to the latter invariably
contains all the substances it requires for the maintenance of its
functions. And not only is this the case, but these elements are to a
great extent combined together in a similar manner,--the fibrine,
caseine, albumen, and fatty matters contained in animals corresponding
in all respects with the compounds extracted from plants under the same
name; and though the starchy and saccharine substances do not form any
part of the animal body, they are represented in the milk, the food
which nature has provided for the young animal. It has been frequently
assumed that the nitrogenous and fatty matters are simply absorbed into
the animal system, and deposited unchanged in its tissues; but it is
probable that the course of events is not quite so simple, although,
doubtless, the decomposition which occurs is comparatively trifling. The
starchy matters, on the other hand, are completely changed, and devoted
to purposes which will be immediately explained.

It is a matter of familiar experience, that if the food be properly
proportioned to the requirements of the animal, its weight remains
unchanged; and the inference to be drawn from this fact obviously is,
that the food does not remain permanently in the system, but must be
again got rid of. It escapes partly through the lungs, and partly by the
excretions, which do not consist merely of the part which has not been
digested, but also of that portion which has been absorbed, and after
performing its allotted functions within the system, has become effete
and useless. When the weights of the excretions, the carbon contained in
the carbonic acid expired by the lungs and the small quantity of matter
which escapes in the form of perspiration, are added together, they are
found in such a case to be exactly equal to the food. If the animal be
deprived of nutriment, it immediately begins to lose weight, because its
functions must continue--carbon must still be converted into carbonic
acid to maintain respiration--and the excretions be eliminated, although
diminished in quantity, because they no longer contain the undigested
portion of the daily food, and the substances already stored up in the
body are consumed to maintain the functions of life. Universal
experience has shewn that, under such circumstances, the fat which has
accumulated in various parts of the body disappears, and the animal
becomes lean; but it is less generally recognised that the muscular
flesh, that is the lean part of the body, also diminishes, although it
is sufficiently indicated by the fact that nitrogen still continues to
be found in the urine, and that the animal becomes feeble and incapable
of muscular exertion. Respiration and secretion, in fact, proceed quite
irrespective of the food, which is only required to repair the loss they
occasion. When the course of events within the animal body is traced, it
is found to be somewhat as follows: The food consumed is digested and
absorbed into the blood, where it undergoes a series of complicated
changes, as a consequence of which part of it is converted into carbonic
acid, and eliminated by the lungs, and part is deposited in the tissues
as fat and flesh. After the lapse of a certain period, longer or shorter
according to circumstances, a new set of actions comes into play, by
which the complex constituents of the tissues are resolved into simpler
substances, and excreted chiefly by the lungs and kidneys. The changes
thus produced are, to a great extent, identical with those which would
take place if the fat and flesh were consumed in a fire; and the animal
frame may, in a certain sense, be compared to a furnace, in which, by
the daily consumption of a certain quantity of fuel and air inhaled in
the process of respiration, its temperature is maintained above that of
the surrounding atmosphere. If the daily supply of fuel, that is of
food, be properly adjusted to the loss by combustion, the weight of the
animal remains constant; if it be reduced below this quantity, it
diminishes; but if it be increased, the stomach either refuses to
digest and assimilate the excess, or it is absorbed and stored up in the
body, increasing both the fat and flesh.

When an animal is fed in such a manner that its weight remains constant,
a balance is produced between the supply of nutriment contained in the
food and the waste of the tissues, the gain from the former exactly
counterpoising the loss occasioned by the latter. If in this state of
matters an additional supply of food be given, this balance is deranged,
and the nutriment being in excess of the loss, the animal gains weight,
and it continues to do this for some time, until it reaches a point at
which a new balance is established, and its weight again becomes
constant; and this is due to the fact that the animal becomes subject to
an additional waste, consequent on the increased weight of matter
accumulated in its tissues. If, after the animal has attained its new
constant weight, the food be a second time increased, a further gain is
obtained, and so on, with every addition to the supply of nutriment,
until at length a certain point is reached, beyond which its weight
cannot be forced. In fact, each successive increase of weight is
obtained at a greater expenditure of food. If, for example, a lean
animal is taken, and its food increased by a given quantity, it will
rapidly attain a certain additional weight, but if another extra supply
of food be given, the increase due to it will be much more slowly
attained, and so on until at length an additional increase can only be
secured by the long-continued consumption of a very large quantity of
food. The great object of the feeder is to obtain the greatest possible
increase with the smallest expenditure of food, and to know the point
beyond which it is no longer economical to attempt to force the process
of fattening. To do this it is necessary first to consider the
composition of the animal itself, then that of its food, and lastly, the
mode in which it may be most economically used.

It has been already observed that the animal tissues are composed of
albuminous or nitrogenous compounds, fat, mineral matters, and water;
but the proportions of these substances have, until lately, been very
imperfectly known. Water is well known to be by far the largest
constituent, and amounts in general to about two-thirds of the entire
weight, and it has been generally supposed that the nitrogenous matters
stood next in point of abundance, but a most important and elaborate
series of experiments by Messrs. Lawes and Gilbert have shewn that they
are greatly exceeded by the fatty matters. The following table contains
a summary of the composition of ten different animals in different
stages of fattening. The first division gives the composition of the
carcass, that is, the portion of the animal usually consumed as human
food; the second that of the offal, consisting of the parts not usually
employed as food; and the third that of the entire animals, including
the contents of the stomach and intestines:--

[Transcriber's note: Column titles are printed vertical, which is not
possible to do here. Therefore they are replaced with a 2-3 character
code, explained here]

     Column titles:
     MM = Mineral Matter
     NC = Nitrogenous Compounds
     TDS = Total Dry Substance
     CSI = Contents of Stomachs and Intestine in moist state.
     Wat = Water

|---------------------------------------------------------------------------
|                   |                        ||Per cent in Offal, excluding |
|                   | Per cent in Carcass    || contents of Stomachs and    |
|                   |                        ||       Intestines.           |
|                   |------------------------||-----------------------------|
|                   | MM | NC | Fat| TDS| WAT|| MM  | NC  | Fat | TDS | WAT |
|-------------------|----|----|----|----|----||-----|-----|-----|-----|-----|
|Fat Calf           |4·48|16·6|16·6|37·7|62·3|| 3·41| 17·1| 14·6| 35·1| 64·9|
|Half-fat Ox        |5·56|17·8|22·6|46·0|54·0|| 4·05| 20·6| 15·7| 40·4| 59·6|
|Fat Ox             |4·56|15·0|34·8|54·4|45·6|| 3·40| 17·5| 26·3| 47·2| 52·8|
|Fat Lamb           |3·63|10·9|36·9|51·4|48·6|| 2·45| 18·9| 20·1| 41·5| 58·5|
|Store Sheep        |4·36|14·5|23·8|42·7|57·3|| 2·19| 18·0| 16·1| 36·3| 63·7|
|Half-fat old Sheep |4·13|14·9|31·3|50·3|49·7|| 2·72| 17·7| 18·5| 38·9| 61·1|
|Fat Sheep          |3·45|11·5|45·4|60·3|39·7|| 2·32| 16·1| 26·4| 44·8| 55·2|
|Extra fat Sheep    |2·77| 9·1|55·1|67·0|33·0|| 3·64| 16·8| 34·5| 54·9| 45·1|
|Store Pig          |2·57|14·0|28·1|44·7|55·3|| 3·07| 14·0| 15·0| 32·1| 67·9|
|Fat Pig            |1·40|10·5|49·5|61·4|38·6|| 2·97| 14·8| 22·8| 40·6| 59·4|
|-------------------|----|----|----|----|----||-----|-----|-----|-----|-----|
|Mean of all        |3·69|13·5|34·4|51·6|48·4|| 3·02| 17·2| 21·0| 41·2| 58·8|
|-------------------|----|----|----|----|----||-----|-----|-----|-----|-----|
|Mean of 8, viz, the|    |    |    |    |    ||     |     |     |     |     |
| half-fat, fat, and|3·75|13·3|36·5|53·6|46·4|| 3·12| 17·4| 22·4| 42·9| 57·1|
|  very fat animals |    |    |    |    |    ||     |     |     |     |     |
|-------------------|----|----|----|----|----||-----|-----|-----|-----|-----|
|Mean of 6, viz.,   |    |    |    |    |    ||     |     |     |     |     |
| of the fat and    |3·38|12·3|39·7|55·4|44·6|| 3·03| 16·9| 24·1| 44·0| 56·0|
|   very fat animals|    |    |    |    |    ||     |     |     |     |     |
|-------------------|----|----|----|----|----||-----|-----|-----|-----|-----|


     |--------------------------------------------------|
     |                   |                              |
     |                   |   Per cent  in Entire Animal.|
     |                   |                              |
     |                   |------------------------------|
     |                   | MM | NC | Fat| TDS| CSI | WAT|
     |-------------------|----|----|----|----|-----|----|
     |Fat Calf           |3·80|15·2|14·8|33·8| 3·17|63·0|
     |Half-fat Ox        |4·66|16·6|19·1|40·3| 8·19|51·5|
     |Fat Ox             |3·92|14·5|30·1|48·5| 5·98|45·5|
     |Fat Lamb           |2·94|12·3|28·5|43·7| 8·54|47·8|
     |Store Sheep        |3·16|14·8|18·7|36·7| 6.00|57·3|
     |Half-fat old Sheep |3·17|14·0|23·5|40·7| 9·05|50·2|
     |Fat Sheep          |2·81|12·2|35·6|50·6| 6·02|43·4|
     |Extra fat Sheep    |2·90|10·9|45·8|59·6| 5·18|35·2|
     |Store Pig          |2·67|13·7|23·3|39·7| 5·22|55·1|
     |Fat Pig            |1·65|10·9|42·2|54·7| 3·97|41·3|
     |-------------------|----|----|----|----|-----|----|
     |Mean of all        |3·17|13·9|28·2|44·9| 6·13|49·0|
     |-------------------|----|----|----|----|-----|----|
     |Mean of 8, viz, the|    |    |    |    |     |    |
     | half-fat, fat, and|3·23|13·3|29·9|46·4| 6·26|47·3|
     |  very fat animals |    |    |    |    |     |    |
     |-------------------|----|----|----|----|-----|----|
     |Mean of 6, viz.,   |    |    |    |    |     |    |
     | of the fat and    |3·00|12·7|32·8|48·5| 5·48|46·0|
     |   very fat animals|    |    |    |    |     |    |
     |-------------------|----|----|----|----|-----|----|

From this table it appears that, in the carcass, the proportion of fat
is, in general, even in lean animals, much greater than that of
nitrogenous compounds. In one case only, that of the fat calf, are they
equal. But in the lean sheep there is more than one and a half times as
much fat as nitrogenous matters, in the half fat sheep twice, in the fat
sheep four times, and in the very fat sheep about six times as much. As
a general result of the analyses it may be stated, that in the carcass
of an ox in good condition, the quantity of fat will be from two to
nearly three times as great as that of the so called albuminous
compounds; in a sheep three or four times, and in the pig four or five
times as great. In the offal, including the hide, intestines, and other
parts not usually consumed as food, the proportion is very
different,--the quantity of fat being much smaller, and that of
nitrogenous compounds considerably larger.

Taking a general average of the whole, the following may be assumed as
representing approximately the general composition of a lean and a fat
animal:--

                                Lean.    Fat.

     Mineral matters              5       3
     Nitrogenous compounds       15      12·5
     Fat                         24      33
     Water                       56      48·5
                                ---     -----
                                100     100·0

The data given in the preceding table, coupled with a knowledge of the
relative weights of the lean and fat animals, enable us to ascertain the
composition of the _increase_ during the fattening process. It is
obvious, from the material diminution of the per centage of water, that
the matters deposited in the tissues must contain a much larger
proportion of dry matters than the whole body; and the reduced per
centage of nitrogenous matters shews that the fat must also greatly
preponderate. This is still more distinctly illustrated by the following
table, giving the per centage composition of the increase in fattening
oxen, sheep, and pigs:--

     ------------------------------------------------
           | Mineral  | Nitrogenous | Fat. | Water. |
           | Matters. |  Compounds. |      |        |
     ------------------------------------------------
     Oxen  |  1·47    |    7·69     | 66·2 |  24·6  |
     Sheep |  2·34    |    7·13     | 70·4 |  20·1  |
     Pigs  |  0·06    |    6·44     | 71·5 |  22·0  |
     ------------------------------------------------

Hence it may be stated in round numbers, that for every pound of
nitrogenous matters added to the weight of a fattening animal, it will
gain ten pounds of fat, and three of water. These are the proportions
over the whole period of fattening, but it is probable that during the
last few weeks of the process the ratio of fat to nitrogenous matters is
still higher.

In considering the composition of the food of animals, it will be
readily admitted that the milk, the nutriment supplied by nature for the
maintenance of the young animal, must afford special instruction as to
its requirements during the early stages of existence, and indicate, at
least, some of the points to be attended to under the altered conditions
of mature life. The following table gives the average composition of the
milk of the most important farm animals:--

                      Cow.    Ewe.    Goat.
     Caseine          3·4     4·50     4·02
     Butter           3·6     4·20     3·32
     Sugar of milk    6·0     5·00     5·28
     Ash              0·2     0·68     0·58
     Water           86·8    85·62    86·80
                   ------   ------   ------
                   100·00   100·00   100·00

In examining these, and all other analyses of food, it is necessary to
draw a distinction between the flesh-forming and the respiratory
elements; the former including the nitrogenous compounds which are used
in the production of flesh, the latter, the non-nitrogenous substances
which produce fat and support the process of respiration. The former,
however much they may differ in name, are nearly or altogether identical
in chemical composition, the latter embracing two great classes--the
fats which exist in the body and the saccharine compounds, including the
different kinds of sugar and starch which are not found in the animal
tissues. It was at one time supposed that these substances were entirely
consumed in the respiratory process, and eliminated by the lungs in the
form of carbonic acid and water, but it has been clearly shewn that they
may be and often are converted into fat, and accumulated in the system.
Careful experiments on bees have demonstrated that when fed on sugar
they continue to produce wax, which is a species of fat, and animals
retain their health and become fat, even when their food contains
scarcely any oil. There is, however, an important difference between
these two classes of substances as regards their fat-producing effect. A
pound of fat contained in the food is capable of producing the same
quantity within the animal; but the case is different with starch and
sugar, the most trustworthy experiments shewing that two and a half
pounds of these substances are necessary for that purpose. Hence we talk
of the fat equivalent of sugar, by which is meant the amount of fat it
is capable of producing, and which is obtained by dividing its quantity
by 2·5. Applying this principle to the analyses of the milk, it appears
that the relative proportions of the two great classes of nutritive
substances stand thus:--

           Flesh   Respiratory, expressed in
          forming    their fat equivalent

     Cow    3·4             6·0
     Ewe    4·5             6·2
     Goat   4·0             5·4

Taking the general average, it may be stated, that for every pound of
flesh-forming elements contained in the food of the sucking animal, it
consumes respiratory compounds capable of producing one and a half
pounds of fat, and this does not differ materially from the ratio
subsisting between these substances in the lean animal. When the young
animal is weaned, it obtains a food in which the ratio of nitrogenous to
respiratory elements is maintained nearly unchanged; but the latter, in
place of containing a large amount of fatty matters, is in many cases
nearly devoid of these substances, and consists almost exclusively of
starch and sugar, mixed most commonly with a considerable quantity of
woody fibre.

A very large number of analyses of different kinds of cattle food have
been made by chemists, but our information regarding them is still in
some respects imperfect. The quantity of nitrogenous compounds and of
oil has been accurately ascertained in almost all, but the amount of
starch, sugar, and woody fibre is still imperfectly determined in many
substances. This is due partly to the fact that the nitrogenous and
fatty matters were formerly believed to be of the highest importance,
and might be used as the measure of the nutritive value of food to the
exclusion of its other constituents, and partly also to the imperfect
nature of the processes in use for obtaining the amounts of woody fibre,
starch, and sugar. These difficulties have now, to a certain extent,
been overcome, and the quantity of fibre and of respiratory elements has
been ascertained, and is introduced, so far as is known, in the
subjoined table:--

TABLE giving the Composition of the Principal Varieties of Cattle Food.

_Note._--Where a blank occurs in the oil column, the quantity of that
substance is so small as to be unimportant. When the respiratory
elements and fibre have not been separated, the sum of the two is given.

+----------------------------+--------+------+--------+-------+------+-------+
|                            |Nitro-  | Oil. | Respir-| Fibre.| Ash. | Water.|
|                            | genous |      |  tory  |       |      |       |
|                            |Com-    |      | Com-   |       |      |       |
|                            | pounds.|      | pounds.|       |      |       |
+----------------------------+--------+------+--------+-------+------+-------+
|Decorticated earth-nut cake | 44·00  |  8·86| 19·34  |  5·13 | 14·05|  8·62 |
|Decorticated cotton cake    | 41·25  | 16·05| 16·45  |  8·92 |  8·05|  9·28 |
|Poppy cake                  | 34·03  | 11·04| 23·25  | 11·33 | 13·79|  6·56 |
|Teel or sesamum cake        | 31·93  | 12·86| 21·92  |  9·06 | 13·85| 10·38 |
|Rape cake                   | 29·75  |  8·63| 38·72  |  7·30 |  8·65|  6·95 |
|Dotter cake                 | 29·00  |  7·99| 27·04  | 16·12 | 12·59|  7·26 |
|Tares, home-grown           | 28·57  |  1·30|      58·64     |  2·50|  8·99 |
|Linseed cake                | 28·53  | 12·47| 35·78  |  6·32 |  6·11| 10·79 |
|Rübsen cake                 | 26·87  | 11·00| 31·47  | 16·95 |  8·00|  5·71 |
|Tares, foreign              | 26·73  |  1·59|      53·04     |  2·84| 15·80 |
|Earth-nut cake (entire seed)| 26·71  | 12·75|      45·69     |  3·29| 11·56 |
|Niger cake                  | 25·74  |  6·58| 42·18  | 11·15 |  8·12|  6·23 |
|Beans (65 lbs. per bushel)  | 24·70  |  1·59|      54·51     |  3·36| 15·84 |
|Lentils                     | 24·57  |  1·51|      58·82     |  2·79| 12·31 |
|Linseed                     | 24·44  | 34·00|      30·73     |  3·33|  7·50 |
|Grey peas                   | 24·25  |  3·30|      57·99     |  2·52| 11·94 |
|Foreign beans               | 23·49  |  1·51|      59·67     |  3·14| 12·21 |
|Cotton cake (with husk)     | 22·94  |  6·07| 36·52  | 16·99 |  6·02| 11·46 |
|Pea-nut cake                | 22·25  |  7·62| 30·25  | 26·97 |  3·71|  9·20 |
|Sunflower cake              | 21·68  |  8·94| 19·05  | 33·00 |  9·33|  8·00 |
|Hempseed cake               | 21·47  |  7·90| 22·48  | 25·16 | 15·79|  7·21 |
|Kidney beans                | 20·06  |  1·22|      62·16     |  3·56| 13·00 |
|Maple peas                  | 19·43  |  1·72|      63·18     |  2·04| 13·63 |
|Madia sativa (seed)         | 18·41  | 36·55|      34·59     |  4·13|  6·32 |
|Clover hay (mean of         |        |      |        |       |      |       |
|different species of clover)| 15·81  |  3·18| 34·42  | 22·47 |  7·59| 16·53 |
|Rye                         | 14·20  |   ...| 81·51  |  2·47 |  1·82| 14·66 |
|Bran                        | 13·80  |  5·56|      61·67     |  6·11| 12·85 |
|Oats                        | 11·85  |  5·89| 57·45  |  9·00 |  2·72| 13·09 |
|Fine barley dust            | 11·49  |  2·92|      71·41     |  2·67| 11·51 |
|Wheat                       | 11·48  |   ...| 73·52  |  0·68 |  0·82| 13·50 |
|Bere                        | 10·25  |   ...| 62·85  | 10·08 |  2·60| 14·22 |
|Hay (mean of different      |        |      |        |       |      |       |
|    grasses)                |  9·40  |  2·56| 38·54  | 29·14 |  5·84| 14·30 |
|Barley                      |  8·69  |   ...| 64·52  |  9·67 |  2·82| 14·30 |
|Coarse barley dust          |  8·46  |  3·47|      69·73     |  7·31| 11·03 |
|Rice dust                   |  8·08  |  2·95|      69·22     |  8·12| 11·63 |
|Oat dust                    |  6·92  |  3·21|      72·86     |  7·70|  9·31 |
|Winter bean straw           |  5·71  |   ...|      67·50     |  6·39| 20·40 |
|Carob bean                  |  3·11  |  0·41| 62·51  | 18·60 |  2·80| 12·57 |
|Potato                      |  2·81  |   ...| 17·30  |  1·07 |  1·13| 77·69 |
|Carrot                      |  1·87  |   ...|  7·91  |  3·07 |  1·11| 86·04 |
|Wheat straw                 |  1·79  |   ...| 31·06  | 45·45 |  7·47| 14·23 |
|Barley straw                |  1·68  |   ...| 39·98  | 39·80 |  4·24| 14·30 |
|Oat straw                   |  1·63  |   ...| 37·86  | 43·60 |  4·95| 12·06 |
|Mangold-wurzel              |  1·54  |   ...|  8·60  |  1·12 |  0·96| 87·78 |
|Cabbage                     |  1·31  |   ...|       4·53     |  1·05| 93·11 |
|Turnips                     |  1·27  |  0·20|  4·07  |  1·08 |  1·71| 91·47 |
+----------------------------+--------+------+--------+-------+------+-------+

It is at once obvious that in many of these descriptions of food the
ratio of the flesh to the fat-forming constituents differ very widely
from that existing in the milk, and this becomes still more apparent
when the latter are represented in their fat equivalent, as is done for
a few of them in the following table:--


                                    Flesh        Respiratory, expressed
                                    forming,     in their fat equivalent,

     Decorticated earth-nut cake    44·0          16·6
     Linseed cake                   28·5          26·7
     Tares                          26·73         18·8
     Clover hay                     15·81         16·8
     Oats                           11·85         28·8
     Hay (mean of grasses)           9·40         17·9
     Potato                          2·81          6·9
     Wheat straw                     1·79         12·4
     Turnip                          1·27          1·8

It is especially note-worthy that those varieties of food, which common
experience has shewn to promote the fattening of stock to the greatest
extent, contain in many instances the smallest quantity of respiratory
or fat-forming elements relatively to their nitrogenous compounds. This
is especially the case with the different kinds of oil cake, the
leguminous seeds, clover, hay, and turnips. On the other hand, in the
grains the ratio is nearly that of one to three, or similar to that
found in fat cattle; while in the straw, the excess of the respiratory
elements is extremely great.

These facts appear at first sight to be completely at variance with the
composition of the increase of fattening animals, as ascertained by
Messrs. Lawes and Gilbert already referred to, and which have shewn that
for every pound of nitrogenous compounds, nearly ten pounds of fat are
stored within the animal; and it might be supposed that those kinds of
food which contain the largest relative amount of respiratory elements
ought to fatten most rapidly, and should be selected by the farmer in
preference to oil-cakes and similar substances. But there are other
matters to be considered, dependent on the complex nature of the changes
attending the absorption and assimilation of the food. It must be
particularly borne in mind that only a small proportion of the food
consumed is stored up within the body, and goes to increase the weight
of the animal. Even in the case of the milk, in which economy in the
supply of nutritive matters has been most clearly attended to by nature,
a considerable proportion escapes assimilation, and in the adult animal
a large amount of the food passes off with the excretions. The justice
of this position is apparent when it is remembered that an ox will go on
day after day consuming from a hundred weight to a hundred weight and a
half of turnips, three or four pounds of bean-meal or oil-cake, and a
considerable quantity of straw, although its daily increase in live
weight may not exceed a couple of pounds. And in this direction a very
fertile field of inquiry lies open to the agricultural experimenter; for
it would be most important to determine whether there are not some
substances from which the nutritive matters may not be more easily
assimilated than from others, and what proportion of each is absorbable
under ordinary circumstances. On this point no information has yet been
obtained applicable to individual feeding substances, but the
experiments of Messrs. Lawes and Gilbert have shewn the quantity of the
total food, and of each of its constituents, stored up in the fattening
animal, and a summary of their results is contained in the following
Table:--

TABLE shewing the Amount of each Class of Constituents, stored in the
increase, for 100 consumed in the Food.

     +------+---------+-------------+------+------------+
     |      | Mineral | Nitrogenous |      | Total Dry  |
     |      | Matters | Compounds.  | Fat. | Substance. |
     +------+---------+-------------+------+------------+
     |Sheep | 3·27    | 4·41        |  9·4 |   8·06     |
     |Pigs  | 0·58    | 7·34        | 21·2 |  17·3      |
     +------+---------+-------------+------+------------+

Hence it appears that the pig makes a better use of its food than the
sheep, retaining twice as much of its solid constituents within the
body, from which may be deduced the important practical conclusion, that
the former must be fattened at a much smaller cost than the latter.
Looking at the individual constituents, it appears that, in the sheep,
less than one-twentieth of the nitrogenous compounds, and one-tenth of
the non-nitrogenous substances contained in the food, remain in the
body; and a knowledge of these facts tends to modify the conclusions
which might be drawn from the composition of the increase in the
fattening animal. Its influence may be best illustrated by a particular
example. If, for instance, the increase in a sheep contained its
nitrogenous and respiratory elements in the ratio of 1 to 10, it would
be totally incorrect to supply these substances in the food in the same
proportions. On the contrary, it would be necessary at the very least to
double the proportion of the former, because one-tenth of the
fat-forming elements are absorbed, and only one-twentieth of the
nitrogenous.

On further consideration, also, it seems unquestionable that the
quantity of the nutritive elements stored up must depend to a large
extent on the nature of the food and the particular state in which they
exist in it. It is probable, or at least possible, that some kinds of
food may contain their nitrogenous constituents in an easily assimilable
state, and their respiratory elements in a nearly indigestible
condition, or _vice versa_, and under these circumstances their
nutritive value would be below that indicated by analysis; but these
points can only be determined by elaborate and long continued feeding
experiments. It is well known, however, that the mechanical state of the
food has a most important influence on its nutritive value. Thus, for
example, the presence of a large quantity of woody fibre protects the
nutritive substances from assimilation, and seeds with hard husks pass
unchanged through the animal, although, so far as their composition
alone is concerned, they may be highly nutritive; and the loss of a
certain quantity of many varieties of food in this way is familiar to
every one.

The proper adjustment of the relative quantities of the great groups of
nutritive elements in the food is a matter the importance of which
cannot be over-rated, for it is in fact the foundation of successful and
economical feeding; and this will be readily understood if we consider
what would be the result of giving to an animal a supply of food
containing a large quantity of nitrogenous and a deficiency of
fat-forming compounds. In such circumstances, the animal must either
languish for want of the latter, or it is forced to supply the defect by
an increased consumption of food, in doing which it must take into the
system a larger quantity of nitrogenous compounds than would otherwise
have been requisite, and in this way the other elements, which are
present in abundance, are wasted, and the theoretical and practical
value of a food so constituted may be very different, and it is only
when the proportions of the different groups are properly attended to
that the most economical result can be obtained. It can scarcely be said
that the experiments yet made by feeders enable us to fix the most
suitable proportion in which those substances can be employed, although
experience has led them to the use of mixtures which are in most cases
theoretically correct; thus they combine oil-cakes or turnips with
straw, which is poor nitrogenous, and rich in fat-forming elements; and
in general it will be found that where different kinds of food are
mixed, the deficiencies of the one are counterbalanced by the other, and
though this has hitherto been done empirically, it cannot be doubted
that as our knowledge advances it will more and more be determined by
reference to the composition of the food.

Although the presence of a sufficient quantity of nutritive compounds in
the food is necessarily the fundamental matter for consideration, its
bulk is scarcely less important. The function of digestion requires that
the food shall properly fill the stomach, and however large the supply
of nutritive matters may be, their effect is imperfectly brought out if
the food is too small in bulk, and it actually may become more valuable
if diluted with woody fibre, or some other inert substance. At first
sight this may appear at variance with the observations already made as
to the effects of woody fibre in protecting the nutritive matters from
absorption; but practically there are two opposite evils to be contended
against, a food having too small a bulk, or one containing so large a
proportion of inert substances as to become disadvantageously
voluminous. The most favourable condition lies between the two extremes,
and the natural food of all herbivorous animals is diluted with a
certain amount of woody fibre. When these are replaced by substances
containing a large quantity of nutritive matters in a small bulk, the
result is that the natural instinct of the animal causes it to continue
feeding until the stomach is properly distended, and it consequently
consumes a much larger quantity of food than it is capable of digesting,
and a more or less considerable quantity passes unchanged through the
intestines, and is lost. On the other hand, if the food be too bulky,
the sense of repletion causes the animal to cease eating long before it
has obtained a sufficient supply of nutritive matter. It is most
necessary, therefore, to study the mixture of different kinds of food,
so as to obtain a proper relation between the bulk and the nutritive
matters contained in the mixture; and on examining the nature of the
mixed foods most in vogue among feeders, it will be found that a very
bulky food is usually conjoined with another of opposite qualities.
Hence it is that turnips, the most voluminous of all foods, are used
along with oil-cake and bean-meal, and if from any circumstances it
becomes necessary to replace a large amount of the former by either of
the latter substances, the deficient bulk must be replaced by hay or
straw.

It has been already remarked that there are three great purposes to
which the food consumed is appropriated; the increase of weight of the
animal--the object the feeder has in view and desires to promote--the
supplying the waste of the tissues, and the process of respiration, both
of which are sources of loss of food, and which it must necessarily be
his aim to diminish as much as possible. The circumstances which must be
attended to in order to do this are sufficiently well understood. It has
been clearly established that the natural heat of the animal is
sustained by the consumption of a certain quantity of its food in the
respiratory process, during which it undergoes exactly the same changes
as those which occur during combustion. It has further been observed,
that the temperature of the body remains unchanged, whatever be that of
the surrounding air; and it is obvious that if it is to continue the
same in winter as in summer, a larger quantity of fuel (_i. e._ food)
must be consumed for this purpose, just as a room requires more fire to
keep it warm in winter than in summer, and hence it naturally follows,
that if the animal be kept in a warm locality the food is economized. It
may also be inferred that, if it were possible, consistently with the
health of the animal, to keep it in a room artificially heated to the
temperature of its own body, this source of waste of food would be
entirely removed. It is not possible, however, to do this, because a
limit is set to it by physiological laws, which cannot be infringed with
impunity; but the housing of cattle, so as to diminish this waste as far
as possible, is a point in regard to the propriety of which theory and
practice are at one.

The old feeders kept their cattle in large open courts, where they were
exposed to every vicissitude of the weather, but as intelligence
advanced, we find them substituting, first hammels, and then stalls, in
which the animals are kept during the whole time of fattening at an
equable temperature. The effect of this is necessarily to introduce a
considerable economy of the food required to sustain the animal heat;
but it also produces a saving in another way, for it diminishes the
waste of the tissues.

It has been ascertained by accurate experiments made chiefly on man,
that muscular exertion is one of the most important causes of the waste
of the tissues, and of increased respiratory activity. We cannot move a
limb without producing a corresponding consumption of matters already
laid up within the body; and it has also been found, that the difference
in the quantity of carbonic acid expired during rest and active
exertion, is very large. The inference to be drawn from this is, that
when it is sought to fatten an animal rapidly, every effort must be made
to restrain muscular motion so far as compatible with health. Hence, the
peculiar advantage of stall-feeding, in which the animal is confined to
one spot, and the more thoroughly it can be kept still, the greater will
be the economy of food. This is gained by darkening the house, and
excluding all persons, except when their presence is indispensable.

An extension of the same principle has led to the use of food
artificially heated, but it is doubtful whether the advantages derived
from it are commensurate to the increased expense of the process; at
least opinions differ among the best informed practical men on this
subject.

Many other matters, besides these mentioned, exercise an important
influence on the feeding of stock, such as the general health of the
animal, the breed, etc. These are subjects, however, which bear more
directly on practical agriculture, and need not be discussed here.

The judicious feeder will not only give due weight to the principles
already discussed in all he does, but he must take into consideration
the extent to which they are liable to be modified in particular cases.
He must also attend to the cost of different kinds of food, and the
value of the manure produced by them, subjects of much importance in a
practical point of view, and which must influence him greatly in choice
of the particular substances he supplies to his cattle.




INDEX.



Acid, apocrenic, 21.
  Carbonic, 10, 15, 20, 37, 57, 115.
  Cerotic, 48.
  Crenic, 21.
  Geic, 21.
  Hippuric, 168.
  Humic, 21.
  Lactic, 168.
  Margaric, 47.
  Nitric, 11, 17, 30, 33, 38, 62, 112.
  Oleic, 47.
  Pectic, 46.
  Phosphoric, 73, 90.
  Stearic, 47.
  Sulphuric, 182, 237.
  Ulmic, 21.
  Uric, 168.

Adulteration of guano, 211.

Agricultural Chemistry Association of Scotland, 6.

Air, influence of, on germination, 55.
  In the pores of soils, 115.

Albite, 86.

Albumen, 48.

Albuminous constituents of plants and animals, 48.

Algoa Bay guano, 208.

Alkaline salts, value of, 260.

Alumina, 73, 86, 103.

Ammonia, absorption of, by plants, 29, 38.
  Absorption of, by soils, 123.
  Carbonate of, 29.
  Composition of, 12.
  Decomposition of, by plants, 61.
  Presence in dew, 17.
    "         rain, 17.
  Production of, 12.
  Properties of, 12.
  Proportion of, in air, 16, 20.
  Proportion of, in drain water, 112.
  Proportion of, in soils, 107.
  Sulphate of, 29, 227.
  Sulphomuriate of, 227.
  Urate of, 205.
  Valuation of, 259.

Ammoniacal liquor, 229.

Amylaceous constituents of plants, 40.

Angamos guano, 207, 210.

Animal charcoal, 224.
  Manures, 204.

Animals, composition of, 281.
  Nitrogenous constituents of, 48, 281.

Apatite, 235.

Ascension Island guano, 208.

Augite, 89.

Australian guano, 207.

Avenine, 50.


Barks, amount of ash in, 66.

Barley, 286.

Barrenness of soils, 109.

Basalt, 92.

Beans, 286.

Bere, 286.

Biphosphate of lime, 237, 260.

Bird Island guano, 208.

Blood as a manure, 220.

Bone ash, 234.

Bone oil, 229.

Bones as a manure, 223.
  Dissolved, 237.

Box-feeding, 183.

Bolivian guano, 207, 210.

Bran, 197, 286.

Burning, improvement of soils by, 146.


Cabbage, 286.

Cane sugar, 43.

Carbon, properties of, 10.
  Proportion of, in plants, 10.

Carbonate of ammonia, 29.
  Lime, 96, 247.
  Magnesia, 96.
  Potash, 232.
  Soda, 232.

Carbonic acid, absorption of, by plants, 37.
  Decomposition of, by plants, 57.
  Evolution of, by plants, 58.
  How obtained, 10.
  Properties, 10.
  Proportion of, in air, 15, 20.

Carburetted hydrogen, 19.

Calcium, sulphuret of, 252.

Caramel, 44.

Carrot, 286.

Caseine, 50, 283.

Castor cake, 195.

Cattle food, composition of, 286.

Cellulose, 40.

Cerine, 48.

Cerotic acid, 48.

Chaff, 197.

Chalk, 96, 245.

Charcoal, animal, 224.

Chilian guano, 207.

China-clay, 87.

Chloride of potassium, 73, 102.
  Sodium, 73, 232.
  Manganese, 182.

Clay, 87.
  Absorbent action of, 121.
  Composition of, 95.
  Source of, 88, 94.

Clay-slate, 95.

Classification of plants, 81.

Coprolites, 98, 235.

Coral sand, 246.

Cotton cake, 195, 286.

Crenic acid, 21.

Crops, Mineral matters in, 270.
  Nitrogen in different, 270.
  Rotation of, 81, 266.


Deep Ploughing, effects of, 144.

Dew, ammonia in, 17.
  Nitric acid in, 19.

Dextrine, 43.

Diastase, 43, 53, 55.

Diorite, 92.

Dissolved bones, 237.

Dolerite, 92.

Dotter cake, 286.

Drainage water, analyses of, 112.

Draining, 138.

Dung, composition of, 170.

Dung heaps, management of, 179.


Earth-nut cake, 286.

Emulsine, 50.

Exhaustion of soils, 81.


Farm stock, feeding of, 276.

Farm-yard manure, 166, 172.
  Application of, 186.

Fat, amount of, in animals, 281.

Fatty acids, 47.
  Matters, 46.

Feeding cakes, 286.

Feeding of farm stock, 276.

Felspar, 86.
  Decomposition of, 88.

Fermentation of manure, 184.

Fire-clay, 95.

Fish manure, 221.

Flesh as a manure, 220.

Fog, ammonia in, 17.
  Nitric acid in, 19.

Food, cattle, 286.

Fruits, amount of ash in, 66.


Gas Lime, 252.

Geic acid, 21.

Germination, 54.

Gluten, 49.

Glutin, 49.

Glycerine, 47.

Gneiss, 91.

Granite, 91.

Grape sugar, 44.

Greenstone, 92.

Green manuring, 198.

Guano, 204.
  Adulteration of, 211.
  Application of, 214.
  Average composition of, 207.
  Fish, 222.
  Peruvian, characters of, 209.
  Phospho-Peruvian, 243.
  Sombrero Island, 236.


Hair, 218.

Hay, 286.

Heat, evolution of, by plants, 60.

Hempseed cake, 286.

Hippuric acid, 168.

Horn, 218.

Hornblende, 89.

Humic acid, 21.

Humin, 22.

Humus, 21, 98, 133.

Hydrogen, 10.


Ichaboe Guano, 207.

Indian guano, 208.

Inorganic constituents of plants, 9, 34.

Inorganic constituents;
  Absorption by plants, 38.
  Proportion in plants, 64.

Inorganic constituents of soils, 85.

Inuline, 43.

Iodine in plants, 76.

Iron, protoxide of, in soils, 107.
  Sulphate of, 182.
  Sulphuret of, in subsoils, 135.


Kaolin, 87.

Kooria Mooria guano, 207.


Labradorite, 86.

Lactic acid, 168.

Latham Island guano, 207.

Leaves, amount of ash in, 65.
  As a manure, 202.

Legumine, 50.

Lichen starch, 42.

Light, influence of, on plants, 57.

Lime, action of, on soils, 248.
  As a manure, 245.
  Bicarbonate of, 122.
  Carbonate of, 96.
  Biphosphate of, 237, 260.
  Humate of, 125
  Phosphate of, 96, 233, 258.
  Sulphate of, 96, 253, 260.

Lime-plants, 82.

Limestone, 96.

Linseed cake, 195.

Liquid manure, 166, 187.


Madia Sativa, 286.

Magnesia, carbonate of, 96.
  Sulphate of, 182, 233.

Magnesian limestone, 96.

Malt-dust, 197.

Manganese in plants, 73.
  Oxide of, 73, 87.
  Chloride of, 182.

Mangold-wurzel, 286.

Manures, animal, 204.

Manures, application of, 165, 186.
  Fermentation of, 184.
  Farm-yard, 166, 172.
  Liquid, 166, 187.
  Mineral, 226.
  Theory of, 156.
  Sewage, 191.
  Vegetable, 195.
  Valuation of, 255.

Manuring, Green, 198.
  Principles of, 152.

Maple peas, 286.

Maracaybo guano, 236.

Margaric acid, 47.

Margarine, 46.

Marl, 245.

Mexican guano, 207.

Mica, 88.

Mica slate, 91.

Milk, composition of, 283.
  Curding of, 51.

Mineral constituents of plants, 9, 63.

Mineral manures, 226.

Mineral matters in different crops, 270.
  In animals, 281.

Moisture, influence of, on germination, 55.

Mucilage, 44.


Natrolite, 90.

New Island guano, 208.

Niger cake, 286.

Night-soil, 217.

Nitrate of potash, 229.

Nitrate of soda, 229, 260.

Nitric acid, absorbtion of, by plants, 30, 38.
  Decomposition of, by plants, 62.
  In drainage water, 112.
  In dew, 19.
  In air, 17.
  In fog, 19.
  Production of, 11, 33.

Nitrification, 11.

Nitrogen, amount in a six-course rotation, 160.
  Amount of, in different crops, 270.
  Presence in the atmosphere, 11.
  Properties of, 11.
  Proportion of, in plants, 11.

Nitrogenous constituents of plants, 48, 286.

Nitrogenous constituents of animals, 48, 281.


Oats, 286.
  Proportion of ash in, 68, 70.

Oil-cakes, 195, 286.

Oils, sweet principle of, 47.

Oily matters, 46.

Oleic acid, 47.

Oleine, 46.

Oligoclase, 86.

Oolitic limestone, 96.

Organic constituents of plants, 8.
  Sources of the, 13, 20.

Organic constituents of soils, 103.

Orthoclase, 86.

Oxide of iron in rocks and soils, 87, 107.
  Of manganese, 87.

Oxygen, evolution of, by plants, 58.
  Influence of, on germination, 55.
  Presence in atmosphere, 12.
  Properties of, 12.
  Proportion of, in plants, 12.


Pacquico Guano, 207.

Paring, improvement of soils by, 146.

Patagonian guano, 207.

Pea-nut cake, 286.

Peas, 286.

Peat, as a manure, 203.

Peat, use of, in dung-heaps, 184.

Pectic acid, 46.

Pectine, 46.

Peruvian Guano, 205, 207, 209.
  Upper, 207, 213.

Phosphate of lime, 96, 233.
  Value of, 258.

Phosphates, insoluble, 258.
  Soluble, 237, 260.

Phospho-Peruvian guano, 243.

Phosphuretted hydrogen in air, 19.

Pigeons' dung, 216.

Plants, Albuminous constituents of, 48.
  Amylaceous constituents of, 40.
  Ash of, 64, 73.
  Classification of, 81.
  Inorganic constituents of, 9, 34, 38, 63.
  Oily constituents of, 46.
  Organic constituents of, 8.
  Proximate constituents of, 40.
  Saccharine constituents of, 40.

Poppy cake, 196, 286.

Potash, carbonate of, 232.
  Muriate of, 231.
  Nitrate of, 229.
  Plants, 82.
  Salts, 231.

Potato, 286.

Poudrette, 217.

Proximate constituents of plants, 40.

Pyroguanite, 236.


Quartz, 86.


Rainwater, 17, 18.

Rape Cake, 196, 286.
  Dust, 195.

Rocks, crystalline, 85.
  Composition of, 91.
  Disintegration of, 85.
  Sedimentary, 86.

Roots of plants, amount of ash in, 65.

Rotation of crops, 81, 266.

Rübsen cake, 286.

Rye, 286.


Saccharine  Constituents of plants, 40.

Saldanha Bay guano, 207.

Salt, common, 232.

Sandstones, 95.

Schübler's experiments, 127.

Sea Bear Bay guano, 208.

Sea weed, 200, 201.

Seeds, amount of ash in, 64.

Sesamum cake, 286.

Sewage manure, 191.

Shell sand, 246.

Silica plants, 82.

Silicate of potash, 233.
  Soda, 233.

Skin, 218.

Soda, carbonate of, 232.
  Nitrate of, 229, 260.
  Salts, 231.
  Silicate of, 233.

Sodium, chloride of, 232.

Soil, the, 20, 83.
  Influence on the composition of the ash of plants, 71.
  Chemical composition of, 98.
  Chemical and physical characters of, 83.
  Improvement of, by mechanical means, 137.

Soil, relation of, to heat and moisture, 127.

Soils, absorbent action of, 122.
  Air in the pores of, 114.
  Analysis, 101, 118.
  Barrenness of, 109.
  Classification of, 135.
  Exhaustion of, 81.
  Inorganic constituents of, 85.
  Mixing of, 150.
  Origin of, 84.
  Organic matters in, 103.
  Physical characters of, 118, 127.

Sombrero Island guano, 236.

Starch, 41.
  Lichen, 42.

Stearic acid, 47.

Stearine, 46.

Stems of plants, ash in, 64.

Straw, amount of ash in, 64.
  As a manure, 197.

Sulphate of iron, 182.
  Lime, 96, 253, 260.
  Magnesia, 182.
  Ammonia, 29, 227.
  Potash, 231.

Sulphomuriate of ammonia, 227.

Sulphur in plants, 78.

Sulphuret of iron, 135.
  Calcium, 252.

Sulphuretted hydrogen, 19.

Sugar, 43.
  Of milk, 283.

Subsoil, the, 134.
  Ploughing, 143.

Sunflower cake, 286.

Syenite, 91.


Tares, 286.

Teelcake, 286.

Temperature, influence of, on germination, 54.

Thomsonite, 90.

Trap rock, 92.

Tubers, amount of ash in, 65.


Ulmic acid, 21.

Ulmin, 22.

Upper Peruvian guano, 207, 213.

Urate, 216.
  Of ammonia, 205.

Urea, 168.

Uric acid, 168, 205.

Urine, composition of, 167.
  Human, 168.
  Sulphated, 216.


Valuation of manures, 255.

Vegetable manures, 195.

Vegetation, influence of light on, 57.

Voelcker's analyses of dung, 174.


Warping, 148.

Water, absorption of, by plants, 35.
  Decomposition of, by plants, 60.
  Exhalation of, by plants, 35.
  Rain, 17, 18.

Wax, 48.

Wheat, 286.

Woods, amount of ash in, 65.

Woody fibre, 41.

Wool, 219.


Zeolites, 90.


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DE QUINCEY'S WORKS.


AUTHOR'S EDITION.

_In Fifteen Volumes, price 4s. 6d. each_,

NEW EDITION OF

THE WORKS OF THOMAS DE QUINCEY

"THE ENGLISH OPIUM EATER"

       *       *       *       *       *

Vol. I. CONFESSIONS OF AN ENGLISH OPIUM-EATER.
    II. RECOLLECTIONS OF THE LAKE POETS.
   III. LAST DAYS OF IMMANUEL KANT.
    IV. THE ENGLISH MAIL-COACH.
     V. DR. SAMUEL PARR, ETC.
    VI. RICHARD BENTLEY, ETC.
   VII. PROTESTANTISM AND OTHER ESSAYS.
  VIII. LEADERS IN LITERATURE.
    IX. THE CÆSARS, AND OTHER WRITINGS.
     X. RHETORIC AND STYLE.
    XI. COLERIDGE AND OPIUM EATING.
   XII.  SPECULATIONS, LITERARY AND PHILOSOPHIC.
  XIII. CONVERSATION, AND OTHER PAPERS.
   XIV. THE AUTOBIOGRAPHIC SKETCHES--1790-1803.

Volume XIV. concludes the series of Mr. De Quincey's Works, as arranged
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SHAKSPEARE, POPE, GOETHE, and SCHILLER, contributed by Mr. De Quincey to
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OF MODERN ENGLAND. Also a Complete GENERAL INDEX to the whole Works.

       *       *       *       *       *

EDINBURGH: ADAM AND CHARLES BLACK




NEW EDITION OF KITTO'S CYCLOPÆDIA.

_In royal 8vo, Volume I. (A to E), pp. 884. Price_ £1: 0: 0.

A THIRD EDITION OF

KITTO'S

_Cyclopædia of Biblical Literature._

Edited by the Rev. WILLIAM LINDSAY ALEXANDER, D.D., with the assistance
of numerous contributors.

Illustrated by Numerous Engravings on Wood and Steel.

In undertaking the New Edition of this work, it was the intention of the
Publishers to complete it in two volumes, but the additions have been so
extensive as to render a third volume necessary.

Although this change will occasion great extra expense to the
Publishers, there will be no additional charge to Subscribers, who will
receive the complete book at £3, the price to which the Publishers
pledged themselves in their prospectus.

_List of Contributors._

Beard, J. R., D.D.

Bell, G. M.

Brown, John, D.D., late Professor of Exegetical Theology to the United
Presbyterian Church of Scotland.

Browne, Henry, M.A., Vicar of Pevensey.

Cairns, John, D.D.

Candlish, James S., M.A.

Credner, Karl August, D.D., late Professor of Theology at Giessen.

Davidson, Samuel, D.D., LL.D.

Denham, Joshua Fred., M.A., F.R.S.

Deutsch, Emanuel, of the University of Berlin, M. Ger. Or. Soc., etc.,
British Museum.

Doran, John William, LL.D., Rector of Beeston, St. Lawrence, Norfolk.

Farrar, Frederic W., M.A., late Fellow of Trinity College, Cambridge.

Geikie, Archibald, F.R.S.E., F.G.S., of the Geological Survey.

Ginsburg, Christian D.

Goold, William Henry, D.D., Professor of Theology to the Reformed
Presbyterian Church.

Gotch, F. W., D.D., President of the Baptist College, Bristol; Examiner
in Hebrew to the London University.

Gowan, Anthony T., D.D.

Hävernick, Heinrich August Christ., late Professor of Theology at
Königsberg.

Holmes, Peter, D.D., F.R.A.S., of Magdalen Hall, Oxford.

Jamieson, Robert, D.D., Minister of St. Paul's, Glasgow.

Jennings, Isaac.

Kitto, John, D.D., F.A.S.

Leathes, Stanley, M.A.

Lyon, William P., B.A.

M'Causland, Dominick, Q.C., LL.D.

Madden, Fred. W., M.R.S.L., Brit. Museum.

Michelson, E., Ph. D. of the University of Heidelberg.

Morren, Nathanael, M.A.

Newman, Francis W., late Fellow of Balliol College, Oxford; Professor of
Latin in the University of London.

Newth, Samuel, M.A., Professor, New College, London.

Nicholson, John, B.A. Oxford; Ph. D. Tübingen.

Nicholson, W. A., M.D.

Poole, Reg. Stuart, British Museum.

Porter, J. Leslie, M.A., Professor of Sacred Literature, Assembly's
College, Belfast.

Royle, J. F., M.D., F.R.S., F.L.S., F.G.S.

Ryland, J. E.

Smith, C. Hamilton, Lieut.-Colonel, K.H. and K.W., F.R.S., F.R.L.S.,
etc.

Smith, John Pye, D.D., F.R.S., F.G.S.

Stebbing, Henry, D.D. of St. John's College, Cambridge.

Tholuck, August, D.D., Professor of Theology in the University of Halle.

Wace, Henry, M.A.

Wright, William, M.A. and LL.D. of Trinity College, Dublin.

       *       *       *       *       *

EDINBURGH: ADAM AND CHARLES BLACK.




NEW EDITION, 1862.

_In folio, half bound morocco, gilt edges, price_ £3,

_A new edition of_

BLACK'S GENERAL ATLAS OF THE WORLD,

Series of Fifty-six Maps, containing all the Latest Discoveries,
beautifully coloured, and accompanied by an Alphabetical Index of 65,000
Names, forming a ready Key to the places mentioned in the Maps.

_List of Maps in the order in which they occur._

PHYSICAL GEOGRAPHY, ETC.

 1.  The World in Hemispheres, with Comparative View of Mountains and Rivers.
 2. The World on Mercator's Projection.
 3. Physical and Ethnographical Charts of the World.
 4. Zoological and Botanical Charts of the World.
 5. Isothermal Chart shewing the Temperature of the Earth's Surface.
 6. Northern and Southern Celestial Hemisphere.
 7. Solar System, Theory of the Seasons, etc.


EUROPE.

 8. Europe.
 9. England (North Part).
10. ....... (South Part).
11. Scotland (North Part).
12. ........ (South Part).
13. Ireland.
14. France.
15. Switzerland.
16. Holland and Belgium.
17. Railway Map of Central Europe.
18. Germany.
19. Austria.
20. Prussia.
21. Denmark.
22. Sweden and Norway, with Baltic Sea.
23. Russia in Europe.
24. Spain and Portugal.
25. Italy (North).
26. ..... (South).
27. Turkey in Europe.
28. Greece and Ionian Islands.


ASIA.

29. Asia.
30. Turkey in Asia.
31. Palestine.
32. Persia, Afghanistan, and Beloochistan.
33. India.
34. China.
35. Indian Archipelago, and Further India,
    including Burmah, Siam, etc.


AFRICA.

36. Africa, with Barth's, Livingstone's, and Burton's Routes.
37. Egypt.
38. South Africa.


AMERICA.

39. North America, with Enlargement of British Columbia.
40. British America and Arctic Regions.
41. Canada East, New Brunswick, Nova Scotia, etc.
42. Canada West.
43. United States of America (General Map).
44. The Eastern or Principal States.
45. The Western States (California, Oregon, Utah, etc.)
46. Mexico, Central America, etc.
47. West India Islands.
48. South America.
49. Venezuela, New Granada, Equador, and Peru.
50. Chili--Argentine Republic, and Bolivia.
51. Brazil, Uruguay, and Guayana.


AUSTRALIA AND ISLANDS OF THE PACIFIC.

52. Australia.
53. New Zealand, Tasmania, and Western Australia.
54. Polynesia and Pacific Ocean.
55. The World as known to the Ancients.
56. The Principal Countries of the Ancient World, with the
    Roman and Persian Empires.

_Accompanied by Sketch Maps of the Federal and Confederate States, and
of a portion of Mexico._

       *       *       *       *       *

EDINBURGH: ADAM AND CHARLES BLACK.






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