CHAPTER V._Elements of Agricultural Chemistry

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--

----------------------------------------------------------------------------

------------------|--------------|-------------|-------------|--------------|

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.

|----------------------|-------------------|-------------------

|----------------------|--------|----------|---------|---------

|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

|Organic matters | 0.1481 | 0.2228 | 0.0608 | 0.1342

| | -------|----------|-------------------

| | 0.2319 | 0.2630 | 0.1191 | 0.2661

|----------------------|--------|----------|---------|---------

|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

| |--------|----------|---------|-------

|Silica | 71.3890| 82.5090 | 63.1400 |61.4200

|Alumina | 4.7810| 3.5120 | 11.3500 |10.3400

|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

| |--------|----------|---------|-------

| | | | |

|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

| |========|==========|=========|=======

| 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.

+-----------------------------+---------+-----------+--------+

+-----------------------------+---------+-----------+--------+

| | | | |

| Lime | 0.07 | ... | ... |

| Magnesia | trace | ... | ... |

| Potash | 0.06 | ... | ... |

| Soda | 0.04 | ... | ... |

| Chlorine | 0.05 | ... | ... |

| Organic matter | 0.15 | ... | ... |

| | ---- | | |

| | 0.37 | | |

| | | | |

| 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 |

| | | | |

| | | | |

| 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 |

| | | | |

| 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--

+-----------------------------+----------+-----------+-----------+

+-----------------------------+----------+-----------+-----------+

| 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:--

+----------------------+-----------------+------------+-------------+

+----------------------+-----------------+------------+-------------+

| 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 |

| 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 |

+-------------------+------+------+------+------+------+------+------+

| 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:--

+--------------------+------------+---------------+---------------------------+

| Soil. | | inches of air +--------+--------+---------+

+--------------------+------------+---------------+--------+--------+---------+

| newly manured | ... | 8.0 | 2.17 | ... | ... |

| 8 days before | ... | ... | 1.54 | 18.80| 79.66 |

| after manuring | turnip | 7.9 | 0.93 | 19.50| 79.57 |

|Very sandy |Vineyard | 9.6 | 1.06 | 19.72| 79.22 |

| stones |Forest | 4.0 | 0.87 | 19.61| 79.52 |

|Loamy | ... | 2.4 | 0.46 | ... | ... |

| of the last | ... | 3.0 | 0.24 | ... | ... |

| after manuring |Trefoil | 7.6 | 0.74 | 19.02| 80.24 |

| manured | ... | ... | 0.85 | 19.41| 79.74 |

| days before | ... | ... | 1.54 | 18.80| 79.66 |

|Heavy clay |Jerusalem | 7.0 | 0.66 | 19.99| 79.35 |

|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:--

-----------------------------------------------------------------------------

------------------------|------------------------|----------|---------------|

|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

Schuebler, 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. Schuebler'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, Schuebler 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, Schuebler 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 | |

+-------------------+----------+-----------+-----------+------------+

| 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. |

--------------------+----------+----------+----------+----------+-------------

| 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 deg.. 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 Schuebler, 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,

Schuebler, and others, where the subject is minutely discussed.

FOOTNOTES:

[Footnote I: Transactions of the Highland and Agricultural Society, vol.

vi., p. 317.]