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

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

Nature of Soil. Crop. No. of cubic inches of air in 34 cubic inches of soil100 VOLUMES OF AIR CONTAIN
Carbonic acid. Oxygen.Nitrogen.
Light sandy soil, newly manured ... 8·0 2·17 ... ...
Do. manured 8 days before ... ... 1·54 18·80 79·66
Do. long after manuringYellow turnip 7·9 0·93 19·50 79·57
Very sandyVineyard 9·6 1·06 19·72 79·22
Sandy, with many stonesForest 4·0 0·87 19·61 79·52
Loamy ... 2·4 0·46 ... ...
Sandy, subsoil of the last ... 3·0 0·24 ... ...
Sandy soil, long after manuringTrefoil 7·6 0·74 19·02 80·24
Do. Recently manured ... ... 0·85 19·41 79·74
Do. manured 8 days before ... ... 1·54 18·80 79·66
Heavy clayJerusalem artichoke 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.