SOIL. Wherever the surface of the earth is not covered with water, or is not naked rock, there is a layer of earth, more or less mixed with the remains of animal and vegetable substances, in a state of decom position, which is commonly called the soil.
The nature and composition of the soil, and consequently its greater or less aptitude to the growth and maturity of vegetable productions, depend on the composition, the proportion and the mechanical structure of the various substances of which it consists. When the soil is favourable to the chemical action by which the elements are combined to form vegetable substances, and admits that quantity of air and moisture without which this chemical action cannot take place in any given climate or temperature, vegetation goes on rapidly, and all the plants which are suited to the climate grow in the greatest perfection, and bear abundant fruits.
It is not however very frequently the case that a poll possesses all those qualities on which great fertility depends. So many circum stances must concur to make a soil highly fertile, that the great majority of soils can only be made to produce abundantly by being improved by art both in their texture and composition. Hence the practice and science of agriculture, which is founded on experience, but to which every progress in science also affords great assistance, by the additional light which every new discovery throws on the true theory of vegetation.
There are various modes of distinguishing soils, without entering into a minute analysis of their component parts. The simplest and most natural is to compare their texture, the size and form of the visible particles of which they are composed, and to trace the probable source of their original formation from the minerals which are fouud around or below them, or the rocks from which they may have been slowly separated by the action of the elements. The science of geology, which teaches the relative position and nature of the minerals of which the outer crust of the earth is formed, is consequently of the greatest utility in aiding us to compare different soils and in ascertain ing their composition.
The knowledge which geology imparts is however not sufficient for the minnter classification of soils; for it is found by experience that the soils which lie over or near the different strata, as they appear near the surface, vary greatly, although they retain some general character which distinguishes them from others. The streams which descend from the hills, and flow towards the valleys, and through them to the sea, carry to a great distance the minuter portions of the minerals which they flow over in their course, while the larger and heavier are deposited much sooner. Hence the heterogeneous mixture
of various earths and stones, and their stratification in thin layers, as is often found when a soil is examined which has never been disturbed by cultivation.
It is not sufficient to class soils according to the substance which predominates, as has been usually done, such as sandy, gravelly, chalky, or clay soils ; for this gives very imperfect information respecting their nature or fertility ; neither is it altogether sufficient to class them according to any particular geological formation.
The soils which have been evidently formed from the rocks which are supposed to be of secondary formation are fertile according to the proportion of the earths of these rocks which they contain.
Argillaceous earth exists in some proportion in almost every rock. It has the property, when mixed with other substances, as silica or lime, of fusing into a stone of great hardness and insolubility. In this state its effect on the soil is not to be distinguished from that of silica; and by burning common clay, or clay mixed with carbonate of lime, a sandy substance is produced resembling burnt brick, which tends greatly to improve the texture of those clays which contain little or no sand in their composition. It must be remembered that the stiffest clays contain a large portion of silica in an impalpable state ; but this, instead of correcting their impermeable and plastic nature, rather adds to it. It is only palpable sand which with clay forms what is commonly called loam, and which, when the sand is in due proportion with a mixture of organic matter, forms the richest and most easily cultivated soils. Some of the rocks of secondary formations contain a considerable portion of alumina and lime ; and when these earths meet with crystallised sand, a compound, or rather a mixture is formed, which has all the requisite qualities, as to texture, to produce the most fcrtilo learns. The only deficiency is that of organic matter ; but this is so readily accumulated wherever vegetation is established, or can he so easily added artificially, that these learns may always be looked upon as the most favourable soils for the usual agricultural operations; and if a considerable depth of loam is found, which neither retains water too long nor allows it to percolate too rapidly, it may be looked upon as a soil eminently capable of the highest degree of cultivation, and on which no judicious outlay of labour will ever cause loss or disappointment to the farmer.