SOILS consist of the disintegrated materials of the hard crust of the earth, mixed with decayed vegetable matter. This disintegration is effected partly by the chemi cal action of oxygen, carbonic acid, and the other acid or alkaline substances brought by the atmosphere to_bear upon rocks, and partly by the wearing action of water in a fluid state or in the form of glaciers, or by its bursting force when frozen in deep clefts. The soils produced by running water, floods, and tides, are found along the banks or at the mouths of rivers, and are generally called alluvial soils; those produced by glacier action 'are known as drift soils; and both are generally found at a great disiance from the rocks of whose disintegrated materials they are composed. But by far the greater mass of soil has been produced in the other way aliove mentioned, by the gradual weathering of rock under atmospheric influence: and it is generally found adjoining or overlying the rocks from which it has been produced. Immediately beneath the soil or stratum of earth which affords nourishment to plants, is a mass of earth or rock, unmixed witlOclecayed vegetable matter, to which the term subsoil is applied. The sub soil may or may not be similar in its geological constitution to the soil; and from the absence of vegetable matter, is generally lighter in color than the latter.
. Every species of rock has produced its soil; but the older formations, from their greater hardness and power of resistance to atmospheric action, produce, in proportion to their exposed surface, less soil than do the secondary and tertiary groups. The fer tility of soils has no relation to the chronological succession of the strata of the earth's crust; thus, igneous rocks produce a naturally fertile soil, though they seldom become thoroughly disintegrated; metamorphic or transition rocks furnish one of poor quality,, as does also the greater portion of the Silurian system; while to the vast mass of the secondary group of deposits, •especially the Devonian system, with its old red sandstone, and limestone, and marl beds, the mountain limestone of the carboniferous system, and the new red sandstone of the Permian and Lriassic systems, belong some of the richest tracts in great Britain, though numerous members of the same group supply barren and ungrateful soils. The has, and oolitic,
and wealden systems generally supply clay-soils of considerable fertility, hut of the densest texture and most intractable character; soils formed from the cretaceous group are extremely variable in qUality; but when the chalk is largely mixed with sand or clay, they exhibit a considerable degree of fertility; however, they have one great gen eral defect, that of not sufficiently retaining moisture. The soils produced from the ter tiary formations possess no general characteristics, being sometimes extremely fertile, and again almost wholly barren; and, in short, we are bound to come to the conclusion that the mere geological composition of soils affords no very reliable criterion by which their economic value can be estimated; the same rock which produces the almost barren soil of Argyleshire, weathers into the fertile soil of the Channel islands; and to the old red sandstone is due at once the rich soil of Hereford, Monmouth, Moray, and Strath more, and some of the most barren heaths and moors in Scotland. These apparent anomalies are no doubt largely produced by the various action of heat, moisture, and other meteorological agencies.
But however soils may vary in a geological point of view, they are all resolvable into a few elements—viz., the various compounds of aluminium, iron, manganese, the four alkaline metals, thesseven alkaline earths, and the four organic elementary sub stances. These eighteen bodies supply, singly or in combination, all the constituents necessary to the growth of plants, each of them having its own portion of the plant to sustain—the silica producing strength and rigidity in the stems; alumina giving tenac ity to the soil, and so rendering it a stable support; magnesia perfecting the seeds; iron absorbing oxygen and ammonia from the atmosphere, and giving it up as required; and so on. Of these ingredients, silica, alumina, lime, along with matter derived from organic bodies, constitute the bulk dot the soil; the _other ingredients existing only in minute quantity, and hence is derived the common quadruple division of soils into siliceous or sandy, argillaceous or clayey, calcareous, and burnous.