Soils. Soils are generally said to be derived from our primitive rocks by that disintegrating process called weathering (see DENUDATION). The doctrine that commonly obtains is that lichens, the first occupants of the thin initial layer so formed, contributed by their life and death in turn to soil formation, and thereby made life possible for the mosses. These in like manner yielded their increase, and rendered it possible for plants of a still higher order to grow and flourish, and so on, until perfect soils were produced in which all plants might luxuriate. It is perhaps convenient to adopt the lichens as the starting-point; but it would probably be more accurate to presume that these were preceded by other forms, for the origin of soils may indeed have been the origin of life itself, and until we can clearly define the one there must of necessity be indefiniteness about the other. Recent experiments go to show that sterilised soils are infertile soils; and if that be an unassailable doctrine, then it follows that micro-organisms aided in the formation of that soil which was sufficient for the growth of the lichen. The origin of soil organisms must be left to the bacteriologist to discuss, but it may not be out of place to state here that the growing, sowing, and feeding of the desirable soil germs is of as much importance to the agriculturist of to-day as is the sowing of seeds, or the growing and feeding—by manuring—of plants; indeed, it may almost be asserted that manures applied to soils do not always act—as they do in water-culture experiments—as direct plant-food, but rather as food for those soil bacteroids which are the great agricultural workers, or preparers of food for plants. It may be affirmed that it is quite as necessary for the agriculturist to have certain conditions of soil—physical and chemical—which are essential to the growth and working of the desirable germs, and accordingly essential for the growth of good crops, as it is for the brewer to have those more or less definite physical and chemical conditions which are essential to the growth and working of the yeast plant in the production of good beer. Moreover, what is universally stated in text-books as being due to a 'selective power' of plants is entirely ascribable to the biological condition of the soil; and far from its being a power of selection or instinct possessed by plants, these have no choice in the matter.
Popularly speaking, the breaking down of rocks—by weathering—results in the building up of soils, and the composition of soils so formed must vary in proportion to the kind and number of minerals employed in the process. The principal minerals so employed are felspar, quartz, mica, talc, lime-stones (including chalks, marls, &c.), hornblende (amphibole, angite, olivine, &c.), clays, and zeolites. Soils formed from the rocks underlying them are designated sedimentary, while transported soils are those derived from rocks at higher levels: thus, if carried down by glaciers they are termed drift soils, if carried by running water they are known as alluvial soils, and the combination of these two agents results in co-alluvial soils. Anderson classified soils according to 'their general physical characters, and the ordinary mode followed in practice of dividing them into clays, loams, &c.' They are also frequently classed thus: siliceous or sandy, calcareous, argillaceous, and vegetable or peaty, while a somewhat elaborate subdivision of these is given in Schuber's classification.
Generally speaking, a mixed soil will possess important advantages over clay, chalk, or siliceous soils, and this mixing is performed by nature herself, as already described, where there is a dual outcropping of rocks; while the art of man effects what is practically the same thing by claying, liming, marling, &c. The chemical composition and physical conditions of soils have until quite recently been about the only features which received consideration, but it is now beyond doubt that the biological condition is of at least equal importance, for, in regard to a well-drained soil, sterility and infertility are synonymous terms. This new doctrine solves at once the problems which for many generations have been insurmountable—in such cases, for instance, as two soils having the same chemical composition, and one being fertile and the other barren. Another highly important consideration is that sterile soils are practically non-retentive; and if that be so, all the hitherto obtaining doctrines which have ascribed to silicates, oxides, &c. such unerring precision in forming new and definite but purely chemical relationships with added substances, such as phosphates, potash and ammonia salts, &c., must fall. It has been (and is still) customary for exponents of agricultural science to remark that it was a curious thing that the valuable nitrates were not retained by soils—were indeed easily washed out, and were more or less always to be found in drainage waters, while phosphoric acid, potash, or ammonia was rarely if ever so; but according to the germ theory there is nothing curious about it, and it could not be otherwise in a fertile soil. It is evident that many of the heretofore established certainties of soil science and of agriculture are destined to be overthrown. Capillarity, for instance, is doomed; for fertility of soil is incompatible with that condition, and it is scarcely compatible with drains operating at the lower end of the capillary tubes. Drainage dogmas also require modification, in so far, at least, as they declare the removal of water—which is surely antithetic to capillarity—and the opening up of a path for the entrance of atmospheric air to be the chief functions of drains. Plants can live in water, but not in an atmosphere of carbon dioxide; a fertile soil is as prolific a source of this gas as the brewer's fermenting tun, and but for the presence of drains—i.e. the removing per descensum of carbonic acid—no plant could grow. It is also maintained that the entrance of carbon dioxide is essential because of its function as a soil solvent; but from what has been said it is evident there is something wrong with the theories. Free entrance of oxygen to soils is necessary for root life, and that is the reason why removal of the over-abundant carbonic acid becomes imperative; but it is not the case that it is necessary to nitrification, and the leguminosæ can grow robustly in what is practically an atmosphere of carburetted hydrogen, so long as calcium carbonate is maintained in the surface soil. Strange as this may seem, it has been demonstrated on fields in Midlothian; and the fact goes to show that the nitrifying organisms in soils can produce from calcium carbonate all the oxygen required by them for their life and work. This, indeed, is one of the great functions of lime in soils. Lime cannot be replaced by magnesia in soils, nor magnesia by lime; thus in fruit formation lime cannot perform the functions of magnesia, while lime—in addition to its all-importance as a salifiable base—becomes the great carrier of food-stuffs into the plant, where again it is of paramount importance as a fixer of the acid product of the oxalic fermentation, in which rôle magnesia is useless.
An article on soil formation would be incomplete if reference were not made to the important part played by earthworms (Lumbricus terrestris especially; see EARTHWORMS); but while they bring up much valuable material from the subsoil, they are great robbers of lime from the surface soil.
The views above stated are more fully treated in a work on the subject by the present writer and Mr A. N. M'Alpine (1892). There are also works by Scott Burn, Fream, Scott and Morton, Johnson, Munro and Wrightson, Brannt, &c. See also AGRICULTURE, MANURES, NITRIFICATION, &c.