It is not sufficient that soil possesses all the ingredients necessary for rendering it fertile. or that these ingredients are in a sufficiently comminuted state to enable them to be absorbed; there is besides a certain physical or mechanical condition necessary. Thus, for example, a soil which possesses too great a proportion of silica is too little retentive of moisture, and has not sufficient consistency of texture to be an effective support of tall plants; one in which calcareous matter abounds is also too dry a soil; while if alumina predominates, it is generally too retentive of moisture; and a great excess of the last-named ingredient renders it so extremely tenacious as to be almost incapable of reduction to a proper mechanical state. The soil which is physically most perfect is composed of about equal proportions of the two great ingredients, silica and alumina, and is generally known as loam, being distinguished into clay loam or sandy loam, according as the alumina or silica sensibly predominates. But the physical quali ties of soils do not wholly depend upon their composition: they are also largely affected by the depth of the soil itself, and the quality of the subsoil. Should the soil and sub. soil be both retentive, or both porous, the defects of these states as to dryness or moist ure are considerably increased; if porous and retentive soils of good depth rest upon subsoils of a contrary character, the defects of the former are to a considerable degree amended. But the advantages and disadvantages of these conditions must to a very large extent be judged by the prevalent character of the climate, a- somewhat porous subsoil in a cold moist district being generally preferable, and vice vend. Each of these classes of soils, when-possessed of the chemical ingredients in quantity sufficient for the wants of plants, and of a texture favorable to their growth, excels in the production of, certain species. Thus, the clay ]Dams are unequaled for the production of wheat and the sandy ]gams for barley, rye, and the various root-crops; while both are well suited for the growth of the other cultivated plants, or for perennial pasture.
Besides the calcareous and manly soils which may be, according to circumstances, classed as a clayey or sandy soil, rarely the former, there is the humous soil, which possesses characteristics peculiarly its own. It is not devoid of consistency like the
sandy, or retentive of moisture like the clayey soils, but in its natural state is spongy and elastic in texture, of a remarkably dark color, and, when dried, becomes inflam triable, and even when much improved by culture retains these characteristics iu a con siderable degree. It consists wholly, or to a great extent, of vegetable mattar, and is found in perfection in forests of ancient date, as the woods of America, and in the peculiar form of peat (q.v.) in many parts of the world. In its ordinarily decomposed condition, it is at once the richest of soils; but in the state of peat it calls for long continued drainage, and the application of decomposing agents, before it can be ren dered of service in the production of crops.
Improvement of a soil must, then, as is seen from the foregoing considerations, be effected either by supplying the substances required by plants to a soil which is deficient in them, by altering its depth and texture, and by removing excess, or supplying deficiency of moisture. The first of these objects is effected by the introduction and incorporation of manures (q v.) with the soil, care being taken that the manure contains the requisite ingredients, and in such a condition as to be assimilable by plants either directly or indirectly through the soil, and by the more thorough exposure of the soil to the action of the atmosphere; the second is effected by the admixture of marl or clay with sandy, chalky, or peat soils, of lime, ashes, or burned clay, with tenacious clay soils, or by the mixture of the subsoil (if differing in quality) with the soil by means of the subsoil plow. or by more complete surface-tillage, and free exposure to the action of. frost; and the third is accomplished by drainage (q.v.) and irrigation (q.v.). The fertility and chemical composition of a soil may be approximately determined by inspection of its color and texture; -but more accurately, as well as its dryness or moisture, excess or defect of silica and alumina, by the predominance of certain species of wild plants or weeds.