The organic part of soils is derived chiefly from the remains of vegetables and animals which have lived and died in or upon the soil, which have been spread over it by rivers and rains, or which have been added by the hand of man for the purpose of increasing its natural fertility. This organic part varies very much in quantity in different soils. In some, as in peaty soils, it forms from 50 to 70 per cent. of their whole weight, and even in some rich long cultivated lands it has been found, in a few rare cases, to amount to as much as 25 per cent. In general, however, it is present in much smaller proportion, even in our best arable lands. Oats and rye will grow upon a soil containing only 1½ per cent., barley when 2 to 3 are present, while good wheat soils generally contain from 4 to 8 per cent. In stiff and very clayey soils 10 to 12 per cent. may occasionally be detected. In very old pasture lands and in gardens, vegetable matter occasionally accumulates, so as to overload the upper soil. To this organic matter in the soil the name of _humus_ has been given by some writers. It contains or yields to the plant the ulmic and humic acids described in a previous chapter. It supplies also, by its decay, in contact with the air which penetrates the soil, much carbonic acid, which is supposed to enter the roots and minister to the growth of living vegetables. During the same decay ammonia is likewise produced,—and in larger quantity, if animal matter be present in considerable abundance,—which ammonia is found to promote vegetation in a remarkable manner. Other substances, more or less nutritious, are also formed from it in the soil. These enter by the roots, and contribute to nourish the growing plant, though the extent to which it is fed from this source is dependent, both upon the abundance with which these substances are supplied, and upon the nature of the plant itself, and of the climate in which it grows. Another influence of this organic portion of the soil, whether naturally formed in it, or added to it as manure, is not to be neglected. It contains,—as we have seen that all vegetable substances do,—a considerable quantity of inorganic, that is, of saline and earthy matter, which is liberated as the organic part decays. Thus living plants derive from the remains of former races buried beneath the surface, a portion of that inorganic food which can only be obtained in the soil,—and which, if not thus directly supplied, must be sought for by the slow extension of their roots through a greater depth and breadth of the earth in which they grow. The addition of manure to the soil, therefore, places within the easy reach of the roots not only organic but inorganic food also. SECTION II.—OF THE INORGANIC PART OF SOILS.