Humus
The name given to the black substance that results from the decay of plants in the soil. Earth containing much Humus is often called Vegetable Mould or Black Earth, on account of its colour. This substance contains all the mineral compounds that existed in the living plants, but it chiefly consists of compounds of carbon, along with oxygen and hydrogen. Nitrogen, also, is present, chiefly in compounds of ammonia. Decaying organic matter becomes broken up into several acids (humic, ulmic, crenic, &c.), which have a great power of absorbing ammonia from the air, or from its less stable compounds in the soil, and of forming with it substances of greater permanence and more suited to yield the nourishment that plants require. Hence Humus, which is largely made up of these acids, acts an important part in storing up ammonia till required by plants; and there seems reason to believe that it may even cause the production of ammonia by decomposing water (H2O), and setting free the hydrogen in it, in a state in which it readily combines with the nitrogen of the air to form ammonia (H3N), which then combines with the organic acids in the soil. Humus absorbs water readily, yielding it up as the plants require moisture. It has also been suggested that it may be of value because of the carbonic acid (CO2) formed in and emitted by it; but it is very doubtful whether the roots absorb that gas. It is certain, however, that the mineral compounds required by plants, and present in their decaying remains, are in a state better suited to be absorbed anew by growing plants than are the same compounds when derived only from the decomposition of rocks or inorganic soils. These properties explain why it is that Humus is beneficial to plant life, though, when present in excess, e.g., in certain kinds of peat, it renders the soil swampy, acid, and unsuitable for the growth of plants, except of a few kinds, chiefly rushes, sedges, and some grasses, none of which are of any value in cultivation. The organic matter (so called because it is derived from the decay of organised beings, i.e., of plants and animals) varies largely in amount in different soils. It is composed chiefly of Humus. In very poor soils, it may hardly be present at all. Good agricultural soils contain from 3 to 8 or 10 per cent. of it by weight. Old gardens, and other soils that have been long under careful cultivation (shown by their dark colour), may contain as much as 25 per cent.; and peaty soils may be almost entirely composed of it.
In the case of ordinary agricultural soils, if the produce is constantly removed, and none is returned to the soil, the Humus becomes exhausted, or so much diminished as no longer to supply the needs of the plants. Therefore, it is necessary to replace the missing substances as far as can be done; and this is most fully effected by the use of farmyard manure, in which is contained decaying organic matter fitted to restore the Humus that the crops have removed. Other manures are frequently employed to hasten the decay of plant remains in soil, and to increase the amount of Humus thereby; while others are added to supply only certain substances in which the soil is deficient, or to afford more stimulating food to plants than they could obtain from the soil for themselves.