Humus (Lat. humus, the soil), vegetable mould, or the product of the decay of vegetable matter. When portions of a decayed stump or the decayed matter of peat is digested in a weak solution of caustic potash or soda, a brown liquid is formed, which on the addition of an acid deposits a dark brown precipitate. This is a mixture, according to Mulder, of three substances, which he considers as compounds of water, or of water and ammonia, with three different acids, viz.: 1, geic acid, C20H12O7; 2, humic acid, C20H12O6; 3, ulmic acid, C20 H14O6. It has been doubted, however, whether 1humus has so definite a composition. Mulder also found that the brown substances formed by the prolonged action of boiling dilute acids upon sugar resemble ulmic and humic acids derived from mould, both in chemical composition and properties. Humus may be regarded as in a state of continuous decomposition or eremacausis, a species of slow combustion (see Eremacausis), in which the hydrogen of the vegetable matter is more rapidly removed by oxidation than the carbon, so that it contains an excess of the latter element.

The formation of water, carbonic acid, and ammonia, and the elimination of mineral constituents in the decay of woody fibre is one cause of the beneficial action of vegetable manures in promoting the growth of plants.