Humus

Humus, decaying organic matter found in soil and derived from dead animal and plant material. During early decomposition, some of the carbon, hydrogen, oxygen, and nitrogen are quickly dissipated as water, carbon dioxide, methane, and ammonia, but the other constituents decompose slowly and remain as humus. The chemical composition of humus varies, because it depends on the action of living soil organisms, such as bacteria, protozoa, fungi, and certain kinds of beetles, but usually it contains varying amounts of proteins and certain uronic acids combined with lignins and their derivatives. Humus is a homogeneous, amorphous, dark-colored, and practically odorless material. The end products of the decomposition of humus are mineral salts, carbon dioxide, and ammonia.

As humus decomposes, plant residues are converted into stable forms that are stored in the soil and are usable by plants as food. The amount of humus in soil also affects such important physical properties of the soil as structure, color, texture, and moisture-holding capacity. The ideal development of crop plants, for example, depends largely on the humus content of the soil. In areas where cultivated plants are grown, humus is constantly being depleted from the soil by the succession of crops, and the organic balance of the soil is restored by adding humus in the form of compost or manure.

See also Mulch; Soil Management.