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 str...