Nonliving, finely divided organic matter in soil, derived from microbial decomposition of plant and animal substances. Ranging in colour from brown to black, it consists primarily of carbon but also contains nitrogen and smaller amounts of phosphorus and sulfur. As it decomposes, its components are changed into forms usable by plants. Humus is classified according to how well it is incorporated into the mineral soil, the types of organisms involved in its decomposition, and the vegetation from which it is derived. It is valued by farmers and gardeners because it provides nutrients essential for plant growth, increases the soil's water absorption, and improves soil workability.

This entry comes from Encyclopædia Britannica Concise.
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