humus, organic matter that has decayed to a relatively stable, amorphous state. It is an important biological constituent of fertile soil. Humus is formed by the decomposing action of soil microorganisms (e.g., bacteria and fungi), which break down animal and vegetable material into elements that can be used by growing plants. Technically, humus, as the end result of this process, is less valuable for plant growth than are the products formed during active decomposition (see fertilizer). Because of its low specific weight and high surface area, humus has a profound effect upon the physical properties of mineral soils with regard to improved soil structure, water intake and reservoir capacity, ability to resist erosion, and the ability to hold chemical elements in a form readily accessible to plants.

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.

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