Humus is the organic matter above the topsoil and below leaf litter which is finely divided having a high surface area. It is derived from decomposition of plant and animal substances. Humus, which ranges in colour from brown to black, is largely composed of Carbon, and contains high amounts of Nitrogen, and smaller amounts of Phosphorus and Sulfur. Humus retains moisture in the topsoil, in particular in soils with a coarse texture (e.g. sand). Humus is the Latin word for "earth" or "ground".

Organisms which feed off the litter layer serve a key role in the formation of Humus, known as Humification. Earthworms are a major contributor to this process, alongside many other invertebrates, microbial processes, and Fungi.

Only the decomposed organic matter in soil is considered humus, while the portion still in the process of decomposition is referred to as detritus. Since the respective stages of transformation are gradual, a precise distinction is not possible. Neither humus nor detritus is dead matter; both are heavily permeated by microorganisms such as soil bacteria and fungi.

In agriculture, "humus" sometimes also is used to describe mature or natural compost extracted from a woodland or other spontaneous source for use as a soil conditioner. It is also used to describe a topsoil horizon that contains organic matter (humus type, humus form, or humus profile). Humus has many nutrients that improve soil health, nitrogen and phosphorus being the most important. The ratio of carbon to nitrogen (C:N) of humus commonly ranges between 8:1 and 15:1 with the median being about 12:1. It also significantly improves (that is, decreases) the bulk density of soil. Humus is amorphous and lacks the cellular structure characteristic of organisms. The solid residue of biological wastewater treatment is also called humus. When not found to be contaminated by pathogens, toxic heavy metals, or persistent organic pollutants according to standard tolerance levels, it is sometimes composted and used as a soil amendment.