Organic matter refers to carbon-based compounds and structures that are derived from living organisms. Organic matter contains carbon in a reduced form as well as other elements that serve as plant nutrients. It includes living organisms, although in soil the term is generally used to mean the breakdown products of organisms and their wastes. The non-living organic matter may be in various stages of decomposition, on a continuum from fresh residue to humus. “Humus” is organic matter in its most decomposed state, retaining only the most basic biochemical structures from the original material and its metabolic byproducts. Most organic matter in native soil is not visible, occurring primarily as humus coatings on mineral particles, except that any given soil tends to appear darker as
organic matter content increases. However, soil organic matter content can be quantified in the laboratory.