The chemical and biochemical breakdown of a complex substance into its constituent compounds and elements, releasing energy, and often with the formation of new, simpler substances. Organic decomposition takes place mostly in or on the soil under aerobic conditions. Dead plant and animal materials are consumed by a myriad of organisms, from mice and moles, to worms and beetles, to fungi and bacteria. Enzymes produced by these organisms attack the decaying material, releasing water, carbon dioxide, nutrients, humus, and heat. New microbial cells are created in the process.

Decomposition is a major process in nutrient cycling, including the carbon and nitrogen cycles. The liberated carbon dioxide can be absorbed by photosynthetic organisms, including green plants, and made into new tissue in the photosynthesis process, or it can be used as a carbon source by autotrophic organisms.

Decomposition also acts on inorganic substances in a process called weathering. Minerals broken free from rocks by physical disintegration can chemically decompose by reacting with water and other chemicals to release elements, including potassium, calcium, magnesium, and iron. These and other elements can be taken up by plants and microorganisms, or they can remain in the soil system to react with other constituents, forming clays.