- Living organisms (soil biomass)
- The remains of microorganisms that once inhabited the soil
- The remains of plants and animals
- Organic compounds that have been decomposed within the soil and, over thousands of years, reduced to complex and relatively stable substances commonly called humus
As organic matter decomposes in the soil, it may be lost through several avenues.
Since organic matter performs many functions in the soil, it is important to maintain soil organic matter by adding fresh sources of animal and plant residues, especially in the tropics where the decomposition of organic residues is continuous throughout the year.