Most soils have three major horizons—the surface horizon (A), the subsoil (B), and the substratum (C).Some soils have an organic horizon (O) on the surface, but this horizon can also be buried.
In this way the nitrogen (nitrogen cycle) and the other nutrients (nutrient cycle) are recycled. Depending on the conditions in which the breakdown is carried out, a fraction of the organic matter does not continue into mineralization, but instead goes in the contrary direction, forming new organic chains (polymers).
These organic polymers are stable, that is resistant to the action of microorganisms, and constitute humus.
Humus should be differentiated from decomposing organic matter.
The latter is rough-looking material and remains of the original plant are still visible.
This process is thought to have been important in the formation of the fertile Amazonian dark earths or Terra preta do Indio.