It is defined as the most stable component of soil organic matter. In biochemical terms, it is a polymeric, amorphous material, a consequence of the decomposition of organic matter, especially lignin.

In other words, the mix described above in which living and dead organic matter coexists, make some parts difficult to degrade. In addition to being insoluble in water, its longevity is very long, estimated to be at around 250 and 1500 for several parts of the humus in different conditions in temperate climate soils.