Humus

Blackish matter composed of organic matter, resulting from the partial decomposition of plant debris, which is formed in the soil. The evolution of humus depends for a large part on the oxygenation of the environment. In an oxygenated environment, it can produce sludge (more or less dependent on the underlying soil, its pH, and the nature of the foliage that feeds it); in a non-oxygenated environment, it can give more or less acid and anoxic peats.