The formation of humus begins when the residues from plants and animals comes in contact with microbial life in the soil. Much of the carbon compounds contained in those residues are proteins, carbohydrates and energy for the various bacteria, fungi and actinomycetes involved in the decay process.
Aerobic microorganisms are the most adept at decomposing organic matter.
They need an environment where there is an adequate amount of free oxygen to live and to be active. The degree to which free oxygen exists in soil plays a major role in regulating the favorable or unfavorable conditions under which humus is formed. The same is true for the amount of moisture, for the soil temperature and for the carbon to nitrogen ratio of the residues being decomposed. As the temperature of a soil increases there is a corresponding increase in microbial activity. Soils that exist in warmer regions of the earth tend to have lower average levels of humus than soils in colder areas.