Soil organic matter (SOM) is the organic matter component of soil, consisting of plant and animal residues at various stages of decomposition, cells, and tissues of soil organisms, and substances synthesized by soil organisms.
The following are factors affecting decomposition of soil organic matters:
- Temperature: Cold periods retard plant growth and OM decomposition. Warm summers may permit plant growth and humus accumulation.
- Soil moisture: Extremes of both arid and anaerobic conditions reduce plant growth and microbial decomposition. Near or slightly wetter than field capacity moisture conditions are most favourable for both processes.
- Nutrients: Lack of nutrients particularly N slows decomposition.
- Soil PH: Most of the microbes grow best at pH 6 – 8, but are severely inhibited below pH 4.5 and above pH 8.5.
- Soil Texture: Soils higher in clays tend to retain larger amounts of humus.
- Other Factors: Toxic levels of elements (Al, Mn, B, Se, Cl), excessive soluble salts, shade, and organic phytotoxins in plant materials.