Soil organic matter is the collective term for all the material found in the soil that comes from microorganisms, plants and animals. Organic matter consists largely of complex molecules of carbon (C), oxygen (O) and hydrogen (H). It also contains other organic substances (e.g. proteins and amino acids) which include nitrogen (N), phosphorus (P) and sulfur (S).
As a guideline, carbon makes up about 50% of the organic matter, however, this percentage varies widely (between 30 and 70%). The actual carbon content depends on factors such as the origin of the organic matter and the type of soil.
Organic matter enters the soil through means such as the supply of crop residues (leaves, stems and roots), animal manure, green manuring and compost. Bacteria, fungi and other soil organisms break it down until indigestible residues remain. The decomposition process occurs in several steps involving all organisms of the soil food web. Decomposition is rapid at first and then slows down. It can take decades for freshly applied material to be fully converted to stable organic matter.