Soil organic matter is the fraction of the soil that includes plant and animal residues.
The residues are contained at various stages of decomposition, plant roots, cells and tissues of soil organisms, and substances synthesised by the soil population.
Fresh or undecomposed plant materials such as litter, straw and animal dung lying on the soil surface are not included in this definition although, upon decomposition, these will eventually become part of the soil organic matter.
Organic matter is largely made up of the elements carbon, hydrogen, oxygen, nitrogen, phosphorus and sulphur. It is expressed as a percentage of the soil mass less than 2 mm in diameter.
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