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Organic Matter

Soil organic matter is an important component of soils; although it constitutes only about 5% of total soil composition of mineral soils (it could constitute as much as 95% in organic soils like peat, muck etc). Along with mineral matter, it constitutes the soil solids.


Soil organic matter (especially humus) is important because its water holding as well as its nutrient holding capacity greatly exceeds that of clay, its inorganic counterpart. Therefore, small amount of humus increases the soil’s ability to support plant growth. In addition, it is involved in many chemical reactions that take place in the soil.

Contributions towards soil organic matter comes from all biological organisms, whose bio-products and/or remains are deposited on or in the soil. This is the reason why soil organic matter levels may be higher in areas with higher population of living organisms.

Soil organic matter is composed of the following:

     –    All freshly deposited bio-materials of plants, animals, microorganisms, and other living organisms,

     –     Deposited bio-materials that are partly decomposed, and

     –    Humus, the fully decomposed product of all deposited bio-materials – it is dark coloured (usually black or brown) and colloidal.


Soil organic matter is dynamic both in amount (percentage of total soil composition) and in its composition (the materials that make up soil organic matter). There are several reasons for this and some of the reasons are:

     –   Deposition of bio-materials is affected by seasonal variations and its effects on living organisms (the depositors).

   –  Soil organic matter is constantly being transformed from freshly deposited and identifiable bio-materials, into humus through the process of decomposition by soil microorganisms.

     –     Mineralization – the release of oxide solids through decomposition of organic matter – reduces the levels of organic matter in soils.

     –     Soil organic matter is also involved in the nutrient cycle of several nutrients such as the Nitrogen, Phosphorus and Sulphur cycles.


The above, are also reasons for the rare/no accumulation of soil organic matter in soils. However, a large accumulation of soil organic matter can only occur under conditions unfavourable to decomposition e.g. anaerobic conditions or extreme cold. The accumulation of organic matter under these conditions often leads to the formation of organic soils.

Some of the major functions of soil organic matter are:

   –     It is a major source of plant nutrients, supplying trace elements like copper (Cu), Molybdenum (Mo), Zinc (Zn), Manganese (Mn), Iron (Fe), Boron (B), and Chloride (Cl) alongside some of the essential nutrients like Nitrogen (N), Phosphorous (P), and Sulphur (S).

     –     It acts as a binding agent, promoting the development of good soil structure.

     –     It increases the water holding capacity of soils through its granulating effect on the soil.

   –  It is the primary source of energy for soil microorganisms, thereby encouraging the biological and biochemical processes operating in the soil.






Further Studies in this theme:

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