Organic matter affects both the biological, chemical, and physical properties of the soil and hence its fertility and health. Some of the soil properties influenced by organic matter are: diversity and activity of soil organisms (beneficial and harmful ones); nutrient availability; soil structure; and water holding capacity.
It also influences the effects of chemical amendments, fertilizers, pesticides and herbicides. Freshly added or partially decomposed plant residues and their non-humic decomposition products usually constitute a bile organic matter pool.
The labile soil organic matter pool regulates the nutrient supplying power of the soil, particularly of nitrogen, whereas both the labile and stable pools affect soil physical properties, such as aggregate formation and structural stability.
When crops are harvested or residues are burned, organic matter is removed from the system. However, retaining plant roots in the soil and leaving crop residues on the surface can minimize the loss.
Organic matter can also be restored to the soil through growing green manures, cuttings from agroforestry species, and the addition of manures and compost. Soil organic matter is the key to soil life and the diverse functions provided by the range of soil organisms.
Effect of organic matter on soil biological properties
Soil microorganisms are of great importance for plant nutrition as they interact directly in the biogeochemical cycles of the nutrients.
Agricultural production systems in which residues are left on the soil surface and roots left in the soil and the use of cover crops stimulate the development and activity of soil microorganisms.
It has been reported that the incorporation of crop residues into the soil is beneficial to soils, improving one or more essential soil attributes.
It has been shown that incorporating crop residue in a 19-year experiment in Brazil resulted in a 129% increase in microbial biomass carbon and a 48% increase in microbial biomass nitrogen.
The roots of most plants are colonized by Arbuscular mycorrhizal fungi (AMF), which form a network of mycelia or threads on the roots and extend the surface area of the roots. This allows absorption of water and nutrients by the plants beyond the depleted zone.
Factors that might stimulate the AMF development are the increase in organic carbon in soil and the rotation of crops with cover crop/green manure species.
In an undisturbed soil ecosystems, e.g. in conservation agriculture, colonization with AMF increases strongly with time compared with colonization under natural vegetation.
Fine roots are the primary sites of mycorrhizal development as they are the most active site for nutrient uptake. This partly explains the increase in mycorrhizal colonization under undisturbed situations, as rooting conditions are far better than under conventional tillage.
Another effect of increased organic matter content in soil is ian ncrease in the earthworm population. Soil moisture is one of the most important factors that determine the presence of earthworms in the soil. Through cover crops and crop residues, evaporation is reduced and organic matter in the soil is increased, which in turn can hold more water.
Residues on the soil surface induce earthworms to come to the surface in order to incorporate the residues in the soil. The burrowing activity of earthworms creates channels for air and water; this has an important effect on oxygen diffusion in the root zone and the drainage of water from it.
Furthermore, nutrients and amendments can be distributed easily and the root system can develop, especially in the acid subsoil in the existing casts. The shallow-dwelling earthworms create numerous channels throughout the topsoil, which increases overall porosity, and thus bulk density.
Read Also: Role of Organic Matter in Soil Fertility
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