Soil organic matter

Soil, a living and dynamic ecosystem, is the basis of the food system. Healthy soil produces healthy crops which in turn is responsible for human health. Soils are categorized as mineral soil or organic soil based on their organic content. Mineral soils contain less than 30% organic matter and are cropping soils thus forming most of the worlds cultivated land. On the other hand, organic soils are rich in organic matter mainly due to climatic reasons.
It is important to understand the difference between organic material and organic matter. A pile plant and animal material such as leaves, manure and other organic wastes dumped into the soil is not called organic matter, but organic material. Organic material, when acted upon by various soil microorganisms and decomposed into humus becomes organic matter. Organic material is unstable whereas organic matter is stable in the soil. Soil organic matter comprises microorganisms (10-40%) and stable organic matter (40-60%). The amount of organic matter is greatly influenced by temperature, soil moisture, water saturation, soil texture, topography, salinity, acidity, vegetation and biomass production.

Main functions of organic matter

The two main functions served by soil organic matter are:

1. It acts as a storehouse of all of the plant nutrients since it is largely derived from plant residues
2. The humus adsorbs and retains nutrients in a form that can be availed by the plant

The Role of Microorganisms in Plant nutrition

The chemical elements that are essential for the plant growth are carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, sulphur, calcium, magnesium, iron, manganese, boron, zinc, molybdenum, nickel, copper, cobalt and chlorine. Except for carbon and oxygen, all essential elements are acquired from the soil and hence majorly governed by the presence of organic matter. The nutrients present in the organic matter cannot be consumed by plants as such since they exist as large organic molecules. Soil organisms play a vital role in breaking down these larger molecules of the organic matter into the forms that can be directly used by the plants through a process called mineralization. The by-products produced in the course time also soil organic matter, though it is not as decomposable as the original plant and animal material
Soil organisms play varied roles in plant nutrition and growth. Maintenance of soil structure, regulation of soil hydrological processes, gas exchange, carbon sequestration, soil detoxification, nutrients cycling, defence against pests, parasites, and diseases, symbiotic and asymbiotic association with plants and roots and plant growth control are some of the essential functions performed by different soil organisms.

Benefits of organic matter

1. Plant nutrition: As discussed previously, organic matter is a powerhouse of nutrients that are released into the soil. It was found that the nutrient release majorly occurs in the spring and summer, so summer crops are more benefitted that winter crops from the mineralization.
2. Water retention: Organic matter has the capacity to absorb and retain water equivalent to up to 90% of its weight. Most of the water held by the organic matter is absorbed by the plants whereas water absorbed by clay is unavailable for the plants
3. Soil structure aggregation: Organic matter results in clumping of the soil to form aggregates. Soil aggregates improve the structure of the soil and consequently its water retention
4. Prevention of soil erosion: It was found that an increase in soil organic matter from 1 to 3 % would result in lessening erosion up to 20-33%. This is again attributed to its ability to form soil aggregates which in turn increases water infiltration.

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