Dr Tasneem Mubarak

Soil organic matter (SOM) refers to all materials of plant and animal origin formed in or added to soil regardless of stage of decomposition. When we talk about sustainable agriculture, organic matter in soil has a key role to play. However, there is still not enough awareness among farmers about the benefits of SOM and use of organic manures. This essay may benefit the readers, especially those interested in agriculture, in understanding the immense value of organic manures in crop production.
The adverse effect on soil health and environment safety of indiscriminate use of chemicals in agriculture can only be countered with best use of natural resources. At the same time, these natural resources must optimise production and minimise cost. Soil organic matter can help maintain soil health, particularly under intensive agriculture. It is like a bank account which pays back with interest. SOM should not be valued only as a source of nutrients but its benefit is the improving of soil health. Chemical fertilisers are more efficiently utilised in healthy soil but continuous use of chemicals alone deteriorates soil health, depletes organic matter and nutrients, and causes soil nutrient balance to become increasingly negative. This leads to declining yield and quality of crop. The soil organic components, together with microorganisms, help in binding small soil particles into larger aggregates. This way they improve soil aggregation, which is important for good soil structure, aeration, water infiltration and resistance to erosion and crusting.
The resistant fraction of soil organic matter (humus) contributes mainly to nutrient holding capacity of soil. Numerous studies suggest that addition of organic matter to soil not only improves soil fertility but also sustains better crop productivity. From a practical standpoint, SOM is important for two main reasons. First it acts as a nutrient reservoir and second, as an agent to improve soil structure. It is the main indigenous source of nitrogen in soil. In non-fertilised soils almost all of the nitrogen is tied up with the organic matter fraction. Commonly, 20%-50% of the total phosphorus in soil is present in organic combination, which can be as high as 75%. The proportion of sulphur in organic fraction varies appreciably but in productive non-saline soil, half or more is likely to be in such combination.
Besides being a source of major nutrients, organic matter also contributes trace elements which are as important, though required in very small quantity. Use of organic manures can help farmers curtail costs otherwise incurred on purchase of expensive multi-nutrients.
Another important property of organic matter is its influence on biological activities in the soil as it adds a variety of microorganisms and stimulates the multiplication of native soil organisms. The metabolism of soil microorganisms produces cementing substances called “bacterial cement”, which helps in soil aggregation. The increased microbial population causes better crop growth and yield due to improvement in soil structure, reduced pest pressure, and production of plant stimulants analogous to vitamins and hormones. Diverse residue sources sustain a microbial community that is efficient and has more diversity. SOM also counteracts the adverse effects of chemical fertilisers and thus improves plant nutrition.
There are numerous other benefits as well. Studies suggest that SOM plays a role in pest management. The mechanism of insect and pest control as exerted by organic matter is, however, less understood. Since SOM adds numerous microorganisms to soil, which are believed to act as efficient competitors for resources for soil-borne pathogens, the chances of diseases caused by such microorganisms may be reduced. SOM may also help in reducing weed population. This may be attributed to fast multiplication of bacteria which degrade and consume weed seeds. Population of predators like crickets and beetles, which are involved in weed seed predation, is also enhanced by increasing ground cover (organic residues). The distribution of weeds in a field also has some links with varying soil properties. In a nutshell we can say that SOM is to soil is what blood is to us.

The writer is Senior Scientist & Head, KVK Kulgam, SKUAST-Kashmir

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