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ABSTRACT

Organic matter plays an important role in soil health. The amount of organic matter in soil is used as an indicator of the potential sustainability of a system. Farming practices that reduce soil organic matter such as burning, tillage, overgrazing and continuous cropping run the risk of contributing to a decline in soil condition. In this article factors related to soil organic matter content in soil and ways to enhance soil organic matter.

Organic matter in soil is the key to soil health. Soil health refers to the condition of the soil and its potential to sustain biological functioning, maintain environmental quality and promote plant and animal health. The soil is composed of approximately 90-98% minerals and only 2-10% organic matter (Gabriel. et al., 2018). The amount of organic matter in soil often used as an indicator of the potential sustainability of a system. The optimal level of soil organic matter for any given soil is one which supports the functional capacity of the soil to hold supply plant available water, store plant nutrients, provide energy for soil fauna, improve crop yields and moderate net greenhouse gas emission. Farming practices that reduce soil organic matter such as burning, tillage, overgrazing and continuous cropping run the risk of contributing to a decline in soil condition which may not become evident for many years.

Indicators of soil organic matter:

·  Soil smell- Earthy (Strong after fresh rain)

·  Soil color- Soil Organic Matter (SOM) can be directly related to a darkening of soil color.

·  High proportion of ground cover.

·  Presence of roots, earthworms etc.

 

Factors effecting soil organic matter:

1.       Climate:

Climate conditions (temperature and rainfall) control microbial activity and thus SOM decomposition, therefore the frequency and the timing of rainfall events in relation to temperature and organic matter placement is important. Moist, hot and well aerated conditions favor microbial activity and the rapid decay of organic additions.

2.  Organic Matter Input and Composition

If the rate of organic matter addition is greater than the ratio of loss, SOM will increase. Conversely, if the amount of organic matter retained is lower than the rate of loss, SOM will decrease. The amount of carbon input through plant root biomass is possibly the greater importance for increasing SOM than above ground biomass, as it is largely protected from loss. The retention of SOM is both soil and climate dependent.

  

3.     Microbial Activity

Soil organisms such as earthworms and ants breaks large pieces of organic debris into smaller pieces and are capable of incorporating surface residues deeper into the soil but the greatest concentration of organic matter remains in the top ten centimeters of soil and is associated with surface residues and prolific root growth. The microbial population rapidly contributes to the organic matter (Kuzyakov and Blagodatskaya, 2015). Soil conditions that decrease microbial activity also decrease the mineralization (breakdown) of organic matter. For example, change in soil pH can alter the biological activity, functions and survival of the microbial community because microorganisms prefer relatively neutral soil pH (6.5-7.5). As soils becomes more acidic, microbial activity and organic matter decomposition slow down.

 

  

4.       Residue load:

Maintaining ground cover through crop and pasture residues above 50% decreases the risk of erosion and subsequent soil loss as well as providing a significant nutrient store. High level of ground cover can decrease evaporation. Farming practices that break up soil aggregates such as cultivation expose new soil organic matter to decomposition and can accelerate the rate of breakdown, decreasing the amount of carbon stored in the soil. Similarly burning can rapidly affect the balance of soil carbon. The retention of stubble, pasture leys and zero tillage are the examples of the practices that increase the potential for carbon sequestration.

 Practices to increase soil organic matter:

Management practices to maintain or increase SOM in agriculture depend on practices increasing biomass production and minimizing SOM decomposition or loss. A combination of the following points should be considered while developing soil management plans that aim to improve soil organic matter. Organic matter is the fuel which drives the soil biota (Kuzyakov and Blagodatskaya, 2015) and should be retained to maximize nutrient cycling and improve soil aggregation. The following is the list of practices for maintaining SOM and developing a healthy soil organism.

 

i)        Increasing soil water:

Increasing soil moisture capture and effective use of rainfall and irrigation stimulates plant growth and consequently provides a greater input of organic matter to the soil.

 

ii)      Produce high biomass crops and retain stubble:

High inputs of organic matter or increased crop frequency will increase SOM (Poeplau and Don, 2015). However, unless maintained the soil will revert to a steady state equilibrium. Leaving stubble standing and in large size pieces will slow the decomposition rate, promote water conservation and reduce soil erosion. The burning of maize, paddy, wheat and other residues in the field is a common practice. These are usually burnt to help control insect/pest incidence and to make field work easier in the following crop season.

Burning destroys the little layer and so diminishes the amount of organic matter returned to the soil. Beside burning a lot of pollution problem persists for a lot of time.  Burning of crop residues is a common practice in Haryana and Panjab. Stubble burning decreases the amount of organic matter returned to soil. Burning removes lot of the soil carbon suitable for microorganisms, limiting microbial activity and decreasing the population size of soil biota. Rotate crop and pastures to provide a more diverse food source to support a larger, more diverse range of microorganisms. Legumes crops and green manures crops like dhaincha, clover, berseem, moong, urd and methi fix N from the atmosphere and provide a disease break as their residues have a lower C:N ratio and are more easily decomposed by soil biota than cereal straw.

 

iii)    Reduce or Eliminate Tillage:

Increased soil disturbance breaks down soil aggregates and exposes residues and other SOM to microbial decomposition and oxidation, accelerating the rate of SOM decline. Reducing tillage will promote soil aggregation and afford greater physical protection of SOM, while as zero tillage will maximize the potential to build up SOM. Cultivation disrupts the soil aggregates and exposes previously protected SOM to decomposition, providing an initial flush in microbial activity but then a longer decline associated with the loss of their SOM food source. Use of new generation technology like zero tillage and happy seeder sowing of wheat in standing stubble of paddy will help improve SOM in the long run.

 

iv)     Application of organic manures:

Crop residues, animal manure, composts and biogas slurry, press mud added to the soil can contributes both organic matter and nutrients. Where tillage is required to incorporate the manure, decomposition of soil organic matter is accelerated and may disrupts macropores, so zero till option should be considered.

 

v)       Appropriate use of Pesticides and fertilizers:

The appropriate use of herbicides may be less harmful to the soil environment and organisms, than traditional weed control techniques of cultivation and stubble burning. Soil microbial populations adapt to organic chemical inputs; therefore, pesticides are unlikely to have a long term negative impact where chemical application rates are low, or reapplication of the same pesticide within a short period is avoided. Reduce the use of copper based fungicides. Foliar applications are less damaging than those applied directly to soil. Use of Neem based botanicals can help in reduction of pest and disease population (Mondal et al., 2015) Fertilizers increase crop growth and residue inputs, indirectly supporting higher microbial population. High rates of mineral nitrogen fertilizers suppress nodulation an N fixation by legumes. Fertilizers input should be used to compliment N cycling from organic matter and enhances the activities of microorganisms. The efficiency of fertilizer use will be high where the organic matter content of the soil is also high. Where soil organic matter levels are restored, fertilizer can help maintain the nutrients in the soil by increasing crop yields and consequently, the amount of residues returned to the soil. 

Conclusion:

Practices in farming that reduce soil organic matter such as burning, tillage, overgrazing and continuous cropping run the risk of contributing to a decline in soil condition which may not become evident for many years. Organic matter helps determining the porosity of soil, aiding in water entry and storage and helps keep soil particles separated by improving soil aggregation and structure. Soils high in organic matter tend to have fewer problems with soil crusting and hard setting. Healthy soils have greater capacity to grow more organic material and are able to retain their healthy status. These soils are more resilient in the face of environmental extremes. A healthy soil is one that is productive and easy to manage under the intended land use. It has physical chemical and biological properties that promote the health of plants, animals and humans while also maintaining environmental quality.

 

References

Gabriel, C. E., Kellman, L., Prest, D. (2018) Examining mineral-associated soil organic matter pools through depth in harvested forest soil profiles. PLoS ONE. 13(11): e0206847. https://doi.org/10.1371/journal.pone.0206847

Kuzyakov, Y., and Blagodatskaya, E. (2015) Microbial hotspots and hot moments in soil: concept and review. Soil Biol. Biochem. 83: 184–199.

Poeplau, C. and Don, A. (2015) Carbon sequestration in agricultural soils via cultivation of cover crops – A meta-analysis. Agri, Ecosys& Environ.200: 33-41.

Mondal, T., Debnath, S., Mukherjee, A., and  Pachauri, S. P. (2015). Neem based botanical pesticides and its potential utilization.  Indian Farmers Digest, Aug, 2005. 36-39.

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Soil Organic Matter and its Importance for Soil Health.pdf