Importance of Soil Organic Matter

 

The role of soil organic matter in agriculture

It is important for farmers to maintain healthy soil because it produces healthy crops that nourish people. Plants obtain nutrients from organic matter and minerals. Organic matters are plants and animal materials that return to the soil and decompose to produce nutrients, bind soil particles into aggregates, improve water holding capacity of the soil and harbor other soil organisms. These soil organisms convert dead materials, decaying matter and minerals into plant nutrients. Their biological activities depend on the organic matter supply and the type of substrate. The exchange of these nutrients between organic matter, water and soil is vital to soil fertility. Organic matter and soil nutrients need to be maintained for the continuous production of food. The soil fertility will decline if the land is cultivated without restoring the organic matter, nutrients and soil structure. This may further destroy the agro-ecosystem and reduce food yield. Organic matter thus plays a vital role in improving soil productivity and sustainable crop production, especially in the tropics. 

Mulching preserves moisture & organic matter

What is soil organic matter?

Soil Organic Matter (SOM) is a wide range of carbon-containing compounds formed from organic material – the products of on-site biological decomposition which keeps the soil healthy. SOM helps to feed plants, increases growth and yield; improves the soil structure, fertility and acts as a mulch to suppress weeds, protects the soil and reduces water lost. Soil organic matter is key to drought-resistant soil, sustaining food and increasing production. It can also be called soil improver, soil conditioner, or humus.

The soil organic matter is governed by the carbon cycle. The plant grows and adds organic matter to the soil through its roots, fallen leaves and dead plants. Most animals feed on plants and animal matter is also returned to the soil.  When they decompose, the organic matter releases carbon dioxide that is used by the plant in photosynthesis and valuable plant nutrients such as Nitrogen, Phosphorus and minerals.

The continuous supply of organic matter also acts as a food source for micro-organisms and helps build up soil carbon in a process called assimilation – which is vital for soil formation, nutrient availability and cycling. Different types of organic matter produce different organic matter fractions because they have different assimilation and decomposition characteristics. So if the rate of decomposition is more than assimilation, the SOM will reduce quickly (e.g. poultry manure); but if the rate of assimilation is more than decomposition, the SOM will increase (e.g. woodchips). The rate at which decomposition occur depends on oxygen, temperature, moisture, the surface area of the particle and its chemical structure. It takes more than 1 or 2 years for most organic matter to decompose and an ideal active soil organic matter contains about one-third of decomposable organic matter.

Avocado planted without tillage 

Why use organic matter?

In a natural forest, organic matter accumulates in soils.  Natural soils contain about 40% more organic matter than cultivated soils. When we cultivate the soil, the SOM declines because planted crops supply less organic residues than natural vegetation and the effects of tillage (digging, hoeing …) further increase the rate of SOM loss. It continues to decline to a level where the soil function is impaired and becomes unproductive. 

SOM influences several critical soil functions. It helps the soil to hold water and nutrient, improves soil structure, enhances productivity and environmental quality, reduces the severity of droughts, soil erosion and atmospheric carbon dioxide that contribute to climate change. SOM binds soil particles into aggregates that hold moisture and nutrients (preventing them from being washed away by rain). When plant roots penetrate these aggregates, the nutrients are released to feed the plants.

What human intervention decreases organic matter?

Repetitive harvesting of crops reduces soil nutrients and SOM. So much effort is needed to replenish these nutrients and restore soil quality. Most agricultural practices like ploughing, tillage and burning of vegetation increase the decomposition of soil organic matter and the soil become infertile and susceptible to erosion. Land use and management practices affect SOM. Below are some human activities that greatly decrease organic matter;

•  Decrease in biomass production such as replacement of perennial crops, replacement of mixed vegetation to monoculture or pastures, high level of harvesting (e.g. corn with stalks) and the use of bare fallow.
• Reduction of organic matter supply like burning of natural vegetation or crop residues (wildfires), overgrazing, and removal of crop residues.
•  Increasing decomposition rate through tillage, draining, fertilizers and pesticide usage.

How do we increase SOM in farms and gardens?

The key to soil restoration is to maximize the retention and recycling of plants' nutrients and organic matter while minimizing the loss of soil components through leaching, erosion and runoff. This will help to maintain, improve and rebuild soil health and sustain agricultural productivity. Farmers or gardeners need to continuously add sufficient organic matter (3-6%) to increase soil nutrients holding capacity, moisture and plant growth. Severe impacts may occur if the organic matter levels fall below 2%. Fortunately, you don’t need to test the soil to know if it needs organic matter or not. Soils that are light in color and compact when wet or dry probably require organic matter.

Some good conservation agricultural practices like zero or minimal tillage, cover cropping, agroforestry, perennial forage crops and crop rotations can help to maintain surface residues, roots and SOM. They can also suppress weeds, enhance soil aggregates and intact large spores which in turn allow water filtration and reduce water erosion and run-offs. The diverse soil organisms in SOM contribute to pests control and other essential ecological processes. Permaculture or well-managed integrated mixed crop-livestock farming systems are able to enhance SOM and restore soil health.

Mango planted with a little digging

Sources of organic matter

There are different sources of organic matter according to the different land use. The most important source of organic matter is biomass production.

•  Agricultural farms can get organic matter from crop residues, animal manure, green manure. The application of livestock manure to agricultural land increases soil organic matter in the soil profile.
•  Home gardens can get organic matter from household waste, commercial waste, old wood, green waste and compost - kitchen compost (from vegetables, fruit and gardening waste) and green compost (from pruning, branches, grass and leaf litter).

Garden compost is less costly and effective. Some compost can be bought as soil improvers or conditioners. Composted animal manure, spent hops from local breweries and spent mushroom compost can also be collected and used. Some councils offer municipal compost to gardeners (beware of weed killer residues). Organic matter can be stored - stacked and cover with rainproof material to prevent nutrients from being washed off by rain.

How to apply organic matter to soil

Into the soil: -Always add only well-composted materials into the soil to avoid soil nitrogen depletion. 5kg of compost per square meter is sufficient.

On the soil as mulch: - Most organic material can be used directly as mulch e.g. shredded wood waste, wood chip, bark, leaves. Coarser materials will take a longer time to break down. 2-5 cm mulch is enough for weed control or no-tillage gardens.

Leaves return to the soil as organic matter

Always wear boots and gloves when handling the organic matter; avoid eating, drinking, smoking and wash hands after application. Apply nutrients a week before planting and don’t over-fertilize to avoid contamination of water bodies.

Organic matter saves on fertilizer costs because manure contains about 0.8% nitrogen, 0.3% phosphate and 0.8% potash. Unlike fertilizers, the nutrients in organic matter are released slowly with little wastage and the organic matter themselves improves soil structure. Organic matter can be applied every 2 years because the nutrients are mainly available in the first 2 years after application and decrease thereafter. Make sure your compost is free of pests, diseases and weed seeds.

 Conclusion

Maintaining levels of SOM and optimizing nutrient cycling is vital in agricultural productivity. Some SOM fractions function as natural plant hormones that improve seed germination, root initiation, uptake of plant nutrients and provide N, P and S. Farmers should aim to achieve optimal agro-ecosystems that are socially, ecologically and economically sustainable by practicing conservation agriculture - a system that is able to convert low-input agricultural systems into a more productive one. Farmers need to understand the linkages between soil life, functions of the ecosystem and the impact of human activities to know how to enhance soil health and capture the full benefits of soil biological activities that lead to a more sustainable and productive agricultural system. A healthy soil ecosystem does the following:

• Decompose organic matter to form humus
• Retain Nitrogen, Phosphorus, Sulphur, Potassium and Calcium.
• Glue soil particles together into aggregates for the best structure
• Protect roots from pests and diseases
• Make retained nutrients available to the plant.
• Produce hormones that help plants grow
• Retain water moisture

SOM at different decomposition levels

Farmers! Let’s benefit from this knowledge and increase productivity. Take appropriate action now and let’s together feed the growing population while maintaining agro-ecosystems. Leave a comment or question below so we know you were here.

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