The soils of the tropics are generally of low fertility, partly because they are composed of low-activity clays that are not rich in cation exchange sites. Hence, one of the major ways of increasing the cation exchange capacity of the soil and soil fertility is to add organic matter.
There are various benefits of adding organic matter to the soil, and some will be explained in this article. This article deals with the explanation of the roles of organic matter in tropical soils.
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Functions of Soil Organic Matter in Tropical Agriculture
Organic matter functions as an exchanger or sorption agent, especially in kaolinitic soils with naturally very low cation exchange capacity (CEC). Organic matter contributes to the storage and release of nutrient ions.
On acidic, weathered soils, organic matter is responsible for almost the entire CEC. On soils with low mineral exchange capacity, organic matter functions as a buffer system for the pH value and ion concentration, regulating the balance of nutrients in the soil solution.
Organic matter is the product of a ‘bio-accumulation’ of nutrients in the topsoil, serving as a nutrient carrier because the small amounts of Ca, Mg, and available P in the oxisols and ultisols of the humid tropics and moist savannas are concentrated in the humic topsoil.
Organic matter also works as a slow-release source of nutrients, reducing the risk of leaching. On sites with a tendency for Fe or Al toxicity, humifying organic matter works to combat toxic metal concentrations by forming stable complexes with high molecular weight.
Organic matter improves the water-holding capacity of soils, capable of holding up to 3-5 times its own weight in water. Organic matter improves soil structure, with humus contributing to the formation of aggregates.
The clay-humus complexes offer good protection against wind and water erosion and promote permeability while improving water storage. Organic matter is essential as a habitat and source of nutrients for microorganisms and soil fauna, such as earthworms and termites.
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Types of Organic Matter in Agricultural Systems
1. Crop residue: These are materials left after harvest, including straw, stalks, and roots.
2. Green manure: Leguminous crops are used for this, e.g., mucuna, crotalaria, cowpea, soybean, etc. Usually, they are fast-growing with substantial vegetative cover and incorporated green into the soil.
3. Animal manure: These wastes can be solid or liquid, e.g., poultry droppings, piggery droppings, livestock dung, etc.
4. Sewage sludge: Municipal wastes that can be treated, concentrated, and made into commercial fertilizers.
5. Compost: A combination of manure, sewage sludge, leaves, grass cuttings, etc.
6. Peat: Naturally occurring organic material that has accumulated over many years in wet and cool places, used as mulches or soil conditioners.
Strategies for Maintaining Soil Organic Matter in Tropical Farming
All efforts must be made to maintain soil organic matter, although continuous cropping tends to reduce its level. Methods include crop residue incorporation (not bush burning), conservation cropping systems such as zero tillage and minimum tillage, use of animal manure.
Alley cropping (strips of arable crops interspersed with leguminous shrubs), use of compost, green manuring, mulching, and encouraging natural symbionts, e.g., vesicular-arbuscular mycorrhiza (VAM) and biological nitrogen fixation. Fallowing is also a key practice.
Organic matter plays a very important role in the soils of the tropics. It can be sourced from both plant and animal origins. There are various ways of maintaining soil organic matter to support sustainable agricultural productivity.