Soil Organic Matter (SOM) determines soil nutrient levels, influencing the availability of cations such as K, Ca, Mg, and micronutrients like Zn, Bo, Cu, and anions like PO43-, NO3-, and Cl-. SOM consists of fresh organic material and humus.
The fresh organic material is transformed by soil organisms into humus, releasing nutrients into the soil. Humus itself is also broken down by soil organisms, which releases even more nutrients.
Cropping with hoe and cutlass or mulching fairly maintains soil organic matter. This article explains the meaning of soil organic matter, its importance in agriculture, its disadvantages, types of soil organic matter, and methods for maintaining soil organic matter.
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Composition and Significance of Soil Organic Matter in Agriculture
Soil organic matter consists of a wide range of organic substances, including living organisms, carbonaceous remains of organisms that once occupied the soil, and compounds produced by current and past metabolism in the soil.
Although the importance of organic matter has been recognized since time immemorial, its need has become even more relevant in modern times due to the following reasons: ever-increasing costs of commercially manufactured fertilizers, increasing organic residue from crops and farms, greater awareness of the need for ecological conservation of resources and environmental safety, the introduction of zero/minimum tillage practices to promote high soil organic matter, and greater agricultural production from crops and livestock to meet the nutritional and fiber needs of the population.
Role of Organic Matter in Soil Fertility and Structure
Organic matter is an active and important portion of the soil. Although most cultivated soils contain 1-5% soil organic matter in the top 25 cm, it can still modify soil physical properties and strongly affect chemical and biological properties.
Chemically, it is nearly the only source of nitrogen, 5-60% of phosphorus, 80% of sulfur, and a major part of molybdenum, boron, and many other micronutrients. It undergoes constant changes, necessitating continuous replenishment to maintain soil fertility and crop production.
Disadvantages of Soil Organic Matter in Crop Production
Organic matter may provide a source of pathogens for root-borne diseases, nematodes, insects, etc. Crop residues low in nitrogen and other essential nutrients can result in immobilization of available nutrients, affecting their uptake for crop growth.
Toxic levels of some substances can be released early in organic matter decomposition, potentially impacting crop growth.
Types and Decomposition of Soil Organic Matter
Most organic matter derived from nature originates from CO2 in the air through photosynthesis. Organic materials (crop and animal residues) and many simple proteins are decomposed quickly by soil bacteria and fungi.
Fats and oils are decomposed next, followed by cellulose and lignin. The most resistant are crude proteins and lignins, which remain as dark brown to dark grey organic matter called humus. Humus imparts a soft feel or friable consistency to all soils.
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Microbial Decomposition of Organic Matter in Soils
Microorganisms obtain energy and nutrients by decomposing soil organic matter, resulting in the use of some carbon, nitrogen, and other elements by the microorganisms, the release of CO2, water, and other elements to the soil solution or atmosphere, and the formation of humus.
Bacteria and fungi are the most active organic matter decomposers but are aided by actinomycetes. Bacteria and fungi work optimally in moist soils at a temperature of about 35°C and in soils with neutral pH.
Organic matter still undergoing considerable decomposition is referred to as active organic matter. Active organic matter releases more nutrients than less active organic matter (humus). Additionally, it releases gums for cementing particles to form stable soil aggregates, which contribute to desirable soil structure.
This underscores the need for continuous addition of active organic matter to ensure a continuous supply of polysaccharides (gum), as these are depleted within a year in the soil.
Soil organic matter is a major source of soil nutrients. It is an active and important portion of the soil, modifying physical properties and strongly affecting chemical and biological properties.
It undergoes constant changes, requiring continuous replenishment to maintain soil fertility and crop production. There are also some disadvantages of soil organic matter.
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