General Importance of Soil Organic Matter

The politicization of the distribution of inorganic chemical fertilizers makes the role of organic matter as a source of plant nutrients more desirable.

Furthermore, the increase in fertilizer prices coupled with its unreliable availability to farmers makes the assessment of the role of organic matter in agriculture more relevant.

As pointed out earlier, the rate of organic matter decomposition is very high owing to the high temperature and humidity in the country. Hence organic matter does not accumulate in the soil to an appreciable extent.

A substantial amount of organic materials will therefore be needed to satisfy the organic matter requirement of soils. Proper use of organic matter ensures sound environmental sanitation as well as the conversion of waste to food.

These nutrients are utilized by crop plants in growth and developmental processes and by the new generations of micro-organisms. Thus yield of crops grown on soils with actively decomposing organic matter is better compared to crops in soil with low organic matter and micro-organism activities.

Organic matter incorporation has been found (Kang and Osiname, 1985)  to reduce the number of micronutrient problems which usually result from the fact that most standard fertilizer formulations are devoid of micronutrients.

The practice of burning crop residue in traditional Agriculture produces large quantities of ash which are equivalent to a good dose of fertilizer. Although burning is believed to volatilize most of the C, S, and N present in crop residues, the incompleteness of the burning still leaves a substantial portion of this nutrient in the soil.

Burning is also said to increase the pH of acid soils due to the abundance of Ca, Mg, and K in the ash of burnt crop residue released into the soil which reduces nutrient losses through erosion of the soil surface.

1. Organic Matter is a Major Source of Cation Exchange Capacity (CEC)

Especially in tropical soils where Kaolinitic clay with very low ion exchange capacity predominates. In most mineral soils, organic matter accounts for about 30-64% of the total CEC while in sandy soils, over 50% of the CEC is possibly due to the organic matter component of the soils.

The more humified fractions of soil organic matter have higher CEC. The CEC of humus, Kaolinite, illite, vermiculite, and montmorillonite clay minerals have been reported to range from 150-300, 3-15, 10-40, 100-150, and 80-150 mol/kg respectively.

The CEC sites of Soil Organic Matter (SOM) are the carboxyl, aliphatic, phenolic, and hydroxyl groups hence the CEC of SOM is pH – dependent.

Owing to this property of SOM, the term ΣCEC is usually used; the ΣCEC is the sum of cations extracted close to the soil’s actual pH, rather than the CEC extracted with buffer solutions at pH 7 or 8.2 which is the most used.

Tropical soils have low ΣCEC and SOM is the major source of ΣCEC in such soils with higher values in the top soils than in the sub-soils. The higher the ΣCEC, the more cationic nutrients the soil can retain against leaching forces.

A number of research workers (Agbim, 1989; Agbede 1984) have shown that in addition to increasing plant nutrients in the soil, organic materials such as rice husk, cassava peels, and cocoa pods, increased soil pH.

One other characteristic of soil organic matter is the formation of chelates with metal ions. Thus organic matter holds metallic ions both by cation exchange and chelation.

That is, organic compounds in soil usually form complexes with metal ions by two or more coordinate bonds. Thus we have zinc-chelate, copper-chelate, and Fe-chelate depending on which metal ion is being complexed.

2. The Buffering Capacity of the Soil is greatly improved by Soil Colloidal Organic Matter (Humus)

The buffering capacity is the ability of the soil to resist large fluctuations in soil pH and cationic and anionic nutrients. Since the buffering capacity is a function of CEC, any material that increases soil CEC will also increase the buffering capacity.

The buffering capacity prevents large ranges in soil pH and osmotic pressure, which, if otherwise, could have been injurious to crops.

3. The Humus Produced from Organic Matter Being Amorphous, Confers Absorptive Capacity of Water to the Soil

The water holding capacity of humus on a mass basis is 4-5 times that of silicate clays. Thus organic matter increases the amount of available water in sandy, loamy, and clay soils while it also increases aeration in clay soils.

The formation of soil humus which is usually a black or brown colloidal substance has a remarkable capacity to hold water and nutrient ions far more than clay. Thus, a very small amount of humus can augment remarkably the soil’s capacity to promote plant production.

Farmers usually estimate the richness of soil by the dark color conferred by the organic matter content. It should be noted that the color development of organic matter (the degree of color development) is influenced by geographical location or climate.

The humus produced during the decomposition of organic matter in the soil has low plasticity and cohesion, therefore, helps to alleviate unfavorable structural characteristics of clayey soils in particular. It encourages granulation, the polysaccharide in humus serving as cementing or stabilizing agent for soil aggregates.

Read Also: Factors Affecting the Decomposition of Organic Matter in Soil

The soil structure is improved through the synthesis of complex organic substances which bind soil particles into more stable aggregates. More stable aggregates enhance moderate water infiltration, percolation, and aeration as well as the moderation of water and nutrient retention.

The problem of soil workability is more acute in heavy soil which is usually plastic and sticky. These forces make the soil difficult to work and when such soils are wet or dry. On the other hand, due to the low elasticity and low cohesion of soil organic matter, clay soils are loosened while sandy soils are bound together.

There is, therefore, good crop performance in that the soil is better aerated, erosion and run-off are reduced and there is less resistance to root penetration.

4. Organic Matter Serves as Source of Energy for Microbes in Soil

Decomposing root tissues provide energy and nutrients to support the growth of organisms such as earthworms, fungi, and bacteria that can improve soil structure.

Fresh and partially decomposed organic matter serves as food for these organisms such as earthworm which burrows into the soil, mixes the organic matter with soil, and makes worm casts rich in plant nutrients.

Soil erosion by water or wind is a common problem in agriculture. Coarse organic materials on the soil surface reduce the impact of raindrops and reduce run-off and wind erosion. A surface accumulation of any type of plant residue helps reduce erosion.

For example, under forest where thick layers of “duff” (Surface organic matter) accumulates, erosion is almost non-existent. An application of coarse surface mulching in farm practices will cause a larger percentage of water to seep more slowly into the soil and thus make water available for plant use.

Conventional good management of soils such as rotations, grass leys, use of crop residues, and minimum or no-tillage, can reduce erosion. Mulch tillage techniques such as zero tillage, which seek to utilize crop residue as surface mulches have been developed for sub-humid ecologies of Nigeria.

Applied as mulches, organic matter helps to regulate soil temperatures, and reduces evapotranspiration losses of water. In effect, organic matter acts as an insulator on the soil surface and buffers the sudden heat change between the soil and the atmosphere.

An increased microbial population from organic matter inputs also has some influence in protecting plants against pathogens, nematodes, and soil-inhabiting insects. Soil microorganisms by their activities antagonize plant pathogens.

There is competition for soil-available carbon and energy materials. Some of the microbes produce toxic substances which inhibit or kill other organisms. This antagonistic relationship is important in the control of the harmful activities of plant root parasites.

The microbial population inhabiting soil organic matter times provides plant hormones such as auxins, gibberellins, and cytokinins.

There is an association of soil microorganisms with higher plants as in the case of the mycorrhiza relationship and in rhizobium/legume symbiosis in leguminous root nodules which usually enhances p-nutrition in cultivated crops.

Read Also: Soil Temperature and Plant Growth

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Benadine Nonye

An Agric. Consultant & a Writer (With over 12 years of professional experience in the agricultural industry) - National Diploma in Agricultural Technology - Bachelor's Degree in Agricultural Science - Master's Degree in Science Education... Visit My Websites On: - It's All About Agriculture, The Way Forward! - The Most Reliable Global Agricultural Forum! - The Most Reliable Nigeria's Agricultural Job Board! - For Everything Premium Agriculture! - For Proper Waste Management and Recycling Practices. Join Me On: Twitter: @benadinenonye - Instagram: benadinenonye - LinkedIn: benadinenonye - YouTube: Agric4ProfitsTV - Pinterest: BenadineNonye4u - Facebook: BenadineNonye

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