Sources of Soil Organic Matter in Agricultural & Horticultural Soils

Soil Organic Matter
Soil Organic Matter

The organic matter contents of a good mineral soil are 2-5% which consists of organic matter in the form of living and dead organic matter. Living organic matter consists of soil microorganisms (fungi, bacteria, nematodes and earthworms), plant roots and insects.

Dead organic matter is added after the death of these living organisms and addition of leaves, bark of trees and dead animals. Farmyard manure, composts and mushroom compost are also source of dead organic matter. All of this organic matter is ultimately broken down into carbon dioxide, water and minerals. Humus is the collection of organic compounds that persists in soil for longer duration.


Living organisms in the soil ecosystem for food webs and food chains.
1. The source of converting sunlight and simpler compounds into organic compounds are the Producers, which are involved in photosynthesis (green plants mostly in agricultural soils). Next are the primary, secondary and tertiary consumers. The most important after producers are ‘decomposers’ which decompose and convert dead organic organisms and organic matter to simpler compounds for reuse; and successfully complete the nutrient cycle of many nutrients which also become available to the plants.

Decomposers can be further distinguished into primary decomposers which attack the freshly dead organic matter such as earthworm and some species of arthropods and fungi. Secondary decomposers live on the wastes decomposed by the primary decomposers and include many species of fungi and bacteria in particular.
1. Plant Roots

Plant roots are the main source of organic matter and aeration in the soil. Roots evade the soil and when the plant is uprooted or dead, the roots decompose and create space for aeration of soil plus they add to the soil organic matter. The penetration of soil by roots is decreased when the soil becomes too compact. Otherwise roots play a vital role in the development and improvement of soil structure.
2. Earthworms
earthwormEarthworms are the ‘farmers’ friends’ which are very important for the primary decomposing of soil organic matter as well as improving the soil structure. The casting activity of soil is only done by few selected species of earthworms. The casting species of earthworms eat both soil and organic matters and the excreta from their intestines consist of intimately mixed, partially digested, finely divided organic matter and soil.

Earthworm burrows created by their activity in the soil are important source of improving soil structure in uncultivated areas and grasslands but sometime the burrows are troubling the lawns. Some species of the earthworms are present in the topsoil layers while others go on to make vertical burrows up to 2 meters depth.

Earthworm activity is largely affected by soil organic matter, soil moisture contents, soil pH, temperature and soil type. Large populations of earthworms and greater activity is seen in the soils with high organic matter contents; slightly higher calcium reserves, basic pH and light and medium loams soils. Small populations of earthworm exist in acidic soils; soils with low organic matter contents, clayey soil, peat and gravelly soil.

Snails, slugs, nematodes and arthropods also have role in the decomposition of the organic matter of soil but some are also important horticultural pests.

3. Bacteria
Bacteria are the most abundant living organisms in soil. About 1000 million bacteria are present in one gram of fertile soil. Bacteria have strong roles in the decomposition of organic matter, weathering of the rocks, detoxification of pesticides and herbicides. Some bacteria are also fixing nitrogen in the soil and others are denitrifying bacteria; root nodulating bacteria form association with the roots of some plants to help uptake of nitrogen. Bacteria are also important for the completing the nutrient cycles in the soil environment.

Bacterial population in the soil increase with the presence of high organic matter contents, high food supply, pH of 5.5-7.5 and temperature close to 350C.

4. Fungi
Fungi along with bacteria are most important decomposers in the soil. Fungi are more tolerant to harsh environment than other micro ogranisms in the soil. They can exist in soils with low calcium; soils with acidic/neutral pH and low organic matter. There are different species of fungi, some can decompose only simpler organic matter, others decompose cellulose and some also have ability to hydrolyze lignin. Hence, fungi are most important primary decomposers in the soil.

Fungi also form mutualistic association with algae in ‘lichens’ and roots of higher plants in ‘mycorrhizae’.

The freshly dead organic matter in the soil has effect on the physical properties of the soil but nutrients are released only after the decomposition of the organic matter. The decomposition of the green leafy organic matter is usually very fast and completes quickly as compared to the decomposition of the fibrous or woody organic matter which takes longer for decomposition and forms brown humus.

Humus forms in the soil as the result of decomposition of fibrous or woody organic matters which does not completely decomposes due to lignin contents and other decomposition-resistant organic matter. These ‘brown’ contents form humic acids which form a collection of black colloidal-gels that surrounds the soil particles and imparts characteristic brown color to the soil.
This gel has cation exchange capacity (CEC) and add to cation holding capacity of the soils. It also holds the soil aggregates together. In sandy soils, with low clay contents, humus holds the soil particles together. The humus is decomposed completely by action of bacteria in the soil so new organic matter must be added to soil to replace the decayed to retain the soil properties.

All the nutrients in the soil are in constant circulation between soil, plants, animals and air.

Mineralization is the process of formation of simpler inorganic compounds such as ammonia, nitrites, nitrates and sulphates.

Humification, on the other hand, is the process for the formation of humus in the soil through decomposition of fibrous/woody dead organic matter.

Nutrient cycles on-going in the soil include;
1. Carbon cycle
2. Nitrogen cycle
3. Sulphur cycle

Carbon to Nitrogen Ratio (C:N)
Carbon and nitrogen are two most abundant elements in soil. The nutrient cycles of both are continued at the same time in the soil. The change in the quantity of one of them will have an effect on the amount of other. C:N ratio is much wider in plant organic matter as compared to other materials; this organic matter is utilized in decomposition by the microbe. Carbon serves as the energy source to the microbes but nitrogen is utilized by microbes in the formation of proteins. Thus there is an imbalance in the C:N ratio. Courtesy Pak Agri Farming

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