Soil Organic Matter: Sources and Composition | Soil Science

In this article we will discuss about:- 1. Sources of Soil Organic Matter 2. Composition of Organic Residues 3. Decomposition 4. Factors Affecting the Rate 5. Role 6. Maintenance.

Contents:

1. Sources of Soil Organic Matter
2. Composition of Organic Residues
3. Decomposition of Organic Compounds in Soil
4. Factors Affecting the Rate of Decomposition
5. Role of Organic Matter
6. Maintenance of Organic Matter in the Soil

Soil contains organic matter which is derived from dead and decaying of plants and animals. Virgin soils contain organic matter in great quantities. Soil containing much amount of organic matter becomes black or brown in colour. The organic matter of the soil is a source of mineral matter for the growth of the crop.

The materials or substances which are produced at the first instances from the organic debris (i.e. plant and animals) by living organism is known as organic matter.

1. Sources of Soil Organic Matter:

There are several sources of soil organic matter as follows:

(i) Plants:

The original source of the soil organic matter is plant tissue. In the forest, organic matter is added in the soil mainly in the form of fallen leaves. Under natural conditions, the tops and roots of trees, shrubs, grasses and other native plants annually supply large quantities of organic residues. The portion of the crop plant (i.e. top, stubbles, roots etc.) left in the soil after harvesting of the crop also serves as a source of organic matter. Thus the plant tissue is the primary sources of organic matter.

(ii) Animals:

Animals are usually considered secondary sources of organic matter. Soil organisms like insects, millipeds, nematodes etc. also contribute some organic matter to the soil. Soil microorganism makes a considerable contribution of soil organic matter after their death.

(iii) Manures:

Manures (i.e. Farm yard manure, compost, green manure, fish, meal, oil cake etc.) are the good source of organic matter to the soil and their addition in the soil increases the organic matter to some extent.

The amount of organic matter in soil varies with nature of the soil as follows:

(i) Virgin Soils:

The plants and animals growing in or on the soil are the main sources of organic matter of this soil. After the death of animals and plants, the dead bodies of animals and the roots of the plants are left on the soil and top portion fall on the surface of the soil are incorporated in the soil. The animals and plants residues are decomposed by soil micro-organisms and formed a part of organic matter. The soil organisms which are inhabitant of soil make a considerable contribution of soil organic matter after their death.

(ii) Cultivated Soil:

The weeds including green manuring crop ploughed into the soil serve as a good source of organic matter in the cultivated soil. After the harvest of the crops, the roots and stubbles are left in the soil which are converted into organic matter by soil organism and make also a considerable contribution of soil organic matter. Manures which are applied time to time in cultivated soils are the good source of organic matter and the organic matter content of soil increases with the application of manures.

2. Composition of Organic Residues:

Organic residues contain different materials as follows:

1. Organic Materials:

I. Nitrogenous Materials:

Water Soluble – Ammonium compounds, nitrates etc.

Insoluble – Protein, peptides, peptone etc.

II. Non-Nitrogenous Materials:

Carbohydrate – Cellulose (Insoluble), hemicellulose, starch, pectin, mucilage (hydrolysable), sugar (soluble) etc.

Ether Soluble – Fats, oils; waxes, resins, steroid etc.

Miscellaneous – Lignin, organic acid, essential oils, tannins etc.

2. Inorganic Materials (Mineral Matter or Ash):

Calcium, Magnesium, Manganese, Sodium, Potassium, Iron, Zinc, Copper, Sulphur, Phosphorus, Chlorine, Carbonate etc.

The per cent composition of organic residues is as follows:

(a) Carbohydrates:

(i) Sugar and starches – 1-5

(ii) Hemicellulose – 10-28

(iii) Cellulose – 20-50

(b) Fat, Waxes and tannins etc. – 1-8

(c) Protein (Simple water soluble and crude protein) – 1-15

(d) Lignins – 10-30

The plant tissue is composed of 75 per cent moisture which varies from 60-90 per cent and 25 per cent of dry matter. The dry matter consists of carbon, oxygen, hydrogen and mineral matter. The carbohydrates are complex substances and range from simple sugar to very complex cellulose and are made of carbon, hydrogen and oxygen. The fats and oils are glyceride of fatty acids such as butyric, steric and oleic.

They are associated with resins of many kinds and are more complex than the carbohydrates and are made of carbon, hydrogen and oxygen. Proteins are most complicated substances and contain carbon, hydrogen, oxygen, nitrogen and sulphur, iron, phosphorus etc. in lesser amounts. Lignins are made of carbon, hydrogen and oxygen. They occur in older plant tissues such as stems and other woody tissue. Lignins are very resistant to decomposition.

3. Decomposition of Organic Compounds in Soil:

Organic Compound is made of various materials and their rate of decomposition vary greatly i.e. some organic tissues are easily decomposed and some organic tissues are resistant to decomposition.

We can arrange them in the following order on the basis of their ease of decomposition:

(i) Materials decomposed rapidly:

(a) Sugar, starch and simple protein.

(b) Crude protein.

(c) Hemicellulose.

(ii) Materials decomposed very slowly:

(a) Cellulose

(b) Lignin, fats and waxes.

Organic compounds in the soil are decomposed by biochemical process through different steps. The organic compounds (Plant and animal debris), when applied in the soil, are at once attacked by a great variety of Soil organism, if the soil is moist, in order to simplify the nutrient material for their own use.

In the decomposition process, carbon is oxidized into carbon dioxide and energy is liberated which is utilized by the soil organisms for their existence for one part of carbon is locked up in the body of the organism and two parts of carbon are lost as carbon dioxide to supply energy to the organism.

The whole decomposition process may be considered as oxidation (i.e. burning) process. The major portion of organic compounds consisting mainly Carbon and Hydrogen are ultimately oxidized to carbon dioxide (CO₂ and water (H₂O). The overall reaction may be stated as follows –

Organic matter decomposition and formation of humic substances in soil are shown as follows:

But this oxidation does not take place in one step, rather it takes place through a number of steps giving various intermediate products.

The activity and population of soil organism depends on the nature of decomposable organic compounds. As for example, the activity and the population of soil organism increases when some easily decomposable organic compounds are added in a moist soil. After all decomposable organic compounds disappear from the soil due to decomposition, there will be a deficiency of food material for the soil organism. As a result, the population of the organism and also their activities will decrease, resulting the decrease in rate of decomposition.

The dead body of soil organism in turn will be attacked by other organism to decompose them into simpler products. When all the decomposable organic compounds are disappeared, a fraction of organic constituents will still remain in the soil for longer periods, because they are restricted by microbial attack. This portion is mainly lignin, fat, waxes etc. Major portion of humus is composed of this fraction.

Carbohydrates and proteins are the main component of organic compounds and their mode of decomposition are as follows:

(i) Decomposition of Carbohydrates:

The simple sugar, a carbohydrate group, is the first to be decomposed as it offer a very readily available source of carbon, nitrogen and energy for the soil organisms. Under aerobic condition, the end products obtained in the decomposition of these substance are carbon dioxide and water.

Sugar + Oxygen → Carbon dioxide + water

After the decomposition of simple sugar, starches, cellulose and hemicellulose are attacked gradually by the soil organism. First these complex CHO are converted into simple sugar by enzymatic hydrolysis and the simple sugars that are obtained are oxidized to similar end product such as carbon dioxide (CO₂) and water (H₂O).

(ii) Decomposition of Proteins:

Proteins are complex organic substances containing nitrogen, Sulphur and sometimes phosphorus in addition to carbon, hydrogen and oxygen. The decomposition of proteinaceous materials takes place through a number of steps. The proteins are first converted to aminoacid by the process of ‘amminization’. The process of conversion of proteins to aminoacid is known as ‘amminization’.

The aminoacids are then converted to ammonia (NH₃) by the process of ‘ammonification’. The transformation of organic nitrogenous compound into ammonia is called ‘ammonification’. Both these processes are brought about by a large number of heterotrophic organism, bacteria, fungi and actinomycetes. The ammonium ion is readily available to microorganism and mostly higher plants. The above process can be represented as follows –

Ammonification proceeds best in well drained soils with plenty of basic material.

Ammoniacal nitrogen released may again be transformed to nitrate under suitable Condition through the process of nitrification. The process of conversion of ammonia into nitrite (NO₂) and then to nitrate (NO₃) is known as ‘nitrification’. The Process of nitrification may be represented as follows –

Nitrification is an aerobic process involving the production of nitrate from ammoniacal salts and this process is carried out by two groups of special purposes and autotrophic bacteria (i.e. nitrosomonas and nitrobacter). The autotrophic bacteria use the energy and leave nitrate as the bye-products. Nitrates thus formed are used by higher plants or are lost in the drainage water. When all most of all the easily decomposable substances are vanished from the soil in the decomposition process, the most resistant portion of organic residues such as lignin, fats, waxes etc. are left in the soil in a slightly modified form.

This modified resistant portion of organic residues combine with proteins especially microbial proteins to form a complex colloidal substance, which is resistant to microbial attack and this mixture is known as ‘humus’. Humus which is found at the end product of decomposition process is said to be highly resistant to microbial attack. But the humus also is decomposed by the activity of the organism under suitable condition at very slow rate producing the similar end product.

Under poorly drained soils or in soil containing nitrites, reduction of nitrates and nitrite takes place in soils and free nitrogen or oxides of nitrogen is formed by the process of ‘denitrification’. Water logging (e.g. paddy field) and high soil pH increases the loss of nitrogen by denitrification.

The organisms involved in mineralization of nitrogen are shown as follows:

(iii) Sulphur Compound:

Plant residues contain sulphur in the form of proteins, amino acid and vitamins. When organic matter is decomposed, sulphur containing proteins are decomposed to sulphur containing amino acid and then these are decomposed to form sulphide.

In well aerated soil, the sulphur of these simplified product is then subjected to oxidation by special autotrophic bacteria (i.e. Desulphovibrio, Desulphotomaculum etc.) The transformation of sulphur may be represented as follows –

The organism involved utilizes the energy by the transfer and leaves sulphur in the form of sulphate.

(iv) Phosphorus Compound:

Phytin, Phospholipids, nucleoprotein nucleic acid etc. are organophosphorus compound of organic matter. The microorganism acts on organic Phosphorus compounds and decomposed them to release the phosphate in available form (i.e. H₂PO₄^–, HPO₄^{– –}) to plants.

Simple Decomposition Products:

As the enzymic changes of the soil organic matter proceed, simple products begin to manifest themselves. Some of these especially carbon dioxide and water appear immediately. Others such as nitrate nitrogen accumulated only after the peak of the vigorous composition is over and the general purpose decay organisms have diminished in number.

The more common simple products resulting from the action of soil organism are as follows:

4. Factors Affecting the Rate of Decomposition:

(i) Composition of organic residues – The decomposition of organic residues depends on its composition. The organic debris rich in sugar, starch, protein etc. are easily decomposed. On the other hand, organic debris rich in lignin, fats, waxes etc. are very slowly decomposed.

(ii) Soil pH – Soil pH affects the activity of soil organism. Soil pH 6.0-8.0 is generally best for most of bacteria.

(iii) Soil moisture – Moisture optimum for plant growth is good for the decomposition of organic compound.

(iv) Soil air – The decomposing organisms do their function well in well aerated soil. Because sufficient aeration is required for the respiration of soil organism.

(v) Soil temperature – Soil temperature has also influence on the function of soil organism.

Carbon : Nitrogen Ratio (C : N Ratio):

The ratio of the weight of organic carbon (C) to weight of total nitrogen (N) in a soil or organic material is known as ‘C : N ratio’. The fresh plant residues are rich in carbon and poor in nitrogen. The content of carbohydrate is high. The C : N ratio in plant material is variable ranging from 20 : 1 to 30 : 1. The C : N ratio in the organic matter of the furrow slice of arable soil commonly ranges from 8 : 1 to 15 : 1, the median being between 10 : 1 and 12 : 1. The C : N ratio of organic matter is 20 : 1.

Significance of C : N Ratio:

(i) C : N ratio will influence the rate of decomposition of organic compound- The decomposition of organic compounds having low C : N ratio will be rapid due to availability of sufficient amount of carbon and nitrogen for the soil organism. The rate of decomposition of organic compound having wide C : N ratio will be slow, because the multiplication of soil organism will be limited due to deficiency of nitrogen. As a result, the activities of soil organism responsible for decomposition process will be at a low level.

(ii) Availability of nitrogen is affected by C : N ratio of organic compound added to the Soil- The period of nitrate depression in the decomposition of organic matter is affected by the C : N ratio of the substances added to the soil. Wider the C : N ratio of the substances added, longer will be the period of nitrate depression. The soil organism responsible for the decomposition of organic compound will get sufficient amount of carbon in comparison to nitrogen, when the organic compounds of wide C : N ratio is added.

As a result, whatever nitrogen is mineralized in the decomposition process is again utilized by soil organism and is locked up in their body. Over and above this, whatever nitrogen is present in the soil in the available form will be utilized by the organism and the period of nitrate (NO₃) depression starts.

The standing crop, if therein the land, is likely to suffer from the deficiency of nitrogen. Because, there will be a competition for nitrogen between plants and soil organism. As the decomposition proceed, the C : N ratio of organic matter is gradually lowered down, because in the decomposition process, carbon is lost and nitrogen is preserved in the soil.

(iii) Amount of humus formed is determined by the C : N ratio of organic compounds. The amount of humus formed in soil after the decomposition of organic compounds is considerably affected by the C : N ratio of the substances added to the soil. The organic compound having narrow C : N ratio is likely to produce more humus after decomposition than the organic compounds having wide C : N ratio. Because the amount of carbon retained as humus is dependent upon the amount of nitrogen.

5. Role of Organic Matter:

(i) Organic matter is the store house of food for the plant. The major plant nutrients namely nitrogen, phosphorus and potassium remain in organic matter and the nutrient releases throughout the year which becomes available to the plants.

(ii) Organic matter helps to improve and conserve the fertility of soil.

(iii) Organic matter imparts a dark colour of the soil and thereby help to maintain soil temperature.

(iv) Organic matter improves soil structure by enhancing the granulation of soil particles. As a result, clay soil becomes porous and aeration and movement of water is improved. On the other hand, water holding capacity of sandy soil increases.

(v) The plant nutrients remain in insoluble and complicated state in organic matter. As a result, loss of nutrient by rain water Or other natural agencies becomes minimum.

(vi) Organic matter increases cation exchange capacity (C.E.C.) of the soil. Thus it prevents the loss of nutrient by leaching and retains them in available form.

(vii) Organic matter increases the water holding capacity of the soil. This is specially important in case of sandy soil.

(viii) Organic matter makes the soil porous and thus help in proper aeration.

(ix) Organic matter serves as a store house of food for the soil microorganism. They make the plant nutrient available to plants.

(x) Organic matter increases the availability of phosphorus by locking up the calcium, iron and aluminium which are responsible for phosphate fixation.

(xi) Organic matter increases the buffering capacity of soil. Buffering checks rapid chemical changes in soil pH and in soil reaction.

(xii) Organic mulching helps to conserve the soil moisture.

(xiii) Organic acid released from decomposing organic matter helps to reduce alkalinity in soils.

(xiv) Organic matter reduces the undesirable properties of clay soil like cohesion and plasticity. It makes the clay soil friable and make it easy for cultivation.

(xv) Organic matter reduces the losses of soil by wind erosion and reduces the surface run-off and makes soil water more available to plants.

(xvi) Soil becomes inert without organic matter and plant cannot grow well in that soil. It may be said that ‘A field without organic matter is as useless as a cow without a calf’.

The discussion on the role of organic matter reveals that “organic matter is the life of the soil”.

6. Maintenance of Organic Matter in the Soil:

Soil contain 5-10 per cent of organic matter of its volume. But the Indian soil do not contain more than -2 per cent organic matter. Because the organic matter gets lost by oxidation due to high temperature in our country. So the maintenance of organic matter in the soil is difficult task.

But the maintenance of organic matter in the soil is possible if the following measures are undertaken properly:

(i) Application of manures – Manures (e.g. compost, farm yard manure, oil cakes etc.) are the good source of organic matter of the soil and their addition increase the organic matter of the soil. The roots and stubbles of the crops also act as good source of organic matter.

(ii) Crop rotation – Adaption of proper crop rotation and inclusion of leguminous crop (e.g. Sannhemp—Crotolaria juncea, Dhaincha—Sesbania aculeata, Cowpea—Vigna catjang etc.) will help to maintain the organic matter level of the soil.

(iii) Good aeration – Good aeration is necessary for proper decomposition of organic compounds added to the soil. Tillage will help in proper aeration of the soil as this process makes the soil loose and friable.

(iv) Application of fertilizers – Fertilizers encourage the growth of crop plants and addition of leaves, stubbles etc. in the soil increases which in turn converted to organic matter by microbiological decomposition.

(v) Application of lime in the soil – The growth of the plant and activity of soil organism are affected in acid soil. Liming helps to improve the physical condition of the soil and the plant grows well in that soil. The roots, stubbles, leaves etc. are added to the soil and they in turn, converted to organic matter by microbial decomposition and the organic matter content of the soil increases. The activity of microorganism responsible for nitrification is adversely affected in acid soil.