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In this article we will discuss about:- 1. Nature of Soil Organic Matter 2. Types of Soil Organic Matter 3. Factors Affecting Organic Matter Decomposition 4. Role of Organic Matter.
Nature of Soil Organic Matter:
Soil Weight (i.e. of furrow slice) = 2×106kg/ha
Nitrogen status of Soil is 0.03-0.05%N means 1000kg of N/ ha.
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Total nitrogen of soil means available N + Fixed N.
Soil O.M. consists of a whole series of products which range from undecayed plants and animal tissues to fairly amorphous brown to black material bearing no terrace of the anatomical structure of the material that is normally defined as soil humus.
In addition to organic constituents present in undecayed plants and animal tissues, soil O.M. contains living and dead microbial cells, microbially synthesized compound and derivatives of these materials produced as a result of microbial decay. The composition of Humus contains 50% C, 35% O, 5% N and 5% H.
Types of Soil Organic Matter:
i. Humic Substances:
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Such materials produced in the soil are either yellow or brown to black, acidic, polydisperse substance of high molecular weight.
On the basis of solubility, Humic substances are divided into 3- classes:
(a) Fulvic acid – Lowest molecular weight and both acid and alkali soluble.
(b) Humic acid – Medium mol-wt. and alkali soluble but acid insoluble.
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(c) Humin – High mol-wt. and both acid and alkali insoluble except under the most drastic conditions.
Fulvic acid is most susceptible to microbial attack whereas humin is most resistant.
ii. Non-Humic Substances:
It includes all those classes of compounds occurring in plants and micro- organisms that appear to have relatively definite characteristics e.g. – carbohydrates, proteins, fats, waxes, resins, pigments and low mol. wt. compounds. Most of these could be relatively easily attacked by soil microorganism and has a rapid turn-over in the soil.
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Humus:
Humus is a complex and rather resistant mixture of brown or dark brown amorphous and colloidal substances modified from the original tissues or synthesized by the various soil organisms. Fulvic acid, humic acid and humin all come under humus. Humus is in dynamic condition.
In humus, 40-45% lignin and 30-33% proteins and rests are fats, waxes and residual materials. Lignin and proteins constitute about 70-80% hence humus is also called Ligno-Protein complex.
In humus, the C:N ratio is 10:1. In most of the Indian soil, C:N ratio is average 14:1. Humus is capable of adsorbing PO43- anions from soil solution but not other anions.
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Nature and Characteristics of Humus:
(i) Tiny colloidal humus particles (micelles) are composed of C, H and O.
(ii) Surface area of humus colloids is very high, generally exceeding that of silicate clays.
(iii) Negatively charged, the sources of charge being carboxylic (-COOH) or phenolic (C6H5OH) groups. The extent of the negative charge is pH- dependent (i.e. high at high pH).
(iv) At high PH, CEC: 150-300C mol/kg soil.
(v) Water holding capacity is 4-5 times that of silicate clays.
(vi) Low plasticity and cohesion, thus favourable effect on aggregate formation and stability.
(vii) Black colour.
(viii) Cation exchange reactions with humus are qualitatively similar to those occurring with silicate clays.
Cellulose is not readily available for the use of bacteria but this is first acted upon by fungi and changed into similar substances and made available for the use of bacteria.
Wood is decomposed by Actinomycetes.
O.M. content of Indian soils is low because of the high rate of decomposition under tropical and sub- tropical climate.
Factors Affecting Organic Matter Decomposition:
(i) Moisture:
When large quantities of O.M. are applied as manure in arid regions, the slender moisture in the soil may be largely used up for the decomposition of O.M. and the crop following may well suffer from lack of moisture.
(ii) Temperature:
The soil organisms are most active at 24-35°C O.M. of Indian soils is low because of the high rate of decomposition under tropical and subtropical climate. Except in a few localised areas in the hilly and high altitudes regions, the O.M. in most of the cultivated soils rarely exceeds 1% .O.M. content in Indian soil is generally 0.5%.
(iii) Aeration:
In clayey soils, decomposition is less rapid.
(iv) C: N Ratio:
Nitrogenous amendments increase CO2 evolution and greater loss of cellulose, hemicellulose and other plant polysaccharides. Low N- content or wide C:N ratio slows down decaying process. Therefore C:N ratio is used for predicting the rate of decomposition. It is important in controlling the available N, total O.M. rate of organic decay and in developing sound soil management schemes. The C:N ratio of arable soil is 8: 1-15: 1. A little variation is due to climatic conditions viz. temperature and rainfall.
The C:N ratio is lower in arid soil and subsoil. C:N ratio in plant- 20-30: 1 in legumes and farm manures and as high as 400: 1 in sawdust. In microbes it is 4: 1 to 9: 1. The C:N value for soil is in between plant and microbes. It is lower in arid soils than that of humid soils when annual temperature is about the same. It is also lower in warmer regions than in cooler ones if the rainfalls are about equal.
Significance of C: N Ratio:
Two major significance are:
(a) Keen competition among micro-organisms for available N results when residues having a high C:N ratio are added to soils.
(b) Because this ratio is relatively constant in soil, the maintenance of ‘C’ and hence O.M. in soil depends largely on the soil Nitrogen level.
When decay occurs, the C/N ratio of the remaining plant material decreases since ‘C’ is being lost as CO2 and ‘N’ is conserved.
The older the plants the larger will be C:N ratio and the longer will be the period of Nitrate suppression. Leguminous tissues have a distinct advantage over non – legumes since it promotes a rapid organic turn over in soils.
Legume stubbles decompose rapidly due to high N-content in the stubble.
The practical significance of this relatively constant ratio is that a soil’s O.M. content cannot be increased without simultaneously increasing its Organic Nitrogen content and vice-versa.
∴ 58g ‘C’ present in 100g O.M.
∴ 1g ‘C’ present in 100/58 = 1.72g O.M.
Therefore C: O.M. = 1:1.72
1.724 is called Bemlen Factor.
Muck and Peat Soils:
Muck soils are having highly decomposed O.M. i.e. well mixed O.M. in the soil. Whereas in peat soils, mostly O.M. are partially decomposed and found under excessive moisture conditions. Peat soils are acidic, pH 3.9 and below, 10-40% O.M., suitable for paddy when water recedes.
Role of Organic Matter:
Organic Matters are sources of plant nutrients which are liberated in available forms during mineralisation. Humus can be considered to be a store- house of various nutrients essential to plant growth. During the slow microbial decomposition of the soil humus, there is a gradual release, with subsequent mineralisation of C, N, S, P and other elements.
It improves soil structure, its drainage and aeration, water holding capacity, buffer and exchange capacities; influences the solubility of minerals and serves as a source of energy for the development of micro – organisms. 95% N and 33%P of soil are obtained from O.M. It has a capacity to control soil temperature.
Mineralisation and Immobilisation:
Mineralisation is the conversion of an element from an immobilised form to an available form as a result of microbial decomposition. Immobilisation is the reverse of mineralisation where available form of an element is fixed as immobile form.
N- 20-40% of total surface soil N is in the form of bound amino acids; 5-10% as combined hexose amines. Usually only 1-3% of the total amount of N present is mineralised during growing season.
When C:N ratio exceeds 30, Immobilisation occurs and when C:N ratio is below 20, mineralisation takes place.
Bartholomew & Kirkhan:
When immobilisation is faster than mineralisation, no available N is present in soil and vice-versa. As a general rule when C: N ratio of O.M. added to soil is greater than 30, there is Immobilization of soil Nitrogen during initial decomposition process.
Bartholomew (1971) suggested about the natural supply of Nitrogen-
(i) Soil organic N → 20-30 kg N/ha
(ii) Rainfall → 6-8 kg N/ha
(iii) Non-Symbiotic N-Fixation → 2-4 kg N/ha
(iv) Dust and organic particles through rainfall → 12-16kg N /ha
Subbhya and Asija (1956):
They used alkaline potassium permanganate method and given the following values for available Nitrogen.
Low Status – When < 250 kg N/ha
Medium – 250-500 kg N/ha
High – when > 500kg N/ha
P- Brammer (1951):
25- 80 % ‘P’ present in soil is in the form of Organic form. Major portion of ‘P’ in humus is in the form of phytin or its diratives (40-80%); nucleic acid (0-10%).
Humus increases the solubility of ‘P’ due to formation of phospho – humic complexes (which are easily assimilated by plants); anion replacement of PO43- by humate ion; coating of sesquioxide particles by humus to form a protective cover which reduces the phosphate fixing capacity of soil and arising of certain organic anion from decompositions to form stable complex with Fe and Al, thus preventing their reaction with ‘P’.
C:N : P = 100 : 10 : 1 and if C: P ratio is more than 100: 1, immobilisation of ‘P’ occurs.
S- Barrow (1960):
C: S ratio of O.M. when below 200; SO42- usually accumulated in soil, and when C: S > 400; SO42- tie up in soil O.M. When the C:S ratio : 200-400; SO42- either released or tied up.
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