Fundamentals of Soil Science (1+1)

Decomposition of Soil Organic Matter

Decomposition of Soil Organic Matter

• The organic materials (plant and animal residues) incorporated in the soil are attacked by a variety of microbes, worms and insects in the soil if the soil is moist.
• Some of the constituents are decomposed very rapidly, some less readily, and others very slowly .The constituents in terms of ease of decomposition are:
• Sugars, starches and simple proteins = easy to decompose
• Crude proteins
• Hemicelluloses
• Cellulose
• Fats, waxes, resins
• Lignins = Very difficult to decompose

The organic matter is also classified on the basis of their rate of decomposition

• Rapidly decomposed : Sugars, starches, proteins etc.
• Less rapidly decomposed : Hemicelluloses, celluloses etc.
• Very slowly decomposed : Fats, waxes, resins, lignins etc.

• The organic/ humic substances are produced when plant residue and other organic debries are broken down or chemically altered. Fungi dominate over others in the initial stages while bacteria are the important agents of decomposition during the later stages.
• At first, the easily decomposable substances like sugars, starches and water soluble proteins are acted upon by the microorganisms and decomposition and digestion is rapid. Crude protein is next in order, followed by hemicelluloses.
• Cellulose, which is more resistant to microbial attack than hemicellulose decomposes much more rapidly than oils, fats, waxes, resins etc. Lignin decomposes very slowly and continues to dominate soil organic matter when the decay process slows down.
• The sugars, starches, hemicelluloses and celluloses are ultimately decomposed to carbon dioxide and water and energy is liberated which is utilized by microorganisms. Some oils, fats, waxes and resins are also slowly decomposed to carbon dioxide and water and some energy liberated for use by microorganisms.
• A small portion of lignin may be slowly decomposed to form aromatic compounds. Other portions may be chemically altered. Some other portions may chemically unite with protein to form part of the soil humus.
• Proteins are gradually decomposed to amino acids and amides which are further decomposed to ammonium compounds by microorganisms. Ammonium compounds are oxidised to nitrites by Nitrosomonas bacteria. Nitrites are further oxidized to Nitrates by Nitrobacter bacteria.
• Phosphorus is present in the organic matter as phytin, nucleic acid and phospholipids, which are decomposed to liberate the phosphorus present in them as orthophosphate ions, H₂PO₄^-.
• Similarly, sulphur containing amino acids like methionine, cysteine etc. Are decomposed by microorganisms to liberate the sulphur contained them as sulphate. When organic matter decomposes, other complex organic forms of nutrients are converted to simple ionic forms like K⁺, Ca⁺⁺, Mg⁺⁺ , etc.
• This process of conversion of complex organic forms of nutrients to simple inorganic forms by microorganisms is called the mineralisation of nutrients.
• A portion of the nutrients thus mineralized is assimilated by the microorganisms themselves for synthesis of their cell protoplasm. Thus the simple inorganic form of the nutrient is recovered to the complex organic form of nutrients. This process of conversion of the simple inorganic form of nutrients to the complex organic form of nutrients is called immobilization of nutrients.
• During the earlier stages of decomposition of organic matter all the simple inorganic forms of nutrients are assimilated by microorganisms which multiply rapidly and continue to decompose the organic matter.
• When almost all the carbon compounds have been decomposed, the microorganisms die due to the lack of sufficient a mounts of energy giving carbon compounds. Their bodies decay when the complex organic forms of the nutrients are reconverted to simple inorganic forms.
• Some proteins combine with organic compounds like lignins, tannins, humic acids etc. Some proteins are absorbed by the clay, especially the expanding ones. All these reactions protect the proteins from microbial decomposition. At this stage, almost all the original organic material has been converted to dark heterogeneous mass called humus.
• Humus is a resistant complex mixture of dark brown to black coloured colloidal and amorphous substances synthesized or modified from the original organic materials by various microorganisms.
• Simple decomposition products under aerobic decomposition are CO₂,NH₄, NO₃, H₂PO₄, SO₄ and H₂O
• Simple decomposition products under anaerobic decomposition are CH4, H2S, dimethyl sulphide, ethylene, ammonium ions, amine residues and organic acids

C: N ratio

• A close relationship exists between organic matter and nitogen content of the soil (Fig.2). The ratio of organic carbon to nitrogen in soils is known as Carbon: Nitrogen ratio or C/N ratio of the soils.
• The C / N ratio of residue affects the rate of decomposition of organic matter.
• The organisms that decompose residues need N (and other essential elements) as well as C, if there is little N in the residue, decomposition is slow
• Also, if there is little N in the residue, microorganisms will utilize inorganic N in the soil to satisfy their N requirement, thereby competing with plants for N and reducing the amount of soil N available for plant growth.
• The C / N ratio in soil is relatively constant and = 12. In plant residues, it is highly variable and increases with maturity.
• The C / N is lower in microorganisms and = 8. Since microbes incorporate only about 1/3 of the C metabolized into biomass, the substrate material must have C / N = 24 to satisfy the N requirement of microbes.
• Generally, when organic substances with C/N ratios greater than 30:1 are added to soil , there is immobilization . For ratios between 20 and 30 , there may be neither immobilization nor release of mineral N. If the C / N ratio of residue is < 20:1 , there is usually release of mineral n.soil N is consumed by microbes and plant- available N decreases