Decomposition of Organic Matter to Form Humus

After reading this article you will learn about the decomposition of organic matter to form humus.

Whenever organic matter decomposes, it is acted upon by varieties of micro­organisms. 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 micro-organisms and decomposition and digestion is rapid. Crude protein is next in order, followed by hemicellulose.

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 micro-organisms.

Some oils, fats, waxes and resins are also slowly decomposed to carbon dioxide and water some energy liberated for use by micro-organisms. A small portion of lignin may be slowly decomposed to form aromatic compounds. Other portions may be chemically altered. Some other portion 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 micro-organisms. Ammonium compounds are oxidized to nitrites by Nitrosomonas bacteria.

Nitrites are further oxidized to Nitrates by Nitrobacteria 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 micro-organisms to liberate the sulphur contained in 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 micro-organisms is called the mineralization of nutrients.

A portion of the nutrients thus mineralized is assimilated by the micro-organisms themselves for synthesis of their cell protoplasm. Thus the simple inorganic form of 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 micro-organisms which multiply rapidly and continue to decompose the organic matter.

When almost all the carbon compounds have been decomposed the micro-organisms die due to the lack of sufficient amounts 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, tannis, humic acids etc. Some proteins are adsorbed 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 micro-organism.