After reading this article you will learn about the decomposition of various organic compounds.
Different organic residues (plants and animals) contain different groups of organic compounds. So different groups of compounds have their different ease of decomposition.
In-spite of having different composition in different organic residues, the ultimate end products of decomposition (particularly of aerobic) are more or less similar.
When fresh organic materials (plants and animals) are incorporated into the soil, three separate processes occur simultaneously as follows:
(i) Plant and animal tissue constituents disappear under the influence of microbial enzymes and appears new biological tissue, new microbial cells bringing about an increase in soil of proteins, polysaccharides and nucleic acids etc.
(ii) Breakdown of so formed organic compounds (according to their ease of decomposition) by different groups of micro-organisms and release of different essential plant nutrients like N, P, S etc. into the soil or immobilization takes place by a series of specific reactions.
(iii) Formation of resultant compounds to microbial action from the original organic-residues and also due to microbial synthesis.
A tentative scheme for the different stages of microbial decomposition of organic residues is shown in Fig. 19.3.
It is evident that different constituents of organic residues decompose at different rates. Simple sugars, amino acids, most proteins and certain polysaccharides decompose very quickly and can be completely utilized within a very short period. Large macro-molecules which make up the bulk of plant residues must first be broken down into simpler forms before they can be utilized further for energy and cell synthesis.
This process is carried out by certain specific enzymes excreted by micro-organisms. The utilization of residue components and their breakdown products (sugars, amino acids, phenolic compounds and others) leads to the production of microbial cells, which are further degraded following death of the organisms.
Soil humus, the complex array of substances left after extensive chemical and biological breakdown of fresh plant and animal remains, makes up 60-70 per cent of the total organic carbon in soils.