Organic Matter: Meaning, Origin and Composition

After reading this article you will learn about:- 1. Meaning of Organic Matter 2. Origin of Organic Matter 3. Nature and Composition.

Meaning of Organic Matter:

Soil organic matter consists of decomposing plant and animal residues. In addition to these materials, soil organic matter contains living and dead microbial cells, microbially synthesized compounds and a number of derivatives produced as a result of microbial activity. It is the store house of all essential plant nutrients.

It protects soils against erosion and helps to form good soil structure. It provides good aeration and better water movement by loosening the soil. For achieving maximum benefit, organic matter must be decomposed and continuously replenished with the addition of fresh organic materials to the soil.

After the active decomposition, organic residues are collectively called humus (organic soil colloid). Most dominant micro-organisms are involved in the decomposition of organic matter namely bacteria actinomycetes and fungi.

Different soil enzymes (protein substances) produced by these micro-organisms, are directly responsible for the decomposition by reducing the activation energy necessary to break the bonds of different organic materials.

Origin of Organic Matter:

Soil organic matter mainly originates from the plant tissue. Leaves and roots of trees, shrubs, grasses and other plants etc. usually supply large quantities of organic materials to the soil. These plant constituents form the primary material both for the food of the soil organisms and for the production of soil organic matter.

Animals are generally considered secondary sources of organic matter. As they break down the original plant tissues, they contribute waste products and leave their own bodies after death. Certain forms of animal life, particularly earthworms, centipedes, insects and ants, also play an important role in the turnover of plant residues.

Nature and Composition of Soil Organic Matter:

The soil organic matter consists of a whole series of products which range from un-decomposed plant and animal tissues to fairly amorphous brown to black material bearing no trace of the anatomical structure of the material that is normally defined as the “soil humus”.

The native fraction of soil organic matter is made up of a heterogeneous mixture of polymerized aromatic molecules, polysaccharides, bound amino acids, uronic acid polymers and various organic phosphorus compounds. Organic residues consist of both organic and inorganic fractions.

A simple outline for the presence of different compounds in organic residues is given below:

On an average the green plant materials contain mainly water and dry matter. The dry matter content of the material and organic matter also contains various substances like different elements and organic compounds which are shown below:

Since various types of plant tissues are found in nature, their composition also varies accordingly. However, the most tentative composition of a plant material has shown in the above scheme. From the scheme it is found that various organic compounds are present consisting of carbohydrates, proteins, lignins etc. Of which carbohydrate group of organic compounds are predominant.

Lignins containing carbon, hydrogen and oxygen are very resistant to microbial decomposition. The crude proteins are perhaps the most complicated which contain carbon, hydrogen, oxygen, nitrogen, sulphur, iron and phosphorus. All these plant and animal remains are of no importance until and unless they are subjected to decomposition by different soil micro-organisms.

The decomposition of such residues in soil constitutes a basic biological process in that carbon (C) is recirculated to the atmosphere as CO₂; nitrogen (N) is made available as NH₄⁺ and NO₃^–, and other associated plant nutrients like P, S and various micronutrients (Fe, Mn, Cu, Zn etc.) appear in forms which will be available to plants.