Role and Harmful Effects of Organic Matter in Soil Fertility

After reading this article you will learn about the role and harmful effects of organic matter in soil fertility.

Role of Organic Matter in Soil Fertility:

The importance of soil organic matter in relation to soil fertility and physical conditions is widely recognised. However, organic matter contributes to the fertility or productivity of the soil through its positive effects on the chemical, physical, and biological properties of the soil.

In the formation of a fertile soil, organic substances play a direct role, as they are the sources of plant nutrients which are liberated in available forms during mineralization. The functions of organic matter depend on the process of decomposition, and hence the production of a large quantity of residues as well as their subsequent decay in the soil is necessary for good soil and crop management.

The role of organic matter in soil is so varied and extensive that it makes one think of the claims printed on old-time patent medicine labels; fortunately, these for organic matter are all true:

1. Organic matter influences the soil colour. Due to presence of adequate amount of organic matter in soil, the colour will be brown to dark brown or black.

2. Organic matter binds soil particles (sand, silt and clay) into structural units called aggregates. These aggregates help to maintain porous, open and granular condition. Hence organic matter has a profound effect on the improvement of soil structure and thereby soil maintains favourable condition of aeration and permeability.

Organic matter supplies polysaccharides (long chain sugars) which also help for the genesis of good soil structure.

3. The rate of infiltration and percolation of water is enhanced by the application of organic matter in the soil. Drainage condition of soils can also be improved with the application of organic matter.

4. Water holding capacity (WHC) is increased by the application of organic matter. The amount of available water in sandy and loamy soils increases with the application of organic matter.

5. Organic matter reduces plasticity, cohesion, stickiness etc. in soils containing appreciable amounts of clay.

6. Organic matter increases the ability of the soil to resist erosion. It also reduces the surface run-off by providing good infiltration.

7. Organic matter affects the densities of soil especially bulk density of soil which in turn influence the soil porosity favorably.

8. Organic matter reduces losses of surface soil by wind erosion by forming granules with soil particles.

9. The presence of coarse organic matter on the soil surface provides mulching and thereby lower soil temperature during the summer season and keeps the soil warmer in winter season. It also acts as an insulator, retarding heat movement between the atmosphere and the soil.

10. During decomposition of organic matter various organic acids and CO₂ liberate in the soil which help to reduce alkalinity of soil.

11. Organic matter has high cation adsorption capacity. It has two to thirty times more cation adsorption capacity than that of mineral soil colloids. Besides this, organic matter accounts for 30 to 90 per cent of the adsorbing power of mineral soils.

Since organic matter possesses different functional groups, it has been estimated that 54 per cent of the exchange positions in soil organic matter were attributable to carboxyl (—COOH) groups, 36 per cent to phenolic and enolic hydroxyl groups, and 10 per cent to imide nitrogen groups.

12. Organic matter also acts as a buffering agent. Due to such buffering effect it reduces the likelihood of damage to plant roots from excessive acids, alkalies or salts, whether naturally present in the soil or added to it in the form of fertilizers and thus stabilizes the soil pH.

13. Organic matter has solubilizing effect on some mineral compounds present in the soil. The simplest form of action of organic substances on the mineral part of the soil is the dissolution of mineral compounds by the root exudates (CO₂ and organic acids) and by various products derived from microbial activity which ultimately bring about the conversion of a number of chemical elements into forms available to plants.

14. Organic matter can be considered as a store house of various nutrients; essential to plant growth. Organic matter is the source of 90-95 per cent of the nitrogen in unfertilized soils.

In addition, organic matter supplies available phosphorus, sulphur, and some other micronutrients like Fe, Mn, Cu and Zn etc. to the soil and thereby increases the- nutrient regime as well as improves the soil health. Organic matter should be regarded as a positive asset to high crop production.

15. Organic matter temporarily absorb heavy metal pollutants (lead, cadmium etc.) which are usually derived from applied waste waters used for irrigation. Adsorption of pollutants helps clean contaminated water.

16. Organic matter serves as a source of energy for both macro- and micro-organisms and helps in performing various beneficial functions in soil (e.g. nitrogen fixation, mineralization etc.).

17. Organic matter acts as a chelate. A chelate (keylate) is any organic compound than can bound to a metal (usually Fe, Zn, Cu, Mn etc.) by more than one bond and form a ring or cyclic structure by that bonding. The soluble chelates probably help mobilize these micro-nutrients, increasing their availability to plants and mobility in soils.

18. Various organic substances like vitamins, antibiotics and growth-promoting compounds namely auxin are produced by different micro-organisms during decomposition of various organic matter. Auxins enhance the growth of plants by influencing seed germination, root growth etc. Fungi toxic substances derived from organic matter may minimise the incidence of diseases.

19. Organic matter interact with organic pesticides and it can sorb N and S oxides from the atmosphere which have implications with respect to both the yields of crop plants and the quality of the environment respectively.

Harmful Effects of Organic Matter in Soil Fertility:

Many beneficial effects of organic matter in the soil are counter balanced by harmful influences under certain conditions.

A few harmful effects are given below:

(i) Organic matter is an energy and carbon source for many disease organisms, ensuring their longer periods of survival in soils.

(ii) Excessive amounts of organic matter create a problem for mixing with the soil thoroughly and obstruct easy planting.

(iii) Burning off crop residues is harmful because:

(a) It removes organic material that protects the soil against erosion,

(b) Some ash that contains the different plant nutrients can be lost by wind or water erosion,

(c) Majority of plant nutrients containing in the ash are soluble and some are easily removed from the soil through leaching and

(d) Binding capacity of soil particles are lost and as a result aggregate formation reduces.

(iv) Various organic residues produce different phyto-toxins during their decomposition which make them undesirable as organic matter. Allelopathy is also active. Allelopathy is any direct or indirect harmful effect of one plant on another through the production and release of toxic substances into the environment. Most of the toxins identified so far are short-chain organic acids e.g. butyric, propionic etc..