Organic Matter: Maintenance and Role

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

Maintenance of Soil Organic Matter:

Carbon: Nitrogen (C: N) Ratio:

The ratio of the weight of organic carbon (C) to the weight of total nitrogen (N) in a soil (or organic material), is known as C : N ratio. When fresh plant residues are added to the soil, they are rich in carbon and poor in nitrogen. The content of carbohydrates is high. This results in wide carbon-nitrogen ratio which may be 40 to 1. Upon decomposition the organic matter of soils changes to humus and have an approximate C: N ratio of 10: 1.

The ratio of carbon to nitrogen in the arable (cultivated) soils (organic matter) commonly ranges from 8 : 1 to 15 : 1. The carbon-nitrogen ratio in plant material is variable, ranging from 20 : 1 to 30 : 1. Low ratios of nitrogen (10 :1) in soil organic matter generally indicate an average stage of decomposition and resistance to further microbiological decomposition. A wide ratio of C : N (35 : 1) indicates little or no decomposition, susceptibility to further and rapid decomposition and slow nitrification.

Significance of C : N Ratio. (1) Keen competition for available nitrogen when organic residues (with high C : N ratio) are added to soils. When organic residues with a wide C/N ratio (50 : 1) are incorporated in the soil, decomposition quickly occurs. Carbon dioxide is produced in large quantities.

Under these conditions, nitrate-nitrogen disappears from the soil because of the insistent microbial demand for this element. And for the time being, little (or no) nitrogen is available to plants, consequently plant growth is affected. As decay (decomposition) occurs, the C/N ratio of the plant material decreases since carbon is being lost and nitrogen conserved. Nitrates (nitrogen) again appear in quantity in the soil, thus, increases plant growth.

(2) Consistency of C : N Ratio. As the decomposition processes continue, both carbon and nitrogen are now subject to loss—the carbon as carbon dioxide and the nitrogen as nitrates which are leached or absorbed by plants. At a point carbon- nitrogen ratio, becomes more or less constant, generally stabilizes at 10:1 or 12:1.

It has always been considered profitable to mix the highly carbonaceous material like straw with a small quantity of ammonium sulphate. With these available sources of nitrogen from ammonium sulphate, the decomposition of fresh organic material is hastened and thus, the release of available nitrogen is affected in a shorter period of time.

Role of Organic Matter:

The effect of organic matter on fertility are as follows:

(1) Organic matter binds soil particles into structural units called aggregates. These aggregates help to maintain a loose, open, granular condition. Water infiltrate and percolate more readily. The granular condition of soil maintains favourable condition of aeration and permeability.

(2) Water-holding capacity is increased by organic matter. Organic matter definitely increases the amount of available water in sandy and loamy soils. Further, the granular soil resulting from organic matter additions, supplies more water than sticky and impervious soil.

(3) Surface run off and erosion are reduced by organic matter as there is good infiltration.

(4) Organic matter on the soil surface reduces losses of soil by wind erosion.

(5) Surface mulching with coarse organic matter, lower soil temperatures in the summer and keep the soil warmer in winter.

(6) The organic matter serves as a source of energy for the growth of soil microorganisms.

(7) Organic matter serves as a reservoir of chemical elements that are essential for plant growth. Most of the soil nitrogen occurs in organic combination. Also a considerable quantity of phosphorus and sulphur exist in organic forms. Upon decomposition, organic matter supplies the nutrients needed by growing plants, as well as many hormones and antibiotics.

(8) Fresh organic matter has a special function in making soil phosphorus more readily available in acid soils.

(9) Organic acid released from decomposing organic matter help to reduce alkalinity in soils.

(10) Fresh organic matter supplies food for such soil life as earthworms, ants and rodents. These microorganism improves drainage and aeration. Earthworms can flourish only in soils that are well provided with organic matter.

(11) Organic matter upon decomposition produces organic acids and carbon dioxide which helps to dissolve mineral such as potassium and make them more available to growing plants.

(12) Humus (highly decomposed organic matter) provides a storehouse for the exchangeable and available cations—potassium and magnesium. Ammonium fertilizers are also prevented from leaching because humus holds ammonium in an exchangeable and available form.

(13) It acts as a buffering agent. Buffering checks rapid chemical changes in pH and in soil reaction.