It is the intimate relationship between organic matter and nitrogen contents of soil. The ratio of the weight of organic carbon to the weight of total Nitrogen in soil or organic material is known as the C: N ratio.

 

Or, It is defined as the ratio of the weight of organic carbon to the weight of total nitrogen in soil or organic material.  The ratio of C: N in normal aerobic soil usually varies 10 or 12:1.

 

Or, C: N ratio is a ratio of the mass of carbon to the mass of N in a substance, C: N ratio is an indicator for nitrogen limitation of plants and other organisms.

 

 When Organic matter or fresh plant residues of high C: N are added to soil, it is gradually degraded or decomposed by microorganism and C is lost as CO₂. As a result of the C: N of the material is reduced and finally attain a more or less stable C: N of 10:1. At this stage, Organic matter is added and converted to humus. C: N plays an important role in the decomposition of OM in the following ways-

 

1. Keen competition for available N: When organic residues with a wide C: N ratio (50:1) are incorporated in the soil, a keen competition occurs between the microorganism and plants for mineral N presence in the soil. High C content of organic matter causes rapid growth of microbes resulting in a disappearance of Nitrate nitrogen from the soil because of the instant for this element being little or no N to plants.  Consequently, plant growth is affected as the decomposing proceed, the C: N of the organic matter decreases since C is being lost as CO₂and N are conserved. N becomes again available to plants through mineralization of N take microbial cells.

 

2. The consistency of C: N: As the decomposition process continues both C and N are now subjected to loss, the C as CO₂ and N as NO₃ which are leached or adsorbed by plants. At a point, C: N becomes more or less constant, generally stabilized 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 sulfate. With this available sources of N from ammonium sulfate, the release of available N is affected in a shorter period of time. The maintenance of carbon and hence soil organic matter depends largely on the soil N­-level.

 

3. This ratio is an important factor determining how easily bacteria are able to decompose organic material.

 

4. The microorganisms in compost use carbon for energy and nitrogen for protein synthesis, just as we use carbohydrates for energy and protein to build and repair our bodies.

 

5. The optimal proportion of these two elements used by the bacteria averages about 30 parts carbon to 1 part nitrogen.

 

6. Given a steady diet at this 30:1 ratio, they can decompose organic materials very quickly.