C:N ratio

Organic materials are composed of a mixture of carbohydrates, lignins, tannins, fats, oils, waxes, resins, proteins, minerals, and other assorted compounds. With the exception of the mineral fraction, the organic compounds are composed of varying ratios of carbon and nitrogen . This is commonly abbreviated to the C:N ratio. Carbohydrates are composed of carbon, hydrogen , and oxygen and are relatively easily decomposed to carbon dioxide and water, plus a small amount of other by-products. Protein-like materials are the prime source of nitrogen compounds as well as sources of carbon, hydrogen, and oxygen and are important to the development of the C:N ratio and the eventual decomposition rate of the organic materials.

The aerobic heterotrophic bacteria are primarily responsible for the decay of the large amount of organic compounds generated on the earth's surface. These organisms typically have a C:N ratio of about 8:1. When organic residues are attacked by the bacteria under appropriate habitat conditions, some of the carbon and nitrogen are assimilated into the new and rapidly increasing microbial population, and copious amounts of carbon dioxide are released to the atmosphere . The numbers of bacteria are highly controlled by the C:N ratio of the organic substrate.

As a rule, when organic residues of less than 30:1 ratio are added to a soil , there is very little noticeable decrease in the amount of mineral nitrogen available for higher plant forms. However as the C:N ratio begins to rise to values of greater than 30:1, there may be competition for the mineral nitrogen forms. Bacteria are lower in the food chain/web and become the immediate beneficiary of available sources of mineral nitrogen, while the higher species may suffer a lack of mineral nitrogen. Ultimately, when the carbon source is depleted, the organic nitrogen is released from the decaying microbes as mineral nitrogen.

The variation in the carbon content of organic material is reflected in the constituency of the compound. Carbohydrates usually contain less than 45% carbon, while lignin may contain more than 60% carbon. The C:N ratio of plant material may well reflect the kind and stage of growth of the plant. A young plant typically contains more carbohydrates and less lignin, while an older plant of the same species will contain more lignin and less carbohydrate. Ligneous tissue such as found in trees may have a C:N ratio of up to 1000:1.

The relative importance of the C:N ratio addresses two concerns: one, the rate of the organic matter decay to the low C:N ratio of humus , (approximately 10:1), and secondly the immediate availability of mineral nitrogen (NH ⁺ 4 ) to meet the demand of higher plant needs. The addition of mineral nitrogen to organic residues is a common practice to enhance the rate of decay and to reduce the potential for nitrogen deficiency developing in higher plants where copious amounts of organic residue which has a C:N ratio of greater than 30:1 have been added to the soil.

Composting of organic residues permits the breakdown of the residues to occur without competition the of higher plants for the mineral nitrogen and also reduces the C:N ratio of the resulting mass to a C:N value of less than 20:1. When this material is added to a soil, there is little concern about the potential for nitrogen competition between the micro-organisms and the higher plants.

[Royce Lambert ]

RESOURCES

BOOKS

Brady, N. C. "Soil Organic Matter and Organic Soils." In The Nature and Properties of Soils. 10th ed. New York: Macmillan, 1990.

Millar, R. W., and R. L. Donahue. "Organic Matter and Container Media." In Soils: An Introduction to Plant Growth. 6th ed. Englewood Cliffs, NJ: Prentice Hall, 1990.