10.5: Carbon/Nitrogen Ratio

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Page ID: 14691

Anna R. Schwyter & Karen L. Vaughan
University of Wyoming via UW Open Education Resources (OER)

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The proportion of carbon to nitrogen in the organic matter is an important factor in controlling microbial activity. Organic material having a high carbon to nitrogen ratio (C/N), such as wheat straw at about 80/1, will decay relatively slowly because the material contains insufficient nitrogen to satisfy the growth requirements of the decay producing microorganisms. Soil microorganisms often retain the available nitrogen for prolonged periods. This nitrogen immobilization by microbes can create nitrogen deficiencies in the soil and lead to reduced plant growth. Legume residues, such as clovers and alfalfa, have low C/N ratios (< 30/1) and decay very rapidly in the soil. They release large amounts of CO₂ and some nutrients for plant growth. Materials from young plants, having low C/N ratios, decompose more rapidly than do materials from old plants, having higher C/N ratios. A list of common organic materials and their C/N ratios is provided in Table 2.

Table 2. The C/N ratio for some common organic materials.
Organic substance	Carbon	Nitrogen	C/N ratio
Microorganisms	50	6.2	8/1
Humus	50	4.5	11/1
Alfalfa	43	2.5	18/1
Clover hay	40	2.0	20/1
Barnyard manures	35	1.4	25/1
Sewage sludge	48	1.7	28/1
Rye grass	40	1.3	30/1
Peat moss	48	0.8	60/1
Corn stalks	44	0.6	73/1
Barley straw	45	0.1	450/1
Fir bark	54	0.1	540/1
Redwood sawdust	51	0.05	1020/1

Microbial decay of organic matter releases copious amounts of carbon dioxide. In this way carbon is cycled from the soil to the air, completing the carbon cycle that began with photosynthesis. More microbial activity is synonymous with a greater production of carbon dioxide by soil microbes.

Composting of organic material is one way to manage the overall decay process. This allows time for the microbes to decompose most of the organic residues.