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10.5: Carbon/Nitrogen Ratio

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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.

    This page titled 10.5: Carbon/Nitrogen Ratio is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Anna R. Schwyter & Karen L. Vaughan (UW Open Education Resources (OER)) via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.