Plants obtain their carbon from atmospheric carbon dioxide. A plant's weight is forty-five percent carbon. Elementally, carbon is 50% of plant material. Plant residues have a carbon to nitrogen ratio (C/N) of 50:1. As the soil organic material is digested by arthropods, and micro-organisms the C/N decreases as the carbonaceous material is metabolized and carbon dioxide (CO2) byproduct is released and finds its way to the atmosphere. The nitrogen, however, is sequestered in the bodies of the live matter. Normal CO2 concentration in the atmosphere is 0.03% which is probably the factor limiting plant growth. In a field of corn on a still day during high light conditions of the growing season, the CO2 concentration drops very low but under such conditions the crop could use up to 20 times the normal concentration. The respiration of CO2 by soil micro-organisms decomposing soil organic matter, contribute an important amount of CO2 to the photo-synthesizing plants. Within the soil CO2 concentration is 10 to 100 times atmospheric but may rise to toxic levels if the soil porosity is low or impeded by a flooded condition.[74]
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