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 metabolised and carbon dioxide (CO2) is released as a byproduct and finds its way out of the soil and into the atmosphere. The nitrogen, however, is sequestered in the bodies of the live matter and so it builds up in the soil. Normal CO2concentration in the atmosphere is 0.03%, which is probably the factor limiting plant growth. In a field of maize on a still day during high light conditions in the growing season, the CO2concentration 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 contributes an important amount of CO2 to the photosynthesising 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 if diffusion is impeded by flooding.

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