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Global population and the nitrogen cycle. Scientific American 277, 76-81.
| Smil V. (1997).
| Nitrogen is of decisive importance, as it is needed for DNA, RNA and protein formation. Because withdrawals caused by the growth of crops and various natural losses, continually remove fixed nitrogen from the soil, preindustrial farmers, and organic farmers today, replace the lost nitrogen by enriching their fields with crop residues, with animal and human wastes and by growing legumes with nigtrogen-fixing symbionts. The combination of all these measures provide anually up to around 200 kg of nitrogen per hectare, which sets a theoretical limit on how many people can be fed from this land. The practical limit is about 5 people per hectare of arble area. A real breakthrough, therefore, was the invention of ammonia synthesis in a process known as Haber-Bosch synthesis and today about one third of the protein in humanity's diet depends on synthetic nitrogen fertilizer. Massive use of fertilizer has now many deterious consequences for the environment, still, there are no means available yet to grow crops without nitrogen.
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