Nitrogen gets circulated in the soil, air and living things as an element or in the form of its compounds. It is necessary for the proper growth of all plants. However, the plants cannot use the elemental nitrogen from the air; they require soluble compounds of nitrogen. Hence, the plants depend on the process known as "nitrogen fixation", which converts nitrogen in the air into nitrogen compounds.

This process is executed both naturally and by man.
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Natural fixation:
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Nitrogen cycle