Nitrogen is a major nutrient essential to life. It is vital for building DNA and proteins in plants and animals in the feedchain and is sometimes likened to the fuel in an engine. Air is 79% nitrogen. Nitrogen also occurs naturally in many compounds in soil and water. Like all matter, nitrogen cannot be destroyed it can only change its form. The movement between these forms is called the Nitrogen Cycle.
Fertile soils hold vast reserves of organic nitrogen in decaying plant matter (humus) and in many soil organisms. To be usable by plants it has to be converted into the inorganic ammonium or nitrate ions which are released when bacteria and fungi breakdown organic matter.
Plants and animals cannot make direct use of nitrogen from the air so the first step in the nitrogen cycle is to convert nitrogen in the air into ammonium ions. This process is called fixation and can take place in various ways such as: –