biological or industrial process by which molecular atmospheric nitrogen is converted into a chemical compound that is essential for plant growth and is also used in industrial chemical production.

Biological Fixation.

The most widely used and most productive of the soil microorganisms capable of nitrogen fixation are symbiotic bacteria of the genus Rhizobium, which colonize and form nodules on the roots of leguminous plants such as clover, alfalfa, and peas (see Legume). These bacteria obtain food from the legume, which in turn is supplied with abundant nitrogen compounds. Soils are sometimes inoculated with a particular species of Rhizobium to increase a legume crop, which is often planted to replenish the nitrogen depleted by other crops.

Much smaller amounts of nitrogen are fixed in the soil by nonsymbiotic (free-living) bacteria such as the aerobes, which function in the presence of oxygen, and bacteria of the genera Klebsiella and Bacillus, which function without oxygen. Some forms of blue-green bacteria also fix nitrogen, such as the genus Anabaena, which, in symbiosis with the water fern Azolla pinnata, is said to markedly increase rice yields, as was the case in paddies in the Thai Binh region of northern Vietnam. The need for fixed nitrogen in agriculture today is far greater than can be supplied by natural biological processes, and the production of nitrogen compounds from atmospheric nitrogen is a major chemical industry.

Industrial Fixation.

The principal industrial nitrogen-fixation process today is the production of ammonia by passing a mixture of atmospheric nitrogen and hydrogen over a metallic catalyst (see Catalysis) at 500°–600° C (932°–1112° F). Ammonia is then oxidized to form nitric acid, which is in turn combined with ammonia to yield ammonium nitrate, used primarily in explosives and fertilizers (see Fertilizer). In another method, cyanamide, which is used as a fertilizer or in the production of cyanides, is produced by passing atmospheric nitrogen over heated calcium carbide in the presence of a catalyst.