Nitrogen fixation is the conversion of nitrogen gas into nitrogen compounds. Nitrogen usually reacts only at high temperatures and pressures. Industrially, it is fixed in the Haber process, the cyanamide process, and the electric-arc process.
Some bacteria (Rhizobium) in the root nodules of leguminous plants, and also some of those living free in the soil (e.g., Azotobacter), can fix nitrogen from the air as ammonia, nitrates, and nitrites. Some fungi and cyanobacteria also fix nitrogen. See also nitrogen cycle.