<P> Nitrogen fixation is a process by which nitrogen in the Earth's atmosphere is converted into ammonia (NH) or related nitrogenous compounds . Atmospheric nitrogen, which is molecular dinitrogen (N), is relatively reactive and is metabilically useless to all but a few microorganisms . The fixation process converts N into ammonia, which is metabolized by most organisms . </P> <P> Nitrogen fixation is essential to life because fixed inorganic nitrogen compounds are required for the biosynthesis of all nitrogen - containing compounds . Therefore, as part of the nitrogen cycle, it is essential for agriculture and the manufacture of fertilizer . It is also, indirectly, relevant to the manufacture of all chemical compounds that contain nitrogen, which includes explosives, most pharmaceuticals, and dyes . Nitrogen fixation is carried out naturally in the soil by a wide range of nitrogen fixing Bacteria and Archaea, including Azotobacter . Some nitrogen - fixing bacteria have symbiotic relationships with some plant groups, especially legumes . Looser relationships between nitrogen - fixing bacteria and plants are often referred to as associative or non-symbiotic, as seen in nitrogen fixation occurring on rice roots . It also occurs naturally in the air by means of NOx production by lightning . </P> <P> All biological nitrogen fixation is effected by enzymes called nitrogenases . These enzymes contain iron, often with a second metal, usually molybdenum but sometimes vanadium . Microorganisms that can fix nitrogen are prokaryotes (both bacteria and archaea, distributed throughout their respective domains) called diazotrophs . Some higher plants, and some animals (termites), have formed associations (symbiosis) with diazotrophs . </P>

Which bacteria helps nitrogen fixation in the soil