Nitrogen is available in atmosphere in high amount in the form of gas. It is converted into combined form of organic compounds by some prokaryotic microorganisms through biological reactions. The phenomenon of fixation of atmospheric nitrogen/by biological means is known as 'diazotrophy' or 'biological nitrogen fixation' and these prokaryotes as 'diazotrophs' or 'nitrogen fixers'. The diazotrophs may be in free living or in symbiotic forms.
It is estimated that the free living microorganisms contribute about 1-3 kg N/ha/annum. During 1974, the total global nitrogen fixation was about 175 x 106 tonnes, of which 90 x 106 tonnes N was fixed in cultivated soil (Schlegel, 1986) and about 45 x 106 tonnes by rhizobial symbiosis.