Nitrogen gas is mainly fixed as nitrates which are immediately absorbed by plants.
There are 3 ways of nitrogen fixation
Atmospheric Nitrogen fixation -during thunder storms the lightning energy is used for combining nitrogen and oxygen to form oxides of nitrogen. This dissolves in rain water to form nitric acid and reach the soil. In the soil the acid is converted to nitrates by soil .
Biological nitrogen fixation- Rhizobium bacteria and Azotobacter can convert Atmospheric Nitrogen into ammonia and fix them in the soil as ammonium compounds
3 .Industrial nitrogen fixation -nitrate and ammonia fertilisers are directly added to the soil.
Ammonium compounds are converted in to nitrites by nitrosomonas and nitrites to nitrates by Nitrobacter.
It is important that ammonia is converted in to nitrites or nitrates as ammonia gas is toxic to plant!