Why does nitrogen have to be "fixed"? what organisms are responsible for nitrogen fixation? (site 2)

Nitrogen needs to be fixed so that they can be converted into ammonium ions and nitrates making the nitrogen available to the plants . Nitrogen is a necessary element for the production of protein but it cannot be taken up by the plants into in its molecular form. Plants obtain the required amount of nitrogen from soil in the form of  either nitrate  or ammonium ions.

Some prokaryotic organism called diazotrophs are responsible for nitrogen fixation. e'g. Azospirilium, Azotobacter, Rhodospirillum, clostridium etc.

Explanation:

Conversion of molecular nitrogen to nitrate or ammonia is termed as nitrogen fixation.Nitrogen fixation is of two types; biological nitrogen fixation and Non-biological nitrogen fixation.Biological nitrogen fixation is carried out by  some prokaryotes called diazotroph while non- biological nitrogen fixation is carried out by lightning and  photo-chemical reaction.Living matter contains a large amount of nitrogen incorporated in protein, nucleic acids and many other bio-molecules.Plants being the producers in the food chain , meet the demand of nitrogen required for their growth by absorbing inorganic nitrogen.Nitrogen fixation is  a vital process that converts the molecular nitrogen into inorganic nitrates  so that it can be absorbed by the plant.