Answer:
Nitrogen fixing bacteria is the organism which is responsible for the fixing of atmospheric nitrogen into usable form for the plant.
Explanation:
Nitrogen fixing bacteria is a type of bacteria which fixes atmospheric nitrogen into ammonium and nitrates form. Plants are unable to take atmospheric nitrogen so it must be converted into available form i. e. nitrates and ammonium. Examples of nitrogen fixing bacteria are Cyanobacteria and Azotobactor etc. These nitrogen fixing bacteria lives in the root of the plant and makes symbiotic association.