1. Conversion of atmospheric nitrogen into nitrogenous salts to make it available to plants for its update is described as nitrogen fixation. 

2. When living organisms are involved in nitrogen fixation process it is known as biological nitrogen fixation. 

3. The process is mainly carried out by prokaryotic organisms, i.e. different kinds of bacteria present in soil. 

4. The nitrogen fixing organisms are known as diazotrophs or nitrogen fixers and about 70% nitrogen is fixed by them. 

5. The nitrogen fixers are either free living bacteria or symbiotic associated with other higher plants e.g. Rhizobium. 

6. The cyanobacteria have specialized cells heterocysts which help in process of nitrogen fixation. 

7. Nitrogen fixation is high energy requiring process and 16 ATP molecules are needed for fixation of one molecule of nitrogen to ammonia. 

8. Soil bacteria like Nitrosomonas, Nitrosocyccus convert ammonia to nitrate and the Nitrobacter convert nitrite to nitrate. This is known as nitrification, biological oxidation. 

9. These bacteria are chemoautotrophic and utilize these processes for their metabolism. 

10. Fabaceae plants like pea, bean have root nodules which harbour symbiotic bacterium Rhizobium which fixes nitrogen. It is host specific, soil bacterium, Nitrogen is made available to host plant.