(a) Azospirillum and Azotobacter (b) They can fix atmospheric nitrogen, due to the presence of Rhizobium /N2​ fixing bacteria in their root nodules. The Nitrogen fixation in leguminous plants starts with nodule formation in the roots. A common soil bacteria, Rhizobium invades the roots and multiplies within the cortex (thus forming nodules). It fixes the atmospheric Nitrogen in the leguminous roots, whereas the legume provides the bacteria with energy and nutrition. Hence, due to the symbiotic relationship between the leguminous plants and the Nitrogen fixating bacteria, the soil does not require Nitrogen enrichment.