Cyanobacteria either free living of symbiotic are considered as biofertiliser because the fix atmospheric nitrogen in form of nitrates and nitrites by biological process. They leaves of Azolla pteridophytes has as many as $80,000$ BGA and fix atmospheric $N _{2}$ in field of rice.
Nostoc with corrolloid roots of Cycas also fix atmospheric nitrogen. Soil nitrogen is maintained by association between Rhizobium and root nodule of legume.