bio-fertilizers is a substance which contains living microorganisms which, when applied to seed, plant surfaces, or soil, colonizes the rhizosphere or the interior of the plant and promotes growth by increasing the supply or availability of primary nutrients to the host plant. bio fertilizers have plenty of rizobium bacteria

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Bio-fertilizers are organisms that enrich the nutrient quality of soil.
The main source of bio-fertilizers are bacteria,fungi and  cyanobacteria (blue-green algae). Bio- fertilizers include nitrogen fixers which are able to fix nitrogen either by living freely in the soil or associated symbiotically with plants.
For getting nitrogen RHIZOBIUM can act as a bio-fertilizer for legume plant.
Rhizobium forms an endosymbiotic nitrogen fixing association with roots of legumes and parasponia.The bacteria colonize plant cells within root nodules;here the bacteria convert atmospheric nitrogen to ammonia and then provide organic nitrogenous compounds to the plant.The plant provides the bacteria with organic compounds made by photosynthesis.
Azobacter/Azospirillum also provide nitrogen for non-legume crops.