There is a symbiotic association between the rhizobia bacteria and the host legume plant where the rhizobia bacteria can obtain their food requirements from the legume, and in return supply the legume with nitrogen.
It is possible that the right kind of rhizobia for plant growth may be absent in the soil for particular legume plants. Therefore to get the plant to fix its own nitrogen effectively (producing healthy nodules on the roots) each legume species require the correct rhizobia strain to be present.
Strains of rhizobia bacteria that suit particular legumes are grown and multiplied under laboratory conditions by inoculant manufacturers. The correct strain of inoculant for each legume species can be purchased from particular companies to inoculate onto legume seeds before sowing, refer to the inoculant group table below. Peat is the most widely used form of inoculant carrier but in recent times inoculants have become available as water soluble powder or in a granular form for some strains.