'Rhizobia' is a collective name given to groups of soil bacteria that can form a symbiotic partnership with legume roots. A rhizobial species that is compatible with a particular legume plant can infect its roots and form globular/spherical structures on the roots termed 'nodules'.
Within these nodules, a chemical reaction takes place that allows atmospheric nitrogen to be converted into plant nutrients. Rhizobia therefore play a significant role in promoting and improving soil fertility and productivity.
The benefit of this 'nitrogen fixation' is not limited to the legume plant alone. Once the plant decomposes, nitrogen is returned to the soil, which is then available to every other plant in the vicinity. In other words, legumes can contribute additional external nitrogen to the crop-soil ecosystem.
Although legumes have long been used in agriculture, their natural process for harvesting nitrogen has largely been replaced with the application of synthetic nitrogen fertilisers that were first introduced around 200 years ago, during the agricultural revolution.