Rhizobium is an important soil bacteria that has the ability to make the atmospheric nitrogen available to the plants. Our atmosphere is abundant in Nitrogen but it can not be utilized directly by the plants. Nitrogen has to be converted into ammonia (NH3) before it can be used by the plants. Rhizobia are soil bacteria that have the ability to do just that - convert nitrogen into ammonia.
They multiply and form nodules attached to the plant roots of leguminous plants such as peas and beans. There are multiple species of rhizobia and not all can form nodules in all leguminous plants. Each rhizobium species is associated with particular species of plants.
The common garden legumes such as different varieties of beans form association with Rhizobium leguminosarum var. phaseoli.
Common clover species such as white clover, crimson clover etc are associated with Rhizobium leguminosarum var. trifolli
Non-leguminous plants do not form these rhizobia root nodules. If you find nodules in non-leguminous plants suspect root knot nematode which is a parasite harming the plant.
These nitrogen fixing bacteria are useful not only for the leguminous plants but also help increase soil fertility for all plants. This is one of the reasons for crop rotation with a leguminous plants. The roots are left in the soil after the legume crop is harvested. The breakdown of these nodules provides nitrogen to the next non-legume crop.
This knowledge is helpful in home gardens too. Interplant the legume plants with the non legume plants and also use cover crops to increase soil fertility in the garden.
Some examples of nitrogen fixing plants:
Texas bluebonnet (a Lupine)