soil bacteria

Rhizobia are soil bacteria which become established inside root nodules of legumes (the pea family: Fabaceae).

Rhizobia bacteria in nodules fix nitrogen
Soybean root nodules, each containing billions of Bradyrhizobium bacteria

They fix nitrogen in a form the plant can use. Rhizobia require a plant host; they cannot independently fix nitrogen. The relationship is a classic example of symbiosis and mutualism.

The word rhizobia comes from the English root for root (rhizo) and the Ancient Greek for life (bios).