Rhizobia are bacteria that can enter into symbiosis with legume plants (soy, alfalfa, peas, etc.).
They fix nitrogen from the atmosphere and make it available to the plant. In exchange, the plant provides the bacteria with energy.
Nitrogen is a mineral element essential to plant growth and must often be added to crops through chemical fertilizer, which is not eco-friendly, or through rhizobium inoculants, which are compatible with organic farming but are less effective, especially with soy.