The rhizobia infect or invade the roots of legumes and produce nodules containing millions of bacterial cells. An exchange of nutrients occurs between the rhizobia and the legume. The rhizobia provide the legume with nitrogen compounds that are in a form that can be used readily by the legume, while the rhizobia obtains nutrients, in particular carbon compounds from the legume. Rhizobia are able to convert the nitrogen from the air into nitrogenous compounds that can be converted into amino acids by plants. This is known as nitrogen fixation.
Legumes are a useful plant to be used in crop rotations for two reasons.
Firstly, because of nitrogen fixation legumes plants contain higher levels of nitrogen than non-leguminous plants. Therefore if the crop residues are ploughed back into the soil after harvest of the seeds, nitrogen is added to the soil, being made available for future crops in the rotation. Secondly, if legumes grow well, then excess nitrogen is released into the soil. These nitrogenous compounds are then made available for future crop growth.