Soya bean root nodule. Coloured scanning electron micrograph (SEM) of a root nodule (upper centre) on the root of a soya bean (Glycine max) plant. Root nodules occur primarily on the roots of leguminous plants (family Fabaceae). They contain nitrogen fixing bacteria (in this case Bradyrhizobium japonicum) in a symbiotic relationship with the plant. The nodule provides food and shelter for the bacteria, which in turn convert nitrogen in the atmosphere into ammonium, a form that can be used by the plant.