The ability to fix atmospheric nitrogen into a form that can be used for plant growth is confined to bacteria and cyanobacteria. Plants fix nitrogen only by virtue of associations with these simple organisms. The best-known associations are the symbioses of Rhizobium bacteria with agricultural legumes, such as clovers, peas, and beans. Rhizobia stimulate the formation of root nodules, which provide a specialized environment within which high rates of nitrogen fixation can occur. Nitrogen fixation not only supports plant growth independent of mineral nitrogen in the soil but also can improve soil nitrogen status as plant residues, notably leaves and fine roots, decay and are mineralized. Accurate estimates of amounts of nitrogen fixed are difficult to obtain, but a well-nodulated, young, densely planted stand of nitrogen-fixing trees may fix 80–200 kg of nitrogen per hectare per year, similar to rates reported for some leguminous crops.