How do legumes fix nitrogen?

Legumes and nodule bacteria. Beans, peas, vetch, lupin and clovers fix nitrogen only with nodule bacteria (Rhizobium) on their roots. Where legumes haven't grown for years there may be none: before sowing, moisten the seed with a bacterial preparation for that very crop (sold as rhizobium inoculant, «rhizotorphin», «nitragin»), sow the same day, out of the sun. It noticeably raises the yield and the nitrogen left for the next crop. Check pink nodules on the roots in midsummer — a sign that it works.