Legumes are plants that bear their seeds in pods. They differ markedly from grasses, cereals and other non-legume crops because much of the nitrogen they require is produced through fixation of atmospheric nitrogen by bacteria in nodules on their roots. As a result, legumes are rich in protein. World-wide more than 16,000 species of legumes are known, including herbs, shrubs and trees, but only about 200 are cultivated. In Minnesota the legume species grown commercially are lentil, soybean, field bean, field pea, alfalfa and sweetclover .
Forage legumes, such as alfalfa and sweetclover, were grown in rotation with cereals soon after the homesteaders first the homesteaders first broke prairie grassland around the turn of the century. In these early years legumes generally failed to improve soil conditions because the recently cultivated soils still had good structure and large reserves of organic matter. Furthermore, levels of available nitrogen in the soil were so high that biological nitrogen fixation in legumes was largely inhibited.
Over time, soil organic reserves declined due to cereal cropping and frequent fallowing. this resulted in an increase in green manuring, thus an increase in the importance of legumes. After World War II, however, farming was rapidly mechanized and traditional "legume-cereal" rotations were abandoned as relatively cheap nitrogen fertilizers became widely available and they appear ed more efficient than legumes in increasing grain yields. Another reason for the declining use of legumes in rotations was the mounting evidence that deep-rooted forage legumes frequently depressed the yields of subsequent grain crops by depleting soil moisture reserves, particularly in drier years and areas.
When the prairie sod was broken the amount of nitrogen released during a fallow period was more than enough to grow a crop. The straw could even have been removed for use elsewhere and enough nitrogen would have been produced from just the humus to satisfy an average crop. Over time, however, tillage and erosion reduced the ability of the humus to supply nitrogen. Now, many fallow fields need nitrogen fertilizer. Growing concerns about declining organic matter, soil fertility and rising energy and nitrogen fertilizer costs have led to renewed interest in legumes.
Legumes have long been recognized and valued as "soil building" crops. Growing legumes improves soil quality through their beneficial effects on soil biological, chemical and physical conditions. When properly managed, legumes will: enhance the N-supplying power of soils, increase the soil reserves of organic matter, stimulate soil biological activity, improve soil structure, reduce soil erosion by wind and water, increase soil aeration, improve soil water-holding capacity, and make the soil easier to till. The extent of these soil improvements depends mainly on the type of legume used, the quantity of plant material returned to the soil, and the soil and climate conditions.
Thus, the role of legumes as a nitrogen supplier in the rotation and as a builder of soil organic matter will likely gain importance in the future.