Legumes generically describes any plant in the pea family, Fabaceae or formally known as Leguminosae. These plants produce seed pods that split along two sides to reveal their seeds. Another key feature of legumes is their ability to add nitrogen to the soil, making it more fertile, a process known as fixation. Not all legumes fixate nitrogen as well as others. Soil bacteria work in beneficial cooperation with the roots in this organic process.
The majority of earth's atmosphere comprises nitrogen, but as a gas, green plants cannot utilize it well. Fixation is a process of converting gaseous nitrogen into molecules of nitrogen that can be absorbed by plant tissues in the soil, like in the form of ammonia. Legumes help fixate nitrogen gas that penetrates the soil and deposits it in usable molecules. According to New Mexico State University, legumes can provide 25 to 75 pounds of nitrogen per acre per year and upwards of several hundred pounds in a densely cropped field.
Legumes on their own do not fixate the nitrogen. A harmonious, mutually beneficial bond occurs between the legume plant and a soil bacterium, such as Rhizobium. The bacteria invade the cortex cells of the plant roots. The plant supplies all the energy and nutrients needed. Within a few weeks, small nodules form on the roots.
Nodules can be either long- or short-lived, based on the legume plant they are fused. Often nodules develop on primary tap roots on perennial legume plants, while short-lived nodules are continually replaced on the roots of annual legumes like garden beans. They can be variable in size and when young and not fixating nitrogen are white, gray or pale green in color. When actively fixating nitrogen, nodules turn pink or red in color and remain so as long as the plant is healthy and supplying ample energy.
Factors affecting Fixation
Energy is supplied to the bacteria for nitrogen fixation by the green plant leaves making food during photosynthesis. Environmental stresses to the legume plant can "shut off" the fixation, such as a drought, lack of light or cool weather that reduces the number of green leaves. An overabundance of nitrogen in the soil, such as after an application of a fertilizer, can also reduce nodule nitrogen fixation. It requires much less energy if the plant can absorb nitrogen directly from a rich soil than expending it through the help of the fixating bacteria nodules. New Mexico State University notes the majority of nitrogen fixed by the nodules is retained within the tissues of the legume plant and not released into the surrounding soil. Thus, farmers allow the nitrogen-rich tissues to decompose to make that nitrogen available for other plants in the soil the next growing season.
Legumes That Fix Nitrogen
Within the large pea family, there are three subfamilies of legumes that are differentiated by the shape of their flowers. According to the International Legume Database & Information Service, nitrogen fixation occurs primarily by those in the subfamilies Mimosoideae and Papilionoideae, which include trees like acacias and crops like peas, beans, clover and alfalfa. The third subfamily, Caesalpinioidae, contains plants like many ornate tropical flowering trees that rarely form nodules and thus do not enrich soils well.