Legume Crops

Legumes are plants in the botanical family Fabaceae, which is also called Leguminosae, or the pea family. Legumes are the second-largest family of flowering plants, with between 14,000 and 17,000 species and as many as 18,000 species that belong to the pea family, with the Composite family being the largest. The Latin origin of the term legume (legumen) is the verb for “to gather,” reflecting the fact that the seed pods of the plants are gathered for food.

The “fruit” of legumes is a seed pod. The seeds (pulses), such as a pea, a bean, or the peanut, split into two valves. The seeds are attached to one edge of the valves. The pod or the seeds are used as food. They include a variety of common and broad beans, as well as acacia, alfalfa, carob, clover, cowpeas, lentils, lupines, mesquites, mimosa, peas, soybeans, tamarind, and peanuts.

The photo shows the small (2 mm) nodules on the roots of a legume plant where the beneficial bacteria rhizobia establishes itself and helps the plant procure nitrogen from air.

Source: U.S. Department of Agriculture Agricultural Research Service/Markus Dubach

Legumes can be found in much of the world and vary so widely that it is often only the trained botanist who recognizes them as legumes. There are three subfamilies of Fabaceae or legumes (Caesalpinioideae, Mimosoideae, and Papilionoideae), all of which produce root nodules. Some are trees, and others are shrubs, herbs, or climbing plants. Papilionoideae are legumes that have flowers that look like butterflies. The sweet pea is a member of this group. The flowers vary from small to large and from regular to irregular.

The ability of legumes to fix nitrogen in the soil makes them an important natural fertilizer. Nitrogen is a gas that makes up much of the Earth's atmosphere. However, it is relatively inert, which creates a problem for soil fertility. Without nitrogen, the fertility of soil is low. Violent lightning can create nitrogen compounds in the atmosphere that can wash into the soil to fertilize it. Much more effective are the legumes that can fertilize [Page 286]the soil by fixing atmospheric nitrogen. The roots of these legumes take in nitrogen from the air with the aid of a beneficial bacterium.

Legumes have a unique relationship with the bacteria rhizobia, which are soil bacteria. They fix nitrogen (diazotrophy) after the bacteria becomes established in the root nodules of legumes. It is a symbiotic relationship: The rhizobia cannot independently fix nitrogen; they can only do so through a host plant. The rhizobia are usually gram-negative, motile, and nonsporulating rods.

Humans have been growing and gathering legumes for around 8,000 years. Where meat is in short supply, legumes make an excellent source of protein. Legumes such as beans are usually eaten by cultures that depend on rice or corn as their main food; combined, legumes and grains provide a nearly complete diet.

Globally, legumes such as peas, beans, and peanuts are very important economically as foods. The legumes alfalfa, clover, and vetch are grown for animal feed in forage or pasture. Many other legumes are valuable in making edible oils, flavors, fibers, gums, fibers, oils, medicines, dyes, poisons, and timber. Some are used naturally, and others are chemically processed and synthesized into plastics or other products. Some are used as ornamentals.

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