Legume

Legumes belong to the pea family, Fabaceae (also called Leguminosae), and the term can refer to the plants themselves or their fruits and seeds. When the seeds are harvested dry for people to eat, they’re known as pulses. Farmers grow legumes mainly for human food, but also for animal feed (as forage or silage) and as green manure to improve soil. The fruit of a legume is botanically distinctive: a simple, dry pod that develops from a single carpel and usually splits open along two seams. Most legumes host symbiotic nitrogen-fixing bacteria called Rhizobia in their root nodules. This fixed nitrogen becomes available to later crops, which is why legumes are important in crop rotation.

The United Nations’ Food and Agriculture Organization (FAO) uses the term pulse strictly for legume crops harvested only for their dry seeds. This excludes green beans and green peas, which count as vegetables, as well as seeds grown mainly for oil (like soybeans and peanuts) and seeds used solely for sowing forage (such as clovers and alfalfa). In everyday language, these distinctions aren’t always clear, and many varieties used for dried pulses are also eaten as green vegetables when the pods are young. The FAO recognizes 11 primary pulses, leaving out green vegetable legumes, oilseed legumes, and those used only as seed.

Dry beans include kidney, navy, pinto, black turtle, and haricot beans (Phaseolus vulgaris); lima and butter beans (Phaseolus lunatus); adzuki or azuki beans (Vigna angularis); mung beans (Vigna radiata); black gram or urad (Vigna mungo); scarlet runner beans (Phaseolus coccineus); ricebeans (Vigna umbellata); moth beans (Vigna aconitifolia); and tepary beans (Phaseolus acutifolius). Dry broad beans cover horse beans, broad beans, and field beans (all Vicia faba). Dry peas include garden peas (Pisum sativum var. sativum) and protein peas (Pisum sativum var. arvense). Other pulses are chickpeas (also called garbanzo or Bengal gram), dry cowpeas (black-eyed peas), pigeon peas (also known as arhar, toor, cajan pea, Congo bean, or gandules), lentils, Bambara groundnuts (earth peas), common vetch, and lupins. Additional pulses not elsewhere specified include lablab or hyacinth beans (Lablab purpureus), jack beans (Canavalia ensiformis), sword beans (Canavalia gladiata), winged beans (Psophocarpus tetragonolobus), velvet beans or cowitch (Mucuna pruriens var. utilis), and yam beans (Pachyrhizus erosus).

Legumes are the third-largest land plant family by number of species, after orchids and daisies, with about 751 genera and roughly 19,000 known species—around seven percent of all flowering plants.

Many legumes host Rhizobia bacteria inside root nodules (though plants in the genus Styphnolobium are an exception). These bacteria convert atmospheric nitrogen (N₂) into ammonia (NH₃) through the reaction: N₂ + 8H⁺ + 8e⁻ → 2NH₃ + H₂. The ammonia then becomes ammonium (NH₄⁺), which some plants can use: NH₃ + H⁺ → NH₄⁺. This makes legume root nodules a source of nitrogen, so the plants are rich in nitrogenous amino acids and protein. When a legume plant dies—say, after harvest—the nitrogen in its amino acids is released into the soil, converted to nitrate (NO₃⁻), and becomes fertilizer for future crops. In traditional and organic farming, farmers often rotate legumes with non-legumes or grow them together, providing enough nitrogen without synthetic fertilizer. Legumes are also used as green manure. In Sri Lanka, a polyculture practice called coconut-soybean intercropping grows grain legumes in coconut groves, either as an intercrop or a cash crop, for their protein, oil, and ability to maintain soil fertility. However, continuous cropping after three to four years significantly reduces grain yields.

A common pest of grain legumes in tropical and subtropical Asia, Africa, Australia, and Oceania is the bean fly—a tiny, destructive fly in the family Agromyzidae. These flies attack a wide range of cultivated legumes from germination to harvest and can wipe out an entire crop early on. Black bean aphids are a serious pest for broad beans and other beans, often found on fathen, thistle, and dock. Pea weevils and bean weevils chew semi-circular notches on leaf margins. Stem nematodes are widespread, especially where host plants grow. Common legume diseases include anthracnose (caused by Colletotrichum trifolii), common leaf spot (Pseudomonas syringae pv. syringae), crown wart (Physoderma alfalfae), downy mildew (Peronospora trifoliorum), root rot (Fusarium spp.), rust (Uromyces striatus), crown and stem rot (Sclerotinia trifoliorum), southern blight (Sclerotium rolfsii), pythium root rot (Pythium spp.), and fusarium wilt (Fusarium spp.).