Beans and peas fertilise the soil by housing bacteria that pull nitrogen from air
Most crops drain nitrogen from the ground. Legumes add it back. Bacteria living in lumps on their roots convert nitrogen gas from the air into ammonia, and when the plant dies that nitrogen enriches the soil for whatever grows next. Neanderthals were already cooking wild pulses 70,000 years ago.
Legumes belong to the pea family, Fabaceae, which is the third-largest family of land plants after orchids and daisies: roughly 19,000 species in about 751 genera, some seven percent of all flowering plants. Their fruit is distinctive, a dry pod formed from a single carpel that usually splits open along two seams. When the dried seeds are eaten they are called pulses, and the UN's Food and Agriculture Organization reserves that word for crops harvested only for dry seed, leaving out green peas and beans as well as oil crops like soybeans and peanuts. It recognises 11 primary pulses.
The nitrogen trick relies on Rhizobia bacteria in root nodules, with a few exceptions such as the genus Styphnolobium. The captured nitrogen makes legumes rich in protein, and after harvest the remaining plant matter breaks down into nitrate that later crops can absorb. That is why rotating legumes with other crops, or growing them together, can supply enough nitrogen without artificial fertiliser, and why they are ploughed in as green manure. Sri Lankan farmers, for example, grow soybeans among coconut palms.
People have relied on them for a very long time. Pulse farming traces around the Ravi River in Punjab date from about 3300 BC, lentils turn up in Egyptian pyramids and cuneiform recipes, and Stone Age dry peas have been found in a Swiss village. In the Americas the oldest domesticated beans, from Guitarrero Cave in Peru, date to around the second millennium BC; the common bean arose in Mesoamerica and spread south alongside maize and squash. Benjamin Franklin sent soybean seeds from France to Philadelphia in 1770.
The family serves many trades: clover and alfalfa for grazing, lupins for flowers, Indigofera for dye and Acacia for gum, while trees such as black locust and plants like Wisteria are poisonous. A popular kitchen belief is also wrong: salting beans before cooking does not stop them softening. Age, hard water or acidic ingredients are the usual culprits, and early salt simply seasons them better.
Source: Legume
