The legume family, known botanically as Fabaceae, includes roughly 19,580 species across 766 genera. It’s one of the largest plant families on Earth, covering everything from the lentils in your soup to the wisteria climbing your neighbor’s fence. What unites them all is a shared trait: they produce seeds inside a pod.
What Makes a Plant a Legume
The defining feature of every legume is the pod. That long, sometimes curved structure that splits open along two seams is technically called a “legume” itself, and it’s where the family gets its name. Pea pods, green bean pods, and the papery shells around peanuts are all variations on this theme. Some pods are flat, others inflated. Some split open when ripe to release their seeds, while others break into segments, each holding a single seed.
Beyond the pod, legumes share another remarkable ability: they partner with soil bacteria called rhizobia to pull nitrogen directly from the air and convert it into a form plants can use. When a legume’s roots detect low nitrogen in the soil, they release chemical signals that attract these bacteria. The bacteria enter through root hairs, multiply, and trigger the plant to grow small bumps on its roots called nodules. Inside these nodules, the bacteria convert atmospheric nitrogen into nutrients the plant needs for growth. In return, the plant feeds the bacteria sugars produced through photosynthesis. A single crop of cowpea, for instance, can deposit 40 to 80 kilograms of nitrogen per hectare back into the soil, naturally enriching it for whatever grows next.
Common Edible Legumes
The legumes most people encounter daily fall into a few broad categories. Pulses, the dried edible seeds of legume plants, include beans (kidney, black, pinto, navy, chickpeas), lentils, and peas. A simple way to think about it: the pea pod is the legume, but the pea inside the pod is the pulse.
Soybeans and peanuts are also legumes, though they’re often grouped separately because of their high fat and oil content. Soybeans deliver about 10.6 grams of protein per one-third cup serving, making them the most protein-dense legume by a wide margin. Lentils and dry peas come in around 7 to 8 grams per half-cup. Even snap beans (green beans), which people think of as vegetables, belong to the family, though they’re harvested young for the whole pod rather than the dried seed.
Here’s how some of the most common edible legumes compare nutritionally per standard serving:
- Lentils (½ cup): 7.8 g protein, 6.8 g fiber
- Dry beans (½ cup): 6.7 g protein, 6.6 g fiber
- Dry peas (½ cup): 7.2 g protein, 7.2 g fiber
- Chickpeas (⅓ cup): 5.4 g protein, 4.6 g fiber
- Soybeans (⅓ cup): 10.6 g protein, 3.5 g fiber
- Peanuts (2 tbsp): 4.6 g protein, 1.5 g fiber, 8.7 g fat
Legumes You Don’t Eat
Food crops represent only a fraction of the legume family. Alfalfa (also called lucerne) is the world’s most commercially important forage legume, grown in many countries as high-quality feed for livestock. White clover, sweet clover, and Austrian winter pea serve similar roles in different regions of the United States. As many as 60 different legume species have been cultivated specifically as animal feed or forage.
Ornamental legumes are everywhere in landscaping. Wisteria, with its cascading purple flower clusters, is a legume. So are laburnum (golden chain tree), robinia (black locust), and the royal poinciana, a tropical street tree with bright red-orange blooms. Acacia species, often called wattle, are legumes used for timber. Some tropical legumes are even being developed as biofuel crops.
Surprising Members of the Family
People are often surprised to learn how far the legume family extends beyond beans. Peanuts grow underground, which makes them seem more like tubers than legumes, but crack open the shell and you’ll see two seeds sitting in a pod. Tamarind, the sour fruit used in pad thai and Indian chutneys, grows in a legume pod. Carob, sometimes used as a chocolate substitute, comes from a legume tree. Licorice root is a legume. Mesquite, the tree that flavors barbecue, is a legume. Even the redbud tree, a common ornamental across eastern North America, belongs to the family.
The bauhinia genus, sometimes called orchid trees for their showy flowers, is part of one of the oldest branches of the legume family tree. Their leaves look nothing like the compound, feathery leaves typical of beans and peas. Instead, they’re simple and bilobed, shaped like butterfly wings. This kind of diversity is part of what makes the legume family so vast.
How Legumes Are Classified
Botanists traditionally divided the legume family into three subfamilies based on flower shape. The Faboideae (also called Papilionoideae) includes most of the familiar food and forage legumes, with irregularly shaped “butterfly” flowers where petals are different sizes. The Caesalpinioideae, which includes tropical trees like cassia, has flowers that are somewhat more symmetrical. The Mimosoideae, now folded into Caesalpinioideae based on genetic analysis, includes plants like mimosa and acacia with tiny, radially symmetric flowers clustered into puffball-like heads.
The Caesalpinioideae group is now understood to be the oldest lineage, the branch from which the other subfamilies evolved. Its members are found worldwide but are primarily woody tropical plants spanning about 160 genera and 4,680 species.
Preparing Legumes Safely
Raw or undercooked legumes contain naturally occurring compounds, sometimes called antinutrients, that can interfere with digestion or nutrient absorption. The main ones are lectins (proteins that can cause gastrointestinal distress), phytates (which bind to minerals and reduce absorption), oxalates, and tannins. Red and white kidney beans are especially rich in lectins and should never be eaten raw or from a slow cooker that doesn’t reach a full boil.
The good news is that standard cooking methods neutralize most of these compounds. Boiling legumes for one hour at 95°C reduces lectin activity by 93 to nearly 100 percent. Boiling kidney beans completely eliminates their lectins. Cooking common beans for 45 minutes cuts oxalate content by about 60 percent, and cooking lentils for just 15 minutes reduces soluble oxalates by nearly 43 percent. For phytates, soaking beans overnight before cooking helps, as does sprouting or fermenting them. Fermenting lentils for 72 hours destroys almost all their lectins.
In practical terms, the standard approach of soaking dried legumes overnight and then boiling them until tender handles the vast majority of these concerns. Canned beans, which are pressure-cooked during processing, are equally safe.
Why Legumes Matter for Soil
Farmers have rotated legume crops with grains for thousands of years, long before anyone understood the nitrogen-fixing chemistry behind it. Because legumes pull nitrogen from the atmosphere rather than depleting it from the soil, planting them between grain harvests restores fertility naturally. Cowpea alone can fix between 70 and 350 kilograms of atmospheric nitrogen per hectare in a growing season. This makes legumes critical for sustainable agriculture, particularly in regions where synthetic fertilizers are expensive or unavailable. Alfalfa, soybean, and cowpea are increasingly studied for their dual potential as both animal feed and soil-building crops.