Legumes are agricultural plants that contain nitrogen-fixing bacteria. Legumes, such as alfafa and beans, are frequently called vegetables, but true vegetables lack the bacteria characteristic of legumes. Other legumes include peas, lentils, soybeans, peas, carob and mesquite.
According to the Mayo Clinic, legumes and vegetables are important nutrient sources for humans and livestock. Legumes are also vitally important because of their nitrogen-fixing bacteria. Atmospheric nitrogen is highly inert, which means that it does not react with most other elements. Most plants cannot derive nitrogen directly from the air, but legumes can. Nitrogen fixation converts inert nitrogen into ammonia, which is significantly more reactive. Ammonia is also a crucial ingredient in fertilizer.
All plants need nitrogen to thrive. Legumes add nitrogen to the soil, but vegetables remove it. If nitrogen is not added back into the soil, it loses the ability to sustain crops. This is why crop rotation is vital in areas where nitrogen fertilizer is unavailable. For example, rotating corn with alfalfa and beans maintains proper nitrogen levels in the soil.
The only legume regularly used for decorative purposes is the Wisteria vine. These hardy, flowering vines grow so rapidly that they are difficult to control. Humans do not use Wisteria for food but frequently cultivate it for aesthetic reasons.