Leguminous plants are of the family Leguminosae. These plants work with a type of bacteria, either introduced or naturally occurring within the soil, to convert nitrogen in the air into usable nitrogen for plant growth. This is done after the bacteria invades, or inoculates, the root system, creating nodules. Within these nodules, the bacteria absorb nitrogen from the air and convert it into a form of nitrogen the plant can use for energy and growth.
Alfalfa (Medicago sativa) is a perennial from the pea family used as a forage for domestic livestock. Some parts of the world use it as a medicinal plant. Alfalfa forms an extensive root system as long as 30 feet or more, so the number of nitrogen-producing nodules on the root system of each plant can be large.
The peanut (Arachis hypogaea) is a popular food crop cultivated for hundreds of years. This member of the pea family creates more nitrogen-producing nodules on its root system than any other legume when enough beneficial bacteria are present. A stalk grows from a self-fertilized flower into the ground and the peanuts, or seeds, are formed at the end of the stalk just below the soil surface.
Soybeans convert as much as 75 percent of their nitrogen requirements from the atmosphere, so a healthy soybean plant has numerous nodules evident on its roots. The large number of nodules is produced with the assistance of the right amount and type of beneficial bacteria. Like all legumes, the nitrogen is not returned to the soil to benefit later crop plantings unless the entire plant is plowed back into the soil. This is because a large amount of the nitrogen held in the plant is in the green top growth.
The annual cowpea (Vigna unguiculata) grows in warm climates. It is also known as black-eyed pea, Southern pea or crowder pea. It is a bushy or vining plant depending on the variety. It is grown as a food, livestock or wildlife forage, or as a cover crop to rebuild depleted soil. Each plant contains a large number of nitrogen-producing nodules on its root system. The number of nodules present depends on the amount of beneficial bacteria present in the soil.