• The pea is among legumes that produce many nodules--and much nitrogen.peas texture image by Jo?¡êo Freitas from Fotolia.com
  
  Legumes are magicians of the plant world. Seemingly out of thin air, they manufacture their own nitrogen fertilizer. In this mutually beneficial arrangement, worked out over millennia, specialized soil bacteria make atmospheric nitrogen available to plant roots as ammonia nitrogen. This magic is a process called nitrogen fixation, and occurs in specialized root tissue called nodules. Some legumes tend to have more nodules, but active bacteria determine the actual number, which is why bacterial seed inoculants are used.
  
  

Alfalfa

• The University of Kentucky Extension notes that colonists first introduced alfalfa, long grown as a forage crop, to North America in the 1700s. Alfalfa's high protein content complements less nutritious silage and forage. Alfalfa is one of the legumes that produces enough usable ammonia to meet all of its nitrogen needs. According to Colorado State University Extension, alfalfa can fix up to 308 pounds of nitrogen per acre. When planting alfalfa in soil where it has never grown before, it is important to inoculate or coat seed with the specific bacteria strain that boosts nitrogen fixation for alfalfa. Each legume requires a different type.
  
  

Peas

• The pea is a versatile vegetable that can manufacture much of its own nitrogen fertilizer. The University of Illinois Extension notes that this sweet or starchy cool-season legume grows in almost all regions of the United States. The English or garden peas come in either smooth-or wrinkled-seeded types. "Smooth" peas have more starch and are typically used to grow dried split peas that are so good in stews and soup. Sweet, wrinklier types are the peas served whole on dinner plates. Snap peas, bred from garden peas, have sweet, low-fiber pods that "snap" and are tasty when lightly steamed whole and eaten along with the young peas inside. Gardeners harvest snow peas when the sweet pods are still flat, before the peas inside even develop. Then there are field peas, often grown as a cover crop to enrich soils or with cereal grains as high-protein livestock forage. Whichever pea you grow, for maximum yields be sure to inoculate pea seed--or buy seed that's already been inoculated--with the correct Rhizobium strain.
  
  

Soybeans

• A high-protein vegetable that provides high-quality protein for vegetarian foods and livestock feed alike, soybeans are a profitable crop, though dedicated gardeners with adequate space can grow their own. University of Georgia Extension advises that soybeans require very warm soil for successful germination--between 70 and 90 degrees Fahrenheit at two inches beneath the surface--so growers must monitor soil temperatures carefully. Also, they should plant varieties chosen for their region in late spring or early summer and be prepared to supply plenty of water, because soybeans need two or three inches of water per week. Again, for highest yields and crop quality they advise using fresh soil inoculants.