In addition, alfalfa fixes its own nitrogen with the help of Rhizobium meliloti bacteria on its roots. The bacteria converts nitrogen gas into a form that plants can use. A five-ton per acre alfalfa crop can fix up to 250 pounds per acre of nitrogen per year. Alfalfa seed is often inoculated with R. meliloti before sale. Inoculated seed should be stored in a dark, cool place to ensure the bacteria survive until seeding.
Improper inoculation can result in poor nitrogen-fixation.
Alfalfa's nitrogen-fixation can decline after four or five years. Poor stands should be broken and seeded to an annual crop for several years before being reseeded with a recommended variety of alfalfa. Researchers at North Dakota State University Extension Service confirmed both the pattern of nitrogen fertilizer favoring grasses and that when nitrogen-fixation is inadequate, nitrogen fertilizer can increase the yield of alfalfa stands. In considering this, we can look at the nutrients removed when alfalfa is harvested. Tim McDermott, MSU soil microbiologist, once said that next to nitrogen, potassium is the most important nutrient for alfalfa.