The plant is seeded once every three to five years and harvested several times each growing season. Side benefits of growing this crop include, improved soil and water quality, reduced erosion and increased soil organic matter. Alfalfa also forms a symbiotic relationship with a soil bacterium that lives in a pouch-like organ on its roots. The bacteria living in this nodule scrub nitrogen from the air and convert it into a form that the plant can use for growth. In other words, alfalfa essentially makes its own nitrogen fertilizer. Better yet, it leaves some of this naturally formed compound behind when the crop is plowed under, which increases soil fertility. In a rotation of alfalfa and corn for instance, a plowed field of alfalfa usually provides all the nitrogen needs for the first year of corn and about half of the nitrogen required by the second crop, says Hans Jung, a dairy scientist with the USDA Agricultural Research Service (ARS) in St. Paul. In addition, "when you have rotations you typically get an increase in yield of the following crop," he explains. "From an environmental standpoint and an agricultural management standpoint there are some real advantages to bringing in a short-lived perennial like alfalfa.