Crops need soil that is rich in many nutrients, especially nitrogen. Plants such as vetch, bell bean, alfalfa, and clover take nitrogen out of the atmosphere and convert it into a form that plants can use. This is called "nitrogen fixing." The little bumps on the root in this photo are "nitrogen nodules" that are formed with the help of beneficial bacteria. Nitrogen-fixing plants are plowed back into the soil before they use up the nitrogen. The nodules break off, leaving the soil rich in nitrogen for other crops.