The nitrogen fixed is not free. Any stress that reduces plant activity will reduce nitrogen fixation. Factors like temperature and water may not be under the control of the farmer. When a nutritional stress is corrected, the legume responds directly to the nutrient, and indirectly to the increased nitrogen nutrition resulting from enhanced nitrogen fixation. Poor nitrogen fixation in the field can be easily corrected by inoculation, fertilization, irrigation, or other management practices.
Some legumes are better at fixing nitrogen than others.
Maximum economic yield for beans in New Mexico requires an additional 30-50 lbs of fertilizer nitrogen per acre. However, if beans are not nodulated, yields often remain low, regardless of the amount of nitrogen applied. Nodules apparently help the plant use fertilizer nitrogen efficiently. Other grain legumes such as peanuts, cowpeas, soybeans, and faba beans are good nitrogen fixers, and will fix all of their nitrogen needs other than that absorbed from the soil. These legumes may fix up to 250 lbs of nitrogen per acre and are not usually fertilized.