Soybeans produce their own nitrogen for crop growth with the aid of nitrogen fixing bacteria in the soil that infect the soybean root and form nodules which contain the bacteria. This is a symbiotic relationship that benefits both the plant and the bacteria. The bacteria are able to take atmospheric nitrogen and convert it into a form of nitrogen that the plant can use. Much of the nitrogen requirement for soybean production begins at seed development. Soybean physiology studies suggest that the amount of nitrogen supplied for the soybean plant from fixation in the nodules is less than the potential needs of the soybean crop during seed fill. The soybean plant adjusts for the deficiency by translocating nitrogen from the leaves and stems to the seeds. This reduces the capacity of the leaves to produce photosynthetic products for filling the seed and has been the subject of many studies. In most cases, the addition of nitrogen has not increased yields or the increase in yields has been too low to be profitable.
Judy, Curt and Murdock, Lloyd W., "Late Season Supplemental Nitrogen on Double-Cropped Soybeans" (1998). Soil Science News and Views. 10.