Nitrogen fixation is the process through which atmospheric nitrogen (N2) is converted into a form useable to plants (NH4+).
B. japonicum bacteria invade the roots and multiply in the cortex cells, forming round nodules that can grow to the size of a large pea. Each nodule contains a colony of bacteria. (photo above)
Healthy nodules that are actively fixing nitrogen are pink or red inside. Nodules are typically active for 6 to 7 weeks and are continually replaced during the growing season. (photo above)
The nitrogen requirement of soybeans is one of the highest among agronomic crops due to the high concentration of protein in the seed. Soybeans require about 3.5 lbs. of N per bushel of yield.