Soybeans have high protein content, which is rich in nitrogen (N), so the crop’s need for N are high, explains John Schmidt, DuPont Pioneer research scientist. Unlike corn, wheat and most other row crops grown in North America, soybeans are able to obtain their own N through the process of N-fixation, achieved through an intricate biological relationship between soybeans and a particular species of soil bacteria, Bradyrhizobium japonicum.
N-fixation combined with the uptake of residual and mineralized N from the soil are usually sufficient to supply most of the N needs of a soybean crop. Traditionally, soybeans have been grown successfully without addition of N fertilizer. N management has been limited to providing sufficient rhizobia inoculation of fields new to soybeans.
However, some soil fertility experts are now suggesting that N fertilizer applications may be needed to reach extremely high soybean yield levels. An N “budget” developed from numerous research studies shows that soil and fixed N are generally sufficient to supply N needs at yields up to 60 bushels per acre. As yields increase to 80 bushels per acre and higher, an N deficit may result. So as higher soybean yields become more common due to improvements in genetics and management practices, N additions may be needed to maximize potential yields.