Where do soybeans derive additional nitrogen needed for yields above 50 bushels per acre in the absence of additional nitrogen application?

Soybeans have a unique ability to fix atmospheric nitrogen thanks to symbiotic relationships with Rhizobium bacteria in their root nodules. However, when considering where soybeans derive additional nitrogen needed for achieving yields above 50 bushels per acre without any supplemental nitrogen application, the most significant source is actually soil NO3 (nitrate).

As soybeans grow, they take up nitrate from the soil to meet their nitrogen requirements, especially during the critical stages of pod development and seed filling when their demand for nitrogen peaks. While some nitrogen is indeed derived from the air due to the biological fixation process, this primarily benefits the crop's growth needs in general rather than specifically addressing the higher yields required over that 50-bushel threshold.

Soil organic matter contributes to nitrogen availability as it decomposes, but the release of nitrogen from organic matter is a slower process and may not suffice for the immediate increased demand of high-yield soybeans. Irrigation does not contribute nitrogen directly to the plants; rather, it serves to help maintain moisture levels, which can impact nutrient uptake but does not provide nitrogen itself.

Thus, soil NO3 is the main source of additional nitrogen that soybeans utilize to achieve high yields without additional nitrogen application.