Once you have determined that enough nitrogen (N) loss has occurred and that supplemental N application is needed, what's next? Rainfall after supplemental N applications will be important for plant uptake. If the applied N is within the active root system, and if there is a need for the N, corn yield can be increased with N applied until shortly after silking. If the supplemental N stays near the soil surface, and is not available to the plant, then the application will be wasted.