Canopy Sensors to Match N Rate to Crop Need and Reduce the Pool of Vulnerable Nitrate.

Nitrate in soil after harvest represents the most vulnerable pool of N for transfer to surface waters and eventually the Gulf of Mexico. Applying precisely as much N fertilizer as the crop needs reduces the pool of vulnerable post-harvest soil nitrate. Achieving this precision requires intensive diagnosis because optimal N fertilizer rate varies widely from field to field and from place to place within a field. Canopy reflectance sensors offer a promising tool for diagnosis of optimal N rate and simultaneous control of variable-rate N applications. This approach can be used in a range of crops. In 55 on-farm trials, using sensors to guide N rates in corn reduced N use by 16 kg N/ha while increasing yield by 110 kg/ha relative to N rates chosen by cooperating producers. In 6 on-farm cotton trials, sensors reduced N use by 7 kg/ha while increasing cotton lint yield by 40 kg/ha. Additional cotton trial outcomes will be added before the meeting. Canopy sensor-based N management appears to be a cost-effective way to precisely meet crop needs while reducing the pool of post-harvest soil nitrate that is vulnerable to loss.