Active Canopy Sensing and Soil Nitrogen Content for Corn Nitrogen Management in Minnesota.

Active canopy sensors have been proposed as a tool to improve corn (Zea mays L.) nitrogen (N) management, but little has been done to improve sensor’s performance using soil N content. The objectives of this study are to compare the effectiveness of various canopy sensing tools and to evaluate the usefulness of soil N content to improve sensor’s predictive power to estimate grain yield and determine in-season N needs. Six to seven N rates at 35 to 45 kg urea-N ha^-1 increments were pre-plant applied in 12 sites throughout Minnesota. Canopy sensing measurements were obtained at V4, V8, V12 and R1 development stages with SPAD, RapidSCAN (RS-NDVI/RS-NDRE) and GreenSeeker (GS-NDVI) and soil (0-30 and 30-60 cm) ammonium- and nitrate-N concentration at V4. Grain yield was determined at harvest. Regardless of the tool, sensing at V4 resulted in poor predictive power for grain yield and N needs. GS-NDVI and RS-NDVI were outperformed by the SPAD meter and RS-NDRE at all the sensing stages. The SPAD meter presented consistent predictive power from V8 to R1 with mean R² values ranging from 0.62 to 0.70 for yield prediction and from 0.57 to 0.60 for in-season N rate predictions. Sensing at V12 with RS-NDRE provided the highest predictive power for grain yield (R²=0.82) and N needs, with slight underestimations of the N requirements (2 Kg N ha^-1) and a sensor relative critical value of 0.99. The 0-30 cm soil nitrate concentration was as good, or better, than any other combination of ammonium, nitrate, and soil depth at improving sensor-based estimations of grain yield and in-season N needs. Soil nitrate as a covariate was especially useful early in development (V4) where yield prediction with RS-NDRE improved from R² of 0.37 to 0.70 and N rate prediction improved from R² of 0.22 to 0.75.  The combination of canopy sensing and soil N status hold promise to improve corn N management.