Crop-based active canopy sensors and soil-based management zones (MZ) are currently being studied as tools to direct in-season variable-rate N application. Some have suggested the integration of these tools as a more robust decision tool for guiding spatially variable N rates. The objectives of this study were to identify (1) soil variables useful for MZ delineation and (2) determine if MZ could be useful in identifying field areas with differential crop response to N and hence be effective in guiding spatially variable N applications in addition to crop canopy sensing. Eight N rates (0 to 274 kg ha-1 in 39 kg ha-1 increments) were applied in replicated small plots across six irrigated cornfields in central Nebraska in 2007 and 2008. Soil variables evaluated for MZ delineation in each field included maps of apparent soil electrical conductivity (ECa), soil optical reflectance, and landscape topography. Crop response to N was determined via active sensor assessments of in-season canopy reflectance (chlorophyll index; CI590) and grain yield measurements. Global (all fields combined) and field-specific approaches were used to evaluate the relationships between soil and crop response variables, and selected soil variables for each approach were used to delineate MZ. Crop response had the highest correlation to soil optical reflectance readings in sandy fields and to ECa in silt loam fields with eroded slopes. Economic analysis showed potential benefits to N management using soil-based MZ compared to current producer N rates in 3 out of 6 fields. Further economic benefits could potentially be achieved by integrating soil-based MZ and in-season sensor-based N application.
