Local Reference: An Approach for Site-Specific Nitrogen Fertilization Using Active Canopy Sensors.

Nitrogen fertilization needs can vary according to differences in soil, topographical features and weather that can influence corn yield response to fertilization. Active crop canopy sensors have been used to guide nitrogen fertilization based on the status of crop with respect to a reference (N rich) strips. Localized reference strips that account for variable growing conditions can provide more accurate fertilization need estimates than other methods that rely on averaged values of rich nitrogen (N) strips. The objectives of this study are: (i) to develop an approach that relies on local soil conditions as well as on active canopy sensor measurements for real-time adjustment of N application rate; (ii) to compare the method developed against other approaches in different field conditions. The experiment was conducted during 2009 and 2010 growing season in three different production fields each year representing different soil types and agroecosystems across Nebraska (6 site years). In each site, center-pivot irrigation was used and 250 small plots with 0, 75, 125, 200 and 250 kg N/ha application rates were established covering different landscape positions and soil types. The small plots were arranged according to Latin square experimental design to have equidistance between the centroids of the plots (6 by 15 m). This was done to compare different interpolated map layers at different management zones delineated based on field topography and apparent soil electrical conductivity maps. The method developed was validated using a set of replicated field-long strip treatments (the traditional University-recommended method versus  the sensor-based approach with either average reference strip or the localized reference method).