Simulation of fixed- and variable-rate application of granular materials

Research has shown that application errors exist with variable-rate technology (VRT) systems. Consequently, using prescription maps for economic and agronomic analyses can generate misleading results. The intent of this article was to develop and validate a spatial data model for generating "as-applied" maps to support the advancement of precision agriculture practices. Previous research modified ASAE Standard S341.2 to include a 2-D matrix of collection pans to assess fixed-rate and variable-rate (VR) deposition of granular fertilizers and agricultural lime from a spinner disc spreader. The "as-applied" spatial data model uses GIS functionality to generate "as-applied" surfaces by merging distribution patterns and a spatial field application file (FAF) into an "as-applied" surface representing the actual distribution of granular fertilizer or agricultural lime across a field. To validate the "as-applied" spatial data model, field studies were conducted by randomly placing collection pans across two fields. Murate of potash was then applied using a VR spinner spreader. The "as-applied" spatial data model was used to predict the amount of material each pan should have received. Comparisons were made between the actual and predicted application rates for two fields, with R2 values of 0.45 (field A) and 0.58 (field B) computed. However, R2 values of 0.16 (field A) and 0.21 (field B) were observed when comparing the actual application rates and prescription maps. These low R2 values indicated poor application by the spinner spreader but demonstrated that the "as-applied" model did a better job of representing the distribution of murate of potash when contrasted with the prescription maps. "As-applied" surfaces provide a means for evaluating fixed-rate and VR application of granular products while enhancing researchers' ability to compare VR management approaches.