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.