MAPPING SOIL TEST PHOSPHORUS AND POTASSIUM FOR VARIABLE-RATE FERTILIZER APPLICATION

Variable-rate fertilizer (VRF) application requires knowledge of the s patial variability of soil test P and K within a field. The objectives of the study were to: (i) evaluate cell (area) vs. point soil samplin g, both on a grid basis, and (ii) compare methods for mapping location specific soil test data. Soils in two central Wisconsin fields were s ampled with two methods; study site soils included two alfisols and on e entisol. The grid-cell method involved dividing fields into 318-ft s quare cells and compositing soil cores to give one sample per cell. Th e grid-point method involved soil sampling at grid intersection points spaced on a 106-ft square grid. Soil test P and K maps were construct ed with nine mapping methods, including: field average, field median, CELL A (five soil cores), CELL B (72 soil cores), Delaunay triangulati on, polynomial trend surface, inverse distance squared gridding, point kriging, and block kriging. Mapping accuracy was determined with map overlay comparisons where the Delaunay triangulation contour maps (106 -ft data) served as the control maps. On average, the CELL A and CELL B methods improved mapping accuracy by 14 and 33 percentage points ove r the field average. Even with the CELL B method, on average, 38% of t he two fields would receive an incorrect application of fertilizer, in dicating that cell mapping methods are not acceptable for making VRF m anagement maps. Grid-point sampling improved soil test P mapping accur acy by 20 percentage points over the CELL A method, even with point sa mpling in a 318-ft grid. Mathematical procedures used to create a 53-f t grid of data, from field sample data, did not improve map accuracy. Soil samples should be collected on a triangular or unaligned systemat ic grid. Sample spacing will depend on field variability, but probably should not exceed 300 ft.