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

Variable-rate fertilizer (VRF) application requires knowledge of the spatial variability of soil test P and K within a field. The objectives of the study were to: (i) evaluate cell (area) vs. point soil sampling, both on a grid basis, and (ii) compare methods for mapping location specific soil test data. Soils in two central Wisconsin fields were sampled with two methods; study site soils included two alfisols and one entisol. The grid-cell method involved dividing fields into 318-ft square cells and compositing soil cores to give one sample per cell. The grid-point method involved soil sampling at grid intersection points spaced on a 106-ft square grid. Soil test P and K maps were constructed with nine mapping methods, including: field average, field median, CELL A (five soil cores), CELL B (72 soil cores), Delaunay triangulation, 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 over the field average. Even with the CELL B method, on average, 38% of the two fields would receive an incorrect application of fertilizer, indicating that cell mapping methods are not acceptable for making VRF management maps. Grid-point sampling improved soil test P mapping accuracy by 20 percentage points over the CELL A method, even with point sampling in a 318-ft grid. Mathematical procedures used to create a 53-ft grid of data, from field sample data, did not improve map accuracy. Soil samples should be collected on a triangular or unaligned systematic grid. Sample spacing will depend on field variability, but probably should not exceed 300 ft.