Assessing Spatial Variation of Soil Phosphorus Critical Level.

Technology advances in application and guidance equipment have facilitated adoption of variable-rate fertilizer application. However, variable rate phosphorus (P) recommendations often assume that critical level is constant across a field and only adjust for spatial variation in soil test P. Spatial and temporal variation of critical P level within a field needs to be examined in order to successfully utilize variable-rate technology. A 9.14 m grid, aligned with the planter path, was established on two Kentucky no-till fields in a corn-soybean rotation. Either 25 or 50% (based on field area) of the grid-cells were randomly selected as plots for the first year of the study.  Unused plots will be used in subsequent years. The plots were divided into subplots using a 3.05 m grid. Prior to planting or fertilizing, soil samples were collected at two depths (0-10 and 0-20 cm) from three adjacent subplots and Mehlich 3 P, pH, and buffer pH were determined. Phosphorus and nitrogen (N) fertilizer were applied through the planter 5 cm below and 5 cm to the side of the seed. Urea ammonium nitrate and ammonium polyphosphate liquid fertilizers were used in combination so that the entire field received 56 kg-N ha^-1, but only the center subplot received P fertilizer at 29 kg-P ha^-1. This design allows evaluation of yield response to P in the context of multiple soil physical, chemical, and biological properties. We will use results from this, the initial year of the study to refine our methods prior to expanding to additional sites within Kentucky and beyond.