Relating Soil Properties to Spatial Variation of Phosphorus Critical Level.

Variable rate (VR) fertilizer application has increased with proliferation of VR capable equipment. Current recommendations for VR phosphorus (P) application often assume that critical level is constant across a field adjusting only for spatial variation of soil P concentration. We conducted this study to determine whether variation in critical level, identified by relative yield of unfertilized to fertilized plots, could be explained by soil properties. In 2016, plots, measuring 9.1 m square, were randomly selected from a grid overlaid on two no-till fields. We subsequently divided main plots into subplots measuring 3.1 by 9.1 m. During corn (Zea mays) planting starter fertilizer was applied approximately 5 cm below and to the side of the seed row. Control subplots received only 56 kg ha^-1 nitrogen (N), while fertilized plots received 56 kg ha^-1 N and 29 kg ha^-1 P. We grouped plot yield response (relative yield of subplots) by soil test P (Mehlich 3) interpretative categories and conducted additional analysis on a subset of plots. Soil samples were taken from the 0 – 10 cm and 0 – 20 cm depths. Additional analyses included Mehlich 3 calcium, potassium, magnesium, P, and zinc, pH, buffer pH, inorganic fractionation of P, total organic matter, total nitrogen, and texture. We will report preliminary results on predicting variable soil critical level using soil properties.