Validation of Soil-Test Based Phosphorus and Potassium Fertilizer Recommendations for Soybean.

Soil-test-based fertilizer recommendations are used to determine how much P and K fertilizer is needed to maximize yields and maintain soil fertility. Our objective was to validate the accuracy of the University of Arkansas’ fertilizer recommendations for irrigated soybeans [Glycine max (L) Merr.] based on existing soil-test P (STP) and K (STK) interpretations. Phosphorus and K trials were established at 18 total sites in 2013 and 2014. Each site contained six treatments that included a combination of two P rates (0 or 32 kg P ha^-1) and four K rates (0, 56, 112, and 148 kg K ha^-1). The recommended P and K rates were determined from composite soil samples collected from the 0-10 cm depth of each replicate (n=6) before treatments were applied. Soil-test P and K were categorized as Suboptimal (<91 mg K and 26 mg P kg^-1), Medium, or Optimal (>130 mg K and 35 mg P kg^-1) levels for interpreting the expected yield response to fertilization. Results from the nine 2013 trials are summarized below, but 2014 results will also be presented. In 2013, STP was Suboptimal at five sites, Medium at one site, and Optimal at three sites. Soil-test K was Suboptimal at two sites, Medium at three sites, and Optimal at four sites. Significant (P≤0.10) yield increases from P fertilization occurred at one site within the Suboptimal and Medium STP categories and from K fertilization at two sites with Medium STK and one site with Optimal STK.  Overall, the existing STP and STK recommendations accurately predicted the anticipated yield response to fertilization at 56% and 67% of the fields, respectively. The interpretation of crop yield response to fertilization based on STP and STK was least accurate for fields having Suboptimal STP and STK levels.