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

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

M.S. Fryer, N.A. Slaton, T.L. Roberts, R.J. Norman, R.E. DeLong, M.R. Parvej, and R.J. Dempsey

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 soybean [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 16 sites are summarized below, but the remaining two sites will also be presented. Across years, STP was Suboptimal at eight sites, Medium at two sites, and Optimal at six sites. Soil-test K was Suboptimal at six sites, Medium at three sites, and Optimal at seven 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 two sites with suboptimal STK. Overall, the existing STP and STK recommendations accurately predicted the anticipated yield response to fertilization at 75% and 78% 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.