Validation of Soil‐Test‐Based Phosphorus and Potassium Fertilizer Recommendations for Irrigated Soybean

Core Ideas The most common soil‐test‐based fertilization recommendation error is a false positive. Surface soil sample P and K concentrations are qualitative indicators of subsoil P and K concentrations. The R2‐stage, leaflet‐K concentration of irrigated soybean is an accurate indicator of yield responsiveness to K fertilization. Soil testing is the best available technology for making crop fertilizer recommendations, and precision agriculture has increased the demand for soil‐testing. Our primary research objectives were to validate the accuracy of soil‐test P (STP) and K (STK) availability (Mehlich‐3) and trifoliolate leaflet‐P and leaflet‐K concentration interpretations for predicting irrigated‐soybean [Glycine max (L.) Merr.] yield response to P and K fertilization at 22 site‐years. Each trial contained six treatments involving two P rates and four K rates including a no fertilizer‐P or ‐K control and the soil‐test recommendation. Yield and tissue nutrient concentration responses were assessed at three significance levels (p ≤ 0.05, p ≤ 0.10, or p ≤ 0.25) and identified as correct or incorrect according to the response predicted by soil‐test recommendations. The accuracy of soil and tissue analyses to predict the correct plant response to fertilization increased as the significance level moved from conservative (p ≤ 0.05) to liberal (p ≤ 0.25). Mehlich‐3 STP accurately identified soils that did not respond to fertilizer P and had an overall accuracy of 40 (p ≤ 0.05) to 48% (p ≤ 0.25) in predicting yield response to fertilization. Existing STK interpretations were 72 (p ≤ 0.05) to 82% (p ≤ 0.25) accurate. Critical leaflet‐K concentrations accurately identified soybean response to K fertilization at 46 (p ≤ 0.05) to 60% (p ≤ 0.25) of the sites. Interpretation errors were most common in the very low and low STP levels, low and medium STK levels, and the low tissue‐K level. The accuracy of the soil and tissue testing and their interpretation process were reasonably accurate for K but substantial improvements are needed for P.