Abstract

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.