Critical Trifoliolate Leaf and Petiole Potassium Concentrations for Soybean Beyond the R2 Growth Stage.

The trifoliolate leaf-K concentration at blooming (R1-2) is used to diagnose K and other nutrient deficiencies of soybean [Glycine max (L.) Merr.]. Critical nutrient concentrations beyond the R2 stage are not available. Our objective was to determine the critical K concentrations of soybean trifoliolate leaves and petioles across reproductive growth stages by evaluating the season-long dynamics of trifoliolate leaf- and petiole-K concentrations and by correlating relative soybean yield with the trifoliolate leaf- and petiole-K concentrations at each reproductive stage. Fifteen uppermost recently mature trifoliolate leaves with petioles were collected 7-12 times from the V5 to R7 stages across multiple site-years, K-fertilization rates, and cultivars. Across site-years, cultivars, and K-fertilization rates, the trifoliolate leaf- and petiole-K concentrations peaked around the R2 stage and then declined linearly with time at a constant average rate of 0.199 g K kg^-1 d^-1 for the leaves and 0.558 g K kg^-1 d^-1 for the petioles. The leaf- and petiole-K concentrations from the R2 to R5 stages accounted for 62-88% and 75-92%, respectively, of the variability in relative soybean yield. Petiole-K concentration was a better predictor of relative yield than the leaf-K concentration at the R2 stage where the predicted critical K concentrations for the petioles (30.4-37.2 g K kg^-1) were twofold higher than the reported critical K concentrations for the leaves (15.0-19.0 g K kg^-1). The greater concentrations and wider range of the critical K concentrations at the R2 stage followed by a greater linear decline rate for the petioles compared to the leaves are advantageous for categorizing petiole-K concentrations into levels (e.g., deficient, low, and sufficient) among reproductive stages. Preliminary critical leaf- and petiole-K concentrations for each growth stage will be presented.