Critical Trifoliolate Leaf and Petiole Potassium Concentrations during the Reproductive Stages of Soybean

The critical K concentration in soybean [Glycine max (L.) Merr.] has been determined only for leaf tissue at the R2 (full bloom) stage. Our research objective was to develop critical K concentrations in soybean for both leaves and petioles across reproductive stages. Fifteen fully-expanded, uppermost trifoliolate leaves with petioles plot-1 were collected 7 to 12 times from the V5 to R7 stages in five research trials that evaluated multiple fertilizer-K rates and/or cultivars from different maturity groups (MGs). Both leaf-and petiole-K concentrations, regardless of site-year, cultivar, and fertilizer-K rate, peaked around R2 stage and declined linearly with time at average rates of -0.198 g K kg(-1) d(-1) for leaves and -0.559 g K kg(-1) d(-1) for petioles. The leaf-and petiole-K concentrations at the R2 to R6 stages explained 48 to 80% and 41 to 85%, respectively, of the variation in relative yield (RY). Petiole-K concentration was a better predictor of RY than leaf-K at the R2 stage where the predicted critical range (CR) concentrations were 14.6 to 19.0 g leaf-K kg(-1) and 30.1 to 38.3 g petiole-K kg(-1). The wider CR of petiole-K at the R2 stage followed by a greater linear decline rate across reproductive stages indicates that growth stage as well as deficiency and sufficiency thresholds for petiole-K could be more easily categorized than for leaves. Overall, the ability to interpret the K nutritional status in soybean tissues at numerous reproductive growth stages will improve K management.