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

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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. Critical soybean tissue‐K concentrations in the trifoliolate leaf and petiole can be developed for growth stages beyond the R2 stage by modeling the rate of tissue‐K decline across time. Petiole‐K concentrations are approximately twofold higher and decline at a greater rate than trifoliolate leaf‐K concentrations and may be equally as good or a better tissue to sample for the diagnosis of K deficiency. The ability to interpret the K nutritional status in leaves, petioles, or both tissues at numerous reproductive growth stages allows plant K status to be monitored and possibly corrected during the growing season across a range of growth stages.