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.

Indeterminate maturity group (MG) IV soybean [Glycine max (L.) Merr.] cultivars may be more susceptible to yield loss from K de ciency due to the shorter growing season and earlier onset of reproductive growth than MG V determinate soybean cultivars. Our objective was to identify whether indeterminate MG IV or determinate MG V soybean are a ected di erently by K de ciency. Seed yield and selected yield components were evaluated from a determinate (MG 5.3) and indeterminate (MG 4.7) soybean cultivar grown under three K fertility levels (low, medium, and high). e trial was conducted in long-term plots that receive 0, 75, or 150 kg K ha -1 yr -1 . Yield and yield components of both the determinate and indeterminate cultivars responded similarly to K de ciency. Seed yield for soybean grown with low K averaged 3.4 Mg ha -1 and was 13 to 15% lower than the yields of soybean grown with medium and high K fertility. e yield loss associated with K de ciency was from fewer pods (16-25%) and seeds (22-30%) plant -1 , higher seed abortion (5-7%), and lower individual seed weight (8-10%) than soybean with medium or high K fertility. Seed K concentration increased with each increase in K fertility level averaging 15.8, 18.7, and 19.8 g K kg -1 for soybean grown under low, medium, and high K levels, respectively. Regardless of growth habit, the yield loss caused by K de ciency was attributed to the same primary mechanisms of reduced pod number and increased seed abortion.

Core Ideas Soil K at 0‐ to 15‐ and 0‐ to 30‐cm depths were excellent predictors of full‐season soybean relative yield.Tissue‐K concentration can be used for calibrating fertilizer‐K rate and in‐season K management.Soil sampling to 30‐cm depth would reduce fertilizer amount and cost for soybean on low cation exchange capacity soils. Quantifying soil‐K availabilities at deeper depths may be necessary to determine optimum fertilizer‐K rate for soybean [Glycine max (L.) Merr.] grown on low cation exchange capacity (CEC) soils that are prone to K leaching. We characterized full‐season soybean response to fertilizer‐K across 19 coarse‐textured low‐CEC sites during 2013 and 2014. Mehlich‐1 soil‐K concentrations at 0‐ to 15‐ and 0‐ to 30‐cm depths better correlated with relative yield and explained 90% of relative yield variation compared to 77% for 0‐ to 60‐cm depth. Critical soil‐K concentrations were similar for relative yield, V5 plant‐K concentration, and R2 leaf‐K concentration, ranging from 48 to 73 mg K kg−1 for 0‐ to 15‐cm and 41 to 63 mg K kg−1 for 0‐ to 30‐cm depths. Soil‐K concentrations less than this critical range accurately predicted positive yield responses to fertilizer‐K 89% of the time for 0‐ to 15‐cm and 80% for 0‐ to 30‐cm depths. Plant‐ and leaf‐K concentrations were equally good in predicting relative yield with critical concentrations of 19 to 22 g plant K kg−1 and 18 to 21 g leaf K kg−1. Plant‐K concentration was better than leaf‐K concentration in diagnosing K‐deficient sites. Calibration model confirmed that soybean requires no fertilizer‐K to maximize yield for soil‐K concentrations above the critical ranges at both depths. However, for K‐deficient soils, soil‐K concentrations at 0‐ to 30‐cm depth resulted in 7 to 32% less fertilizer‐K requirements than 0‐ to 15‐cm depth, indicating the value of deeper sample in recommending fertilizer‐K for soybean grown on coarse‐textured low‐CEC soils.