Defining the Potassium Nutritional Requirements and Distribution among Plant Parts of Representative Soybean Cultivars from Different Maturity Groups

The potassium (K) requirement of soybean [Glycine max (L.) Merr.] was investigated to determine whether cultivar sensitivity to K deficiency was affected by growth habit (determinate or indeterminate) and how cultivars from each growth habit accumulate and distribute K among plant structures. We also diagnosed K deficiency across reproductive growth stages (R2-6) using trifoliolate leaf- and petiole-K concentrations and at harvest (R8) using seed-K concentration. Soybean responded similarly to K deficiency in terms of yield, selected yield components, and seed-K concentration, regardless of growth habit. The yield loss from K deficiency was greatest on the middle to upper nodes of the indeterminate cultivar and bottom and upper-middle nodes of the determinate cultivar. Seed-K concentration decreased from the bottom to the top nodes of K-deficient plants compared to K-sufficient plants, suggesting K concentration of seeds collected from the upper nodes would be of value for diagnosing K deficiency. We found that both growth habits accumulated maximal K at the R5.5-6.0 stage with the peak accumulation rate at the R3-4 stage. Soybean accumulated 35-45% of the maximum K by the R2 stage and 65-70% by the R4 stage, suggesting that K deficiency of soybean could possibly be corrected by timely fertilization during the early reproductive stages. Trifoliolate leaf- and petiole-K concentrations from the R2 to R5.5 stages were highly correlated with relative soybean yield. The K concentrations in both tissues peaked near the R2 stage and declined linearly at a constant rate with increasing plant age regardless of cultivar, site-year, and K fertility level, indicating that the critical K concentration at each stage beyond the R2 is a negative linear function of time. Mature seed-K concentration across 100 site-years in North America was strongly correlated with relative soybean yield and may seed analysis be an effective post-season tool for diagnosing K deficiency. The threshold of deficient seed-K concentration (<16.5 g K kg-1 ) accurately predicted that soybean yield would be increased by fertilizer-K at 77% of the sites. These research findings will provide growers with more comprehensive tools to identify fields where K availability to soybean may limit yield.