Soybean Yield Components and Seed Potassium Concentration Responses among Nodes to Potassium Fertility

Soybean [Glycine max (L.) Merr.] yield loss by K deficiency has been reported extensively, but very little research has evaluated how the yield loss is distributed among nodes. We evaluated soybean seed yield, individual seed weight, pod and seed numbers, seed abortion, and seed-K concentration among nodes of an indeterminate and determinate cultivar grown under three K fertility levels (low, medium, and high represented by 0, 75, and 150 kg K ha(-1) yr(-1), respectively). Chlorosis along upper leaf margin was observed during seed-filling period in every low K fertility plot. Soybean grown with medium and high K fertility averaged 28 and 43%, respectively, greater predicted seed yield on the top seven (of 10) node segments for the indeterminate soybean and 72 and 101% greater seed yield on the node segments 2, 3, 4, and 7 (of seven) for the determinate soybean than plants having low K fertility. Yield loss was attributed to reduced individual seed weight, fewer pod and seed numbers, and increased seed abortion. The seed-K concentration of soybean grown with low K fertility was lowest (11.6 [indeterminate] and 15.2 [determinate] g K kg(-1)) for seeds located on the top nodes and increased (17.8 g K kg(-1)) quadratically to the bottom of the plant. The largest proportion of seed yield and the greatest yield loss from K deficiency come from the middle and upper nodes of indeterminate plants and the combination of the bottom nodes, due to branching, plus the upper-middle nodes of determinate plants.