Soybean Seed Yield and Composition Responses Among Nodes to Potassium Availability.

The specific effect of K deficiency on soybean [Glycine max (L.) Merr] yield across the nodes of soybean plants is needed to better understand the K requirements for high yield. We evaluated seed yield, selected yield components, and seed-K concentration among nodes of an indeterminate soybean under three K availability levels (Deficient, Sufficient, and High) in 2012 and 2013. The trial was conducted in long-term plots that received 0, 75, or 150 kg K ha^-1 yr^-1. Four mature, whole plants plot^-1 were collected and dissected into ten node segments (two nodes and internodes segment^-1). Seed yield for soybean grown with Deficient K averaged 16 g plant^-1 and was 24-33% lower than the yields of soybean grown with Sufficient and High K availability. The yield loss associated with K deficiency was due to fewer pods (58) and seeds (133) plant^-1, and greater seed abortion (18%) compared to soybean with Sufficient or High K availability (65-73 pods and 161-180 seeds plant^-1, and 9-11% total seed abortion). More specifically, seed yield, pod and seed number, and seed abortion differences among K-availability levels occurred on the top four (2012) to seven (2013) node segments. Soybean grown with Deficient K produced 31-42% less yield, 12-23% fewer pods, 25-35% fewer seeds, and exhibited 33-55% more seed abortion compared to soybean grown with Sufficient or High K. Regardless of K-availability, seed-K concentration decreased quadratically from the bottom nodes towards the top nodes. The decline was greatest between the middle and top nodes for soybean grown with Deficient K [18.2 (bottom node) - 11.5 (top node segment) g K kg^-1] compared to Sufficient (20.2-17.5 g K kg^-1) or High (21.5-19.7 g K kg^-1) K availability. Results suggests that the yield loss from K deficiency begins at the top of plant and proceeds downward as the deficiency severity increases.