Soybean Yield and Seed Potassium Concentration Response to Long-Term Potassium Fertilization.

The specific effects of K deficiency on soybean [Glycine max (L.) Merr] yield and seed composition across the nodes of soybean plants is needed to better understand the nutritional requirements for high yield. We evaluated the seed yield and seed K concentration among soybean plant nodes as affected by annual K-fertilization rate. Research was conducted during 2012 in a long-term K fertilization trial on a Calhoun silt loam. Armor 48-R40, an indeterminate soybean variety was planted across three annual K rates (0, 75, and 150 kg K ha^-1). To evaluate seed yield and K concentration, four representative whole plants were collected at maturity and dissected into ten nodal segments where each segment consisted of two nodes and internodes. The cumulative seed yield four^-1 plants receiving 75 or 150 kg K ha^-1 was similar ranging from 83 to 88 g, and 28-37% greater than soybean receiving no K. Yield differences among K rates occurred at nodes on the top one-half of the plant where soybean fertilized with 75 and 150 kg K ha^-1 produced 40 to 51% greater seed yield. Individual seed weight varied among nodes and K rates. Soybean receiving no K produced the greatest individual seed weights on the bottom four nodes and the lowest seed weights on the top six nodes. Seed K concentration increased linearly from the top nodes towards the bottom nodes. The greatest rate of seed K concentration increase occurred for plants that received no K fertilizer (%K = 0.069x + 1.28, where x = node segment) compared to soybean fertilized with 75 (0.029x + 1.89) and 150 (0.024x + 2.12%) kg K ha^-1. Results suggest that K deficiency results in yield loss from nodes on the top one-half of plants and that seeds produced on the lower nodes receive K preferentially.