Corn and Soybean Tissue Potassium Content Responses to Potassium Fertilization and Relationships with Grain Yield

Research on relationships between K fertilization, crop yield, and tissue K concentration is needed for modern corn (Zea mays L.) hybrids and soybean (Glycine max L. Merr.) varieties. Twenty 2-yr trials with these crops in Iowa evaluated K effects on plant dry weight (DW), K concentration, and accumulation at the V5-V6 stage; leaf K concentration (R1 stage in corn and R2 in soybean); and grain yield, K concentration, and K removal. Five K rates (0–168 kg K ha-1) were broadcast the first year (10 sites for each crop). Potassium increased corn or soybean yield at 16 site-years, which had soil K ≤ 173 mg K kg-1 (15-cm depth, CH3COONH4 test). On average across first-year responsive crops, 91 and 103 kg K ha-1 maximized corn and soybean yield, respectively, and across second-year crops the response was linear. Potassium fertilization increased grain K concentration and K removal to a greater extent and more frequently in soybean than in corn (10 vs. 5 site-years, respectively, for concentration and 11 vs. 3 site-years, respectively, for removal). The magnitude and frequency of responses for both crops were greatest for vegetative tissue K concentration and smaller (in decreasing order) for grain yield and K removal, grain K concentration, and early DW. There was large luxury K accumulation in vegetative tissues. Critical tissue K concentration ranges defined by two models were 20.2 to 25.1, 10.2 to 11.0, and 17.6 to 20.0 g K kg-1 for corn plants, corn leaves, and soybean leaves, respectively, and could not be determined for soybean plants.