Low potassium (K) concentrations in subsurface soil and reduced water content in the K-enriched surface soil are thought to cause K-related yield reductions in rain-fed soybean (Glycine max L. Merr.). Our objectives were to examine root-to-shoot ratios through ontogeny and yield components in relation to stratified soil water and K. Soil and plant samples were collected at V6, R2, R4, and R6 from a 2-year field study. Additionally, a greenhouse split-root pot study in a complete factorial design, with two soil exchangeable K (Ex-K) levels (160 mg kg-1 and 80 mg kg-1) and two water contents (85 to 95% field capacity and 55 to 65% field capacity) was conducted. In the field, initial soil Ex-K levels ranged from 60 to 290 mg kg-1 at the 0 to 0.1 m depth and from 50 to 90 mg kg-1 at the 0.1 to 0.2 m depth. Seed yield of plants grown on low-K soils was significantly lower when compared to plants grown on K-sufficient soils. Tissue K content revealed that there was no translocation of K from tissues into the seed, and luxury K consumption did not result in higher seed K content. Greatest root length density occurred in the surface 0.1 m of soil and root density was unaffected by K stratification. Greater root-to-shoot ratios were observed for low-K soils compared to medium- and high-K soils. The response of soybean to the interrelationship of soil water and K levels in a stratified profile will be presented.
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