Previous research has shown trifoliate leaf K concentration at the R1-R2 growth stage to be highly correlated with yield and soil test K for soybean varieties with a determinate growth habit (Maturity Group 5, MG5), but not for indeterminate varieties (MG4). Tissue K concentrations of MG4 varieties at the R2 stage is often lower than the 1.5-1.8% threshold defining low and deficient leaf K concentrations. Our mission is to determine a specific growth stage, critical leaf K concentration, and sampling protocol for indeterminate varieties that are highly correlated with soybean yield potential. We believe the problem with the current critical leaf K concentration that indeterminate varieties...