The project showed that the concentrations of P and K in young plants sampled at the V5-V6 growth stage and mature leaves sampled at the R2-R3 stage do provide an indication of the potential soybean grain yield response to fertilization. The project identified critical concentration ranges, which distinguish between conditions of nutrient deficiency and likely response to fertilization from conditions of adequate levels and unlikely response. The determined tissue critical concentration ranges for P were 0.33 to 0.41 % P for young plants sampled at the V5-V6 growth stage and 0.35 to 0.47 % P for mature leaves sampled at the R2-R3 stage. The determined tissue critical concentration ranges for K were 1.89 to 2.26 % K for young plants and 1.56 to 1.99 % K for mature leaves. Previous research in Iowa or the north-central region had not determined P or K critical concentrations for soybean plant tissue at the V5-V6 stage of growth, so the concentrations determined by this project are the first reported. Literature from other regions for soybean leaves at early or intermediate reproductive stages suggest sufficiency ranges of 0.26 to 0.6% P and 1.5 to 2.5% K. Therefore, the critical concentration ranges found by the project for P or K in soybean leaves is narrower and more useful than sufficiency ranges suggested in the literature.