A greenhouse pot study was conducted on three soils with varying clay contents to establish K critical value at -3 and -1/3 bar soil moisture levels. Four K fertilizer treatments (0, 0.07, 0.14, and 0.3 g/pot) replicated three times were arranged in a complete randomized block design. The Cate-Nelson graphical procedure revealed that K critical values for corn and soybeans were not equal at the two designated moisture levels. The corresponding percentage yield however was higher at -1/3 bars for corn and at -3 bars for soybean. Statistical regression analysis showed that there was a minor response in plant tissue K to changes in soil test K at both soil moisture levels. The ANOVA at 0.05 significance level showed the observed data were consistent with the null hypothesis that there was no significant difference between plant percentage yield and the increasing K fertilizer application rates at the two moisture levels.