Soil-test K values (STK) and estimates of K removal with crop harvest are used to recommend K fertilization of crops. Soil-test interpretations and fertilizer recommendations should be based on field soil-test calibrations and also on reliable estimates of yield and nutrient concentration in harvested plant parts. This project assessed relationships between STK, grain yield, and K removal with grain harvest for Iowa corn-soybean rotations. Grain yield, STK, grain K concentration, and K removal data were collected from replicated response trials including several K fertilization rates that were conducted in seven locations for a total of 60 site-years. Grain yield ranged from 4.52 to 13.97 Mg ha^-1 for corn 1.66 to 4.66 Mg ha^-1 for soybean, and STK ranged from 69 to 384 mg K kg^-1. Results showed that K fertilization had little or no effect on grain K concentration, almost always increased K concentration of vegetative tissue, and often increased grain yield when STK was less than 150-180 mg K kg^-1 (ammonium-acetate test, 0-15 cm depth). Grain K concentration (mean 3.3 and 16.8 g K kg^-1 for corn and soybean) was not correlated with grain yield or STK. However, K removal with grain harvest was linearly correlated to yield (r = 0.75 for corn and r = 0.88 for soybean). Average grain K concentration estimates used in Iowa for STK maintenance (4.2 and 22.3 g K kg^-1 for corn and soybean) were among the highest concentrations observed for each crop. Overall, results showed that grain K concentration was variable but not correlated to yield or STK, and that yield variation had a much larger impact on K removal than variation of grain K concentration.