Two studies were conducted in Iowa to better understand how potassium (K) fertilization affects grain yield, vegetative growth, and K concentration of different parts of modern corn (Zea mays L.) hybrids and soybean (Glycine max L. Merr.) varieties. One study was based on one- to three-year strip-trials (63 site-years) conducted from 2001 to 2007 to evaluate the effects of K fertilization on corn and soybean grain yield and post-harvest soil-test K (STK) measured with the ammonium acetate extractant. Replicated treatments were 0 and 168 kg K ha^-1. Potassium increased yield as evaluated by strip averages in 21 of 63 site-years, and only when STK of significant portions of a field tested ≤ 171 mg K kg^-1. Yield responses for field areas testing within different STK interpretation classes showed a differential response in seven site-years with no strip-average response. Determined critical concentrations by the linear-plateau model ranged from 178 to 200 mg K kg^-1 for corn and 155 to 246 mg K kg^-1 for soybean for four data handling methods. The other study was based on 20, two-year field trials conducted from 2003 to 2006 to evaluate the effect of K fertilization on corn and soybean early growth; K concentration in young plants, mature leaves, and grain; and grain K removal. Replicated treatments were 0, 28, 56, 112, and 168 kg K ha^-1 broadcast before the first crop. Potassium increased first-year crop yield at eight sites, and residual effects increased yield in eight sites when STK was ≤ 171 mg K kg^-1. On average across yield responsive sites, rates of 91 and 103 kg K ha^-1 maximized yield of corn and soybean, respectively, at first-year sites while 168 kg K ha^-1 maximized yield of both crops at second-year sites. The magnitude and frequency of responses to K were highest for (1) plant K concentration and uptake, (2) grain yield and K removal, (3) grain K concentration, and lowest for (4) early plant growth. The research demonstrated that new technologies are useful to improve assessments of K fertility in highly variable fields, and that large amounts of K absorbed early often is in excess of K needed to maximize crop yield, have little influence on K removed with grain harvest but would significantly affect K removal with harvest of vegetative plant parts.