Shortages od potassium in Iowa soils aren't as widespread as those for nitrogen and phosphorus. But deficiences are common, and severe in certain areas, Here's the Iowa situation and some help in finding out the needs of your corn.
Dumenil, Lloyd; Hanway, John; Meldrum, H. R.; and Pesek, John
"Your Corn May Need Potassium,"
Iowa Farm Science: Vol. 13
, Article 2.
Available at: https://lib.dr.iastate.edu/farmscience/vol13/iss11/2