The Iowa wine and grape industry underwent a rapid phase of growth at the turn of the 21st century that is continuing to evolve and develop today. Cultivar trials across the state found that `Marquette' grapevines were performing poorly in eastern Iowa while other cultivars performed well. A preliminary investigation suggested a magnesium induced potassium deficiency and/or above optimum soil pH as the cause of poor growth of `Marquette'. Soils in the upper Mississippi Valley are derived from limestone and dolomite bedrock resulting in their characteristically high pH and high magnesium properties, which often inhibits potassium uptake. Recommendations to amend these soil types for grape production do not exist but are essential for optimizing grapevine yield. This study was undertaken to determine how to amend vineyard soils with a low potassium/magnesium concentration and above optimum pH. The multi-year pot culture study included two cultivars, Marquette and St. Croix, and four soil amendment treatments. Soil amendment treatments compared all combinations of potassium/magnesium concentration (0.24 and amended to 0.50) and soil pH (7.2 and amended to 6.2) in a two-by-two factorial. Results indicated potassium additions increased the soil potassium/magnesium concentration as well as increased the petiole potassium concentration. Decreasing the pH alone decreased available soil magnesium but had no effect on the potassium/magnesium concentration. This suggests that it is only necessary to add potassium to increase the potassium/magnesium concentration in these soils.