Technical Abstract: Ca and Mg are often treated in agriculture as having the same influence on soil physical properties. However, there is a possibility of the adverse effect of Mg on soil structure and water intake rate. Less stable soil structure can lead to surface sealing, decreased infiltration, increased runoff and erosion during rainfall events. In this paper, we studied the influence of modified Ca/Mg ratio on surface sealing of four soils varying in organic matter content, clay content and mineralogy from the US Midwest. Part of the study was a laboratory experiment in which flocculation behaviors of soil clays were studied at different Ca/Mg ratios and solution electrolyte. A rainfall simulation study was conducted to examine differences in soil structure and consequently surface sealing and infiltration when soil Ca/Mg ratio was modified. It was found that Ca/Mg ration had a significant effect on clay dispersion and surface sealing. There was a linear relationship between Ca percentage in solution and optical transmittance of clay suspension as an indicator of clay flocculation. In rainfall experiments, Ca-treated well-structured soils had final infiltration rates twice that of Mg-treated soils, total infiltration was increased significantly as well. Ca saturation decreased soil losses to half of those from the Mg treatment. Ca/Mg exchange isotherms obtained in the course of modifying Ca/Mg ratio of the soils indicated a preference for Ca.