The boundary between the humid eastern and the arid western United States is of great economic interest and historic intrigue. Areas to the east of this geographic boundary have historically enjoyed more favorable rainfall, fertile soil, and reliable surface water, permitting conventional agriculture to flourish over a surprisingly large percentage of the eastern Great Plains. The expansion of population and agriculture during the nineteenth century across the Great Plains of North America tested the extent to which row crop agriculture could be successful in areas where year-to-year rainfall was unreliable. In this paper, we have quantified the historic variability of soil moisture and hydrologic conditioning resulting from climatic variability in the Great Plains by showing regions that historically demonstrate reliable precipitation, identifying an extent for the arid regions of the United States based on modeled soil moisture. We demonstrate that long-term climate and climate variability are reflected in the depth and concentration of the calcic soil properties. From a practical standpoint, our work suggests that soils with higher water holding capacity can dampen periodic short-term rainfall deficits likewise soils with lower water holding capacity can exhibit edaphic drought during otherwise normal years.