Plymouth County is located in western Iowa, being separated by the Big Sioux River from South Dakota on the west, and is in the third tier of counties south of the Minnesota State line. It lies entirely in the Missouri loess soil area and hence the soils of the county are mainly loessial in origin.
Stevenson, W. H., "Soil Survey of Iowa, Report No. 54—Plymouth County Soils" (1929). Soil Survey Reports. 66.