Johnson County is located in eastern central Iowa in the third tier of counties west of the Mississippi river and in the fourth tier north from the Missouri state line. It is partly in the Mississippi loess soil area and partly in the Iowan drift and the soils are therefore of loessial and glacial origin.
Stevenson, W. H. and Brown, P. E., "Soil Survey of Iowa, Report No. 32—Johnson County Soils" (1923). Soil Survey Reports. 44.