The mean total Rb content in 223 surface horizons of upland soils was 88.8 mg kg⁻¹ and contents were normally distributed. Rubidium tended to be highest at sites where loess is very thin or absent and illitic Wisconsinan till is present. Surface horizons of soils in areas of thick loess commonly have about 80 mg Rb kg⁻¹. The pattern of increasing soil development associated with decreasing loess thickness characteristic of Illinois is reflected by Rb concentrations that first increase with declining loess thickness and then decrease in the most intensely weathered soils. Minerals derived from the basin of Des Moines River of Iowa, which were incorporated in loess below its confluence with Mississippi River, are especially rich in Rb and gave rise to the highest levels observed (110–115 mg kg⁻¹).
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