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Soil Formation on Loess in Southwestern Indiana: II. Distribution of Clay and Free Oxides and Fragipan Formation¹

ABSTRACT

Fourteen soil profiles from southwestern Indiana were studied to determine the effects of loess thickness and natural soil drainage on the profile distribution of clay and free oxides (CBD-extractable) of Fe, Al, Mn, and Si. In all soils, Fe and Al curves follow the clay curve, suggesting that the oxides are adsorbed on clays and move with them. Natural soil drainage had little effect on profile distribution of clay, Si or Fe. Free Al₂O₃ contents increase slightly with poorer drainage. With thinning loess, clay and free Fe₂O₃ distribution changes little, free Al₂O₃ contents increase slightly, free MnO₂ curves have more pronounced minima and maxima, and free SiO₂ increases significantly in fragipan horizons. We propose that free silica moves down the profile in solution and precipitates, possibly with aluminum hydrous oxides on clays. The resulting silica or alumino silicate binds particles together to form the hard, brittle soil material of fragipans.

¹ Contribution from the Agronomy Dep., Purdue Univ. Agric. Exp. Stn., West Lafayette, IN 47907. J. Pap. no. 6250.

² Former Graduate Assistant, currently Assistant Professor, Agronomy Dep., Univ. of Nebraska, Lincoln, NE 68583; Professor; and Associate Professor, respectively.

Received for publication April 13, 1976. Accepted for publication September 13, 1976.

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