Role of Free Silicon, Aluminum, and Iron in Fragipan Formation

Abstract

Fragipans form by various physical and chemical processes. The objective of this study was to elucidate the role of free Si, Al, and Fe in weak cementation of fragipan soils. We studied a toposequence of soils in which natural drainage ranged from poor to good on a dissected pre‐Wisconsinan plain covered with loess in southern Indiana. Soils were sampled in 10 to 15 cm layers. In the laboratory, strength was determined by a rupture test on disturbed samples, and Si, Al, and Fe were determined on citrate–bicarbonate–dithionite (CBD) extracts (with d subscripts). Strength reached a profile maximum in the fragipan. With molar Sid/(Sid + Ald) ratio of ≤0.5 there was no correlation between this ratio and strength, but in horizons in which the ratio was >0.5, mainly in fragipans, the correlation was strong. Strength was also correlated with exchangeable Mg. In fragipans the molar Ald/(Ald + Fed) ratio was ≈0.20, and above the fragipan the ratio was greater. We propose that in the fragipan most of the Ald is derived from Al substituted for Fe in Fe oxides, but in horizons above the fragipan there is more Al than the Fe oxides can accommodate. The extra Ald combines with free Si in these horizons to form aluminosilicates. In the fragipan, however, most free Al is tied up in the Fe oxides and cannot react with Si, so the free Si that is translocated from upper horizons bonds to clay minerals, and especially to Fe oxides, to cause cementation.