Fragipan Genesis in Two Alfisols in East Central Missouri

Fragipan formation has frequently been related to the thickness of loess, with the development of gray vertical seams in deeper loess mantles and full fragipan development restricted largely to loess thickness of less than 2 meters. One body of research has suggested that fragipan formation is related to the position of a paleosurface after burial by loess, whereas other research suggests that fragipan formation may be unrelated to loess stratigraphy. Silicon has been implicated in promoting the moisture-dependent consistence of fragipans: consistence is hard when the moisture content is near the permanent wilting point and brittle when the moisture content is near field capacity. The processes responsible for the moisture-dependent consistence remain a matter of active research. Two fragipan containing soils developed in moderately deep loess were selected to determine the role of loess stratigraphy on fragipan genesis and to determine if silicon, possibly in concert with either aluminum or iron, is an effective cementing agent. The clay mineralogy, Al to clay ratio, Fe to clay ratio, and trace element contents suggest that the soils are developed in a single layer of loess overlying intensively weathered residuum. Fragipans formed within the basal mixing zone of these two parent materials. An ammonium oxalate extraction and an aqua-regia extraction did not reveal the presence of appreciable quantities of noncrystalline material in the fragipan or elsewhere in the soil. The Si activity indicates that noncrystalline materials have not affected Si solubility materially. In general, fragipan horizons seem to be associated with the basal mixing zones of the loess and underlying residuum. Silt and clay illuviation into the fragipan seems to be the dominant soil process associated with fragipan formation in the basal mixing zone. No evidence for Si as a binding agent was observed, and we speculate that fragipan consistence is related to the architecture of the soil separates within the fragipan.