Topography and drainage effects on loess-derived fragipan horizon morphology, physical and chemical properties in western Kentucky and USA

The fragipan horizon is a common diagnostic soil horizon globally and is found throughout the eastern United States. Fragipans form in a range of transported parent materials and at multiple topographic/landscape positions. Typically, topographic influence on fragipan horizon morphology is related to drainage class, but these relationships remain uncertain. We investigated the influence of topography on fragipan morphology, physical properties, and extractable metals, with a focus on extractable silicon (Si) concentration, in seven loess-derived pedons in western Kentucky. We expanded this local data set with a regional synthesis of available literature to assess the influence of landscape position and USDA drainage class on the soil properties of loess-derived fragipan horizons from the Ohio and lower Mississippi River valleys. In western Kentucky, we found that fragipan prism diameter correlated with site elevation (ρ = -0.7) and slope (ρ = 0.7). We also found that dithionite extractable Si (Si_d) and ratios of Si_d to dithionite extractable aluminum (Al_d) and iron (Fe_d) positively correlated with loess thickness (ρ = 0.71–0.90). The regional data synthesis indicated that landscape position also has a significant influence on properties of fragipan horizons such as: soil texture, bulk density, pH, oxalate extractable Fe, and dithionite extractable Si molar ratios. We also observed differences in soil texture, Si_d, and Si_d/Al_d and Si_d/Fe_d between fragipan horizons of different drainage classes, with moderately well drained fragipans exhibiting greater Si_d. This was consistent with the hypothesis that fragipan horizons with frequent wet-dry cycles would support greater accumulation of Si. Across landscape positions and drainage classes, Si_d/Al_d and Si_d/(Si + Al_d) were not consistent which may complicate the use of these ratios as metrics for fragipan horizons. In the loess-derived fragipans of the central US, topography and drainage exerted significant influence on fragipan properties and must be considered when addressing fragipan genesis.