Location and expression of fragic soil properties in a loess-covered landscape, Southern Illinois, USA

Abstract Fragic soil properties often develop in thin loess units overlying residual parent materials in the midwest USA. Morphologic indicators of these fragic properties are often weakly expressed, making consistent mapping of soils difficult. This study was initiated in a small watershed in Union County, Illinois to determine location and degree of development of fragic materials and fragipans on the landscape. We also sought to assess factors influencing the degree of expression of these features, and document accessory soil properties that reflect the presence of these fragic soil materials. Nine pedons from two catenas were sampled and selected chemical, physical, and mineralogical analyses were conducted. Results indicate that loess thickness across the watershed ranged from about 2 to 4 m. Fragic soil properties were observed in argillic horizons on side slope and head slope geomorphic components at depths ranging from 51 to 111 cm. Depths to horizons meeting fragipan criteria ranged from 140 to 175 cm. Discontinuity between two loess units (Peoria and Roxana Silts) was verified by both particle size analysis (fine:coarse silt ratios, clay-free fine and coarse silt) and magnetic susceptibility. These data suggest that the development of fragic soil properties is not controlled solely by the discontinuity between loess deposits. Fragipans are weakly developed based on rupture resistance, morphological features such as structure, bulk density − 3 , and a uniform depth function of citrate dithionite-extractable Fe. The best developed fragipan occurs where loess is thinner (~ 2 m) overlying less permeable material suggesting that loess thickness does influence degree of development of fragic properties. Horizons above fragipans do have increased kaolinite, a greater noncrystalline proportion of the Fe oxyhydroxides, and redoximorphic features suggesting some perching of water. These features suggest that fragic soil properties and fragipans in these soils, though weakly expressed, impact water movement within landscapes.