Location and Expression of Fragipans in an MLRA Benchmark Catena.

Soils in Major Land Resource Area 115 (Central Mississippi Valley Wooded Slopes) in southern Illinois, Indiana, and Missouri are developed from a thick mantle of loess in a udic soil moisture regime. Soils in a representative 10 ha watershed in Union County, Illinois were studied to evaluate the location and expession of fragipan soils in the watershed. Loess thickness on the summits was measured at > 5 m. Deep, well drained soils of the Menfro series (Typic Hapludalfs) compose convex summit and shoulder positions. On shoulder and backslope positions, loess thickness decreased and bisequal soils of the Hosmer series (Oxyaquic Fragiudalfs) are found. These soils have a fragipan at 50 to 100 cm depth beginning in the Peoria-aged loess unit and extending into the Roxana loess unit. Redox concentrations are present above the pan indicating a hydrologic discontinuity. Fragipans are >100 cm thick on shoulder and backslope positions and occurance is related to the presence of the discontinuity between the two loess units. Fragipans do not form on steeper slopes likely due to thinning loess of the overlying Peoria loess and soil instability (e.g. erosion, creep, turbation). Fragipans are also absent in lower bacsklope/upper footslope positions consisting of colluvial and alluvial material.