Aquic conditions and hydric soil indicators for aquolls and albolls

The identification of soil morphologies associated with seasonally-saturated and reduced conditions in Mollisols (wet prairie soils) is particularly challenging due to high organic matter contents in the upper solum and surface accumulations of erosional sediment. Redoximorphic features resulting from Fe reduction are frequently hidden or absent within the upper 30 cm. In this chapter, we summarize soil horizon, profile, and landscape characteristics associated with wet prairie soils and provide examples of soil hydrology and morphology for hydrosequences on hillslopes in the upper midwestern USA. Pathways and timing of water movement and accumulation within wetland landscapes affect specific soil characteristics (e.g., clay and carbonate distribution, organic matter contents, and others) on hillslopes and associated wetland basins. Limited monitoring data collected on seasonal water table, soil water potential, and redox potential fluctuations support the hypothesis of saturation and reduction at shallow (=30 cm) depths in Mollisols despite the lack of clearly-visible morphological features usually associated with soil saturation and reduction. Consequently, the morphological characteristics of subsurface soil horizons at depths of >30 cm and landscape data must be considered for field identification of aquic conditions or hydric soils in landscapes composed of Mollisols. A profile darkness index based on the depth, chroma, and value of dark colors (=3 chroma and value) was used to discriminate soils subject to aquic conditions from upland soils on a hillslope in western Minnesota. Given the complexity of the processes contributing to soil genesis at the wetland fringe in Mollisol landscapes, an approach that considers multiple aspects of soil and landscape morphology is needed to interpret the hydrologic history of these soils.