Hydrology and morphology of a toposequence of three Claypan soils in Central Texas

Abstract

Aquic conditions are defined in terms of saturation and reduction, but they are often evaluated based on soil redoximorphic features. Interpretations of redoximorphic features are complicated by the presence of relict features in some areas. In this study three Alfisols forming a toposequence in the Claypan Major Land Resource Area were evaluated for redoximorphic features and actual current moisture conditions. The purpose of this study was to analyze the Silawa-Rader-Edge toposequence to determine which redoximorphic fealures in these soils are associated with contemporary aquic conditions and which are relict features. Saturation, matric potential, and redox potential were measured at several depths with piezometers, tensiometers, and permanently installed Eli electrodes. Redoximorphic and other features were described in the field and from thin sections. Supporting laboratory analyses included physical and chemical characterization, x-ray diffraction, and elemental analysis. None of the three soils studied had a Ground water table within 5 meters of the surface. The high chroma colors in Silawa reflected current well drained conditions which were confirmed by field monitoring. The Fe-Mn nodules and concretions in Silawa were relict features either transported with the terrace deposit or formed in place during a previous wetter moisture regime. The low chroma colors and few nodules in Edge were also relict features from previous wetter environments and did not reflect current well drained conditions. in Rader there was a perched water table at I meter for 6 to 7 months each year during the study period. Thus, Rader met the requirements for episaturation. Water also perched above 60 cm for shorter periods immediately following rainfall events. From Eh readings, Rader appeared to have sufficient periods of reduction in some microsites to mobilize Fe. Therefore, redoximorphic features within perched water tables reflected current as well as previous aquic conditions, while features below BC horizon upper boundary, at 153 cm, were relicts. Taxonomic classifications of the three pedons representing Silawa, Rader, and Edge series were evaluated. All three pedons studied had good evidence for argillic horizons, but each pedon had a few characteristics outside the range of the established series taxonomic placement. The Rader site appeared to be borderline between aquic (as classified) and oxyaquic sub-groups. There were significant microsite variations indicated by replicate readinos for piezometers in Rader and tensionieters and Eh electrodes as in all three soils. Replicates often indicated similar trends but different degrees of saturation and reduction. The designation of aquic or oxyaquic subgroup for Rader hinges on whether reduction is required throughout most of the soil mass to meet aquic condition requirements or reduction in only some microsites is sufficient. The aquic conditions section of the most recent Key to taxonomy (Soil Survey Staff, 1992) does not address this issue.