Hydrology and genesis of claypan soils in eastcentral Texas

Abstract

Claypan soils at four sites were studied to assess moisture relations, soil-forming processes, classification, and soil interpretations. Hydrological conditions, redox potentials and soil solutions were monitored during a 17- month period. Wetness conditions sufficient to allow anaerobic conditions to persist were diagnosed by saturation values >80%. Each of the soils studied reached >80% saturation in some part but never throughout. The duration of near saturation was longer for surface horizons and upper B horizons and decreased with depth. Because the definition of aquic moisture regime requires that the entire soil be saturated (and reduced) by groundwater, these claypan soils do not have aquic moisture regimes even though two of the pedons have the appropriate morphological properties. The intensity and duration of reducing conditions followed similar temporal and spatial trends to soil saturation. Each soil was sufficiently reduced at some time to cause redox potentials to reach values suggesting Fe(II) was present in solution. Analyses of soil solutions, however, indicated that little Fe reduction occurred. Differences in depth and duration of saturation among soil horizons are not closely correlated with observed macro- and micro-morphological properties. Low chroma soil colors are associated with low free Fe contents; both properties are inherited from soil parent materials. Gley mottles (5GY 5/1) in the Elmina subsoil are the best visual markers of reduction. Small amounts of other pedological features, such as albic materials, glaebules, or cutans which have formed by redox processes and redistribution of sesquioxides were found. Major pedogenetic changes in claypan soils are apparently a consequence of past hydrologic regimes. Changes include organic matter accumulation, leaching of bases from upper horizons and accumulation of salts and carbonates in lower B horizons. Textural differentiation between E and Bt horizons occurred through illuviation, neoformation of clays and differential sediment transport. A sedimentary discontinuity may partially responsible also. Formation of aluminous interlayers in clay minerals, Fe reduction and formation of secondary sesquioxide segregations indicate progressive acid weathering.