Genesis and Physical Behavior of Soils on Sandstone and Shale in Southern California

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Sandstone- and shale-derived soils are common in southern California where they function in watersheds, support wildland ecosystems, and are subject to residential and infrastructure development. Because little is known about these soils on texturally contrasting parent materials, this study was designed to interpret their pedogenesis and accompanying soil physical behavior. The study area is in the chaparral-covered foothills of the San Gabriel Mountains, where the 400-mm mean annual precipitation comes as rain during the winter and summers are hot and dry. The soil on sandstone is an Alfisol with a fine-loamy argillic horizon and mixed mineralogy. It is leached of carbonates and has pH values of 5.7 to 6.8. The shale-derived soil is a clayey, smectitic Vertisol with accumulated calcium carbonate, indicating less leaching than the Alfisol. Soil organic carbon storage in the Alfisol (7.96 kg m−2) and Vertisol (9.87 kg m−2) is similar, but the Vertisol has additional carbon (4.88 kg m−2) in the form of calcium carbonate. The Vertisol has high shrink-swell potential, with coefficient of linear extensibility values mostly 0.09 to 0.12. The Alfisol has minimal shrink-swell potential. Available water capacities for the Alfisol soil horizons (4%–20%) range higher than those of the Vertisol (4%–11%), but the thicker regolith profile of the Vertisol means it can store more plant-available water overall. Field saturated hydraulic conductivity was similar for the soils during the wet season (0.2–1.6 cm h−1), but was higher in the extensively cracked Vertisol during the dry season. The available water capacities (4%) and saturated hydraulic conductivities (0.2–0.4 cm h−1) of the Cr horizons of both soils indicate that these weathered bedrocks are functional parts of the soil hydrologic system. The divergent properties of the soils are related to the nature of the parent material and its evolution in response to soil-water dynamics.

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