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Slickspot Soil Genesis in the Carrizo Plain, California

Woodward-Clyde Consultants, 2020 East First Street, Suite 400, Santa Ana, CA 92705

R. C. Graham and C. Amrhein

Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92521-0424

R. J. Southard

Dep. of Land, Air and Water Resources, Univ. of California, Davis, CA 95616

*Corresponding author.

ABSTRACT

Barren, shallow depressions known as slickspots are common features on sodic soil landscapes. Our objective was to determine the conditions and processes responsible for slickspot soil genesis in a closed basin grassland in the Central Coast Range province of California. The slickspots are irregularly shaped, 0.5 to 15 m long, and devoid of all vegetation throughout the year. The adjacent soils under grass have strong columnar structure below a thin (< 5 cm) A horizon. In the 0.5-m-wide transition zone between grass-covered soils and slickspots, the grass is sparse and stunted, the soil is eroded to the Bt horizon, the columnar structure is degraded, and salt efflorescences rise from > 150 cm deep in the vegetated soil to within about 10 cm of the slickspot soil surface. Infiltration is severely impeded in the crusted slickspot soils, but not in the structured vegetated soils. Thus, leaching predominates over evapotranspiration in the vegetated soils (fine, montmorillonitic, thermic Typic Haploxeralfs), but is insignificant in the adjacent slickspot soils (fine, montmorillonitic, thermic Typic Natrargids). Water and dissolved salts leached through the vegetated soils move along a water potential gradient into the drier slickspot subsoils, then upward, following a gradient established by evaporation at the slickspot surface, resulting in salinization of the slickspot soils. Carbon-dated relict grass roots from the slickspot soils indicate that the slickspots have expanded at the expense of grassland within the last few tens to hundreds of years. Water and wind erosion at slickspot margins is increasing the size of these barren areas, which are too dry, compacted, and saline for vegetation to become reestablished.

Contribution of the Dept. of Soil and Environmental Sciences, Univ. of California, Riverside.

Received for publication March 4, 1992.