Soils in Ultramafic Sediments on a Sequence of Alluvial Surfaces in Baja California

Chronosequences of soils are commonly characterized on fluvial terraces. Few watersheds are so completely dominated by ultramafic rocks that fluvial deposits from them are predominantly ultramafic sediments. Soil properties were evaluated in a sequence of soils on four arid (mean annual precipitation = 97 mm) fluvial surfaces with pebble counts of 90% serpentinized peridotite. Subsoil textures ranged from very gravelly loamy sand in a Torriorthent on a recent floodplain to very gravelly sandy clay in a Duric Petrargid on the uppermost of four alluvial terraces. There were no calcareous horizons and duripans were well developed and practically continuous at about one meter depths in the three terrace soils. Mass ratios of Ca:Mg from aqua regia digestion were about 0.05 (0.03 molar ratios) in all of the soils. Molar exchangeable Ca:Mg ratios were about 1.5 in the floodplain soil. They average 0.7 mol/mol in the surface and 1.0 mol/mol in subsoils on the terraces, declining slightly with soil age. Very fine sand contents were two to three times greater in the surfaces than in terrace subsoils. The extra sand was mostly quartz and feldspars, but serpentine dominated the fine sand and quartz and feldspars were absent from the coarser sand fractions. Supposedly, the Ca input from dust and other aerosols was sufficient to maintain exchangeable Ca:Mg that are high for ultramafic soils, but insufficient to produce Ca-carbonates. Clay minerals evident in the floodplain soil were serpentine, chlorite, smectite, and palygorskite. Palygorskite was the dominant clay mineral in terrace subsoils.