Ultramafic Aridisols on a sequence of fluvial terraces in Baja California

Abstract

Practically all arid ultramafic soils in North America are in Baja California. An arid watershed in Baja California was found to have terraces with soil parent materials dominated by serpentinized peridotite. The soils on a sequence of four fluvial surfaces in the watershed were studied in order to trace the path of serpentine soil development in an arid climate. Flood-plain soils in the arid watershed are Torrifluvents and terrace soils are Argidurids (Durisols) and Duric Petroargids (Luvisols). There were no calcareous horizons in them. Duripans are well developed and practically continuous at about one meter depths in the terrace soils. Mass ratios of Ca:Mg from aqua regia digestion were about 0.05 in all of the soils. Average molar exchangeable Ca:Mg ratios were about 1.5 in the flood-plain soil, and 0.7 and 1.1 in the surface and subsoils on the terraces. Very fine sand contents in the terrace soils were two to three times greater in the surfaces than in the subsoils. The extra sand was mostly quartz and some feldspars, but serpentine dominated the fine sand and quartz was absent from the coarse sand fractions. Apparently, the Ca input from dust and other aerosols was sufficient to maintain exchangeable Ca:Mg ratios that are high for ultramafic soils, but insufficient to produce Ca-carbonates. Clay minerals in the flood-plain soil were serpentine, chlorite, smectite, and palygorskite. Palygorskite was a minor mineral in the flood- plain soil to prominent in terrace soils. There was substantial quartz in the clay <4 μm fraction.