Old Neogene summer-dry soils with ultramafic parent materials

Old, strongly weathered soils that have developed from ultramafic, or serpentine, parent materials in summer-dry temperate climates are sparse. Five serpentine soils with kandic horizons have been described and sampled in summer-dry coniferous forests of northern California and southwestern Oregon. Although the dispersion of clay in the subsoils is difficult, and they are neither very sticky nor very tough, 1.5 MPa water retention indicates that all but one have at least 50% clay and two of them have subsoils with about 85% clay. Thus the “effective” CECs in the Bt horizons are much < 120 mmol +/ kg of clay and at pH 7, CECs are much < 160 mmol +/ kg of clay. The kandic horizons are moderately to slightly acid and net positive charges occur in four of them. In all of the soils, the KCl extractable acidity is negligible. Variable charge (pH 7.0 to 8.2) in the subsoils is closely related to “free” iron contents that range from 14 to 33% Fe (0.33 g/g). Even though all five of the soils have kandic horizons, only in three of them have basic cations been leached deeply enough for the soils to be Ultisols, rather than Alfisols. Magnesium is the dominant basic cation in the soil parent materials, and it remains the dominant cation in the subsoils, although Ca has replaced it as the dominant exchangeable cation in surface soils. Intensive weathering to develop kandic horizons appears to be more prevalent than intensive deep leaching of basic cations in these summer-dry soils with mesic soil temperature regimes. Because the major minerals in serpentinized peridotite are highly susceptible to weathering, kandic horizons may form in them within one or two million years. ► Soils with ultramafic parent materials weather relatively rapidly and iron released from the primary minerals accumulates in the soils. ► As Fe-oxides accumultate in the soils, the ion exchange sites become predominantly positive, even in only slightly to moderately acid soils. ► The old soils that are deeply weathered and leached are Ultisols, Kandihumults.