Potassium‐Selective, Halloysite‐Rich Soils Formed in Volcanic Materials from Northern California

Abstract

Several subsoil horizons of Andic Haploxeralfs, formed in volcanic ejecta under a xeric moisture regime in northern California, retain large amounts of exchangeable K+ and show high K+ saturation. The relationships between clay mineralogy, mineral charge characteristics, and exchangeable K+–Ca2+ selectivity were examined. Clay mineralogy and surface charge were assessed by x‐ray diffractometry (XRD), differential thermal analysis (DTA), transmission electron microscopy, x‐ray photoelectron spectroscopy (XPS), 27Al‐nuclear magnetic resonance (NMR) spectroscopy, total elemental analysis, surface area measurements, and determination of K+–Ca2+ selectivity coefficients. Kaolin minerals with a tubular morphology comprise 75 to 91% of the clay‐size fraction in the subsoil horizons. Kaolinite was prevalent in the surface horizons, while halloysite concentrations and the degree of halloysite hydration increased with depth. No detectable amounts of 2:1 layer silicates, 1:1–2:1 mixed‐layer clays, or zeolites (e.g., clinoptilolite) were found in the clay‐size fraction of the subsoil horizons. Soil samples dominated by halloysite showed a strong selectivity for K+ The clay fractions (<2 μm) have cation‐exchange capacity (CEC) values ranging from 19 to 26 cmolc kg−1 and surface areas from 90 to 112 m2 g−1 The variable and permanent charge components were 11 and 20 cmolc kg−1, respectively. The 27Al‐NMR spectrum of the halloysite‐rich clay indicates a poorly ordered kaolin and a tetrahedral Al content of ≈2%. While a disordered halloysite may be responsible for the high surface area, CEC, and K+ selectivity displayed by these soils, the contribution from 2:1 layer silicates and 1:1–2:1 mixed‐layer clays in the silt and sand fractions and Fe oxide surface coatings must also be considered.