Potassium-selective, halloysite-rich soils formed in volcanic materials from northern California

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 micrometer) have cation-exchange capacity (CEC) values ranging from 19 to 26 cmol(c) kg-1 and surface areas from 90 to 112 m2 g-1. The variable and permanent charge components were 11 and 20 cmol(c) kg-1, respectively. The 27Al-NMR spectrum of the halloysite-rich clay indicates a poorly ordered kaolin and a tetrahedral Al content of approximately equal to 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.