Potassium-Selective, Halloysite-Rich Soils Formed in Volcanic Materials from Northern California
2001
Takahashi, T. | Dahlgren, R.A. | Theng, B.K.G. | Whitton, J.S. | Soma, M.
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.
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