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DIVISION S-9-SOIL MINERALOGY

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

^a Akita Prefectural University, Nakano, Shimo-Shinjo, Akita 010-0195, Japan
^b Dep. of Land, Air and Water Resources, Univ. of California, Davis, CA 95616
^c Landcare Research, PB 11052, Palmerston North, New Zealand
^d Institute for Environmental Sciences, Univ. of Shizuoka, Shizuoka 422-8526, Japan

Corresponding author (radahlgren{at}ucdavis.edu)

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 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 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.

Abbreviations: CEC, cation-exchange capacity • DTA, differential thermal analysis • NMR, nuclear magnetic resonance • XPS, x-ray photoelectron spectroscopy • XRD, x-ray diffractometry

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