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This article in SSSAJ

  1. Vol. 65 No. 2, p. 516-526
     
    Received: Apr 12, 1999


    * Corresponding author(s): radahlgren@ucdavis.edu
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doi:10.2136/sssaj2001.652516x

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

  1. T. Takahashia,
  2. R.A. Dahlgren *b,
  3. B.K.G. Thengc,
  4. J.S. Whittonc and
  5. M. Somad
  1. 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

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.

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Copyright © 2001. Soil Science SocietyPublished in Soil Sci. Soc. Am. J.65:516–526.