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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

View larger version (33K): [in a new window]   Fig. 1. X-ray diffractograms (Co K radiation) of H2O2/citrate-dithionite treated clay (<2 µm) samples. Mg = Mg-saturated; Mg-F = Mg-saturated, formamide solvated

View larger version (180K): [in a new window]   Fig. 2. Transmission electron micrograph of the clay-size (<2 µm) fraction of the LeTrab 2Bt3 horizon. Reference scale is 0.5 µm

View larger version (43K): [in a new window]   Fig. 3. X-ray diffractograms (Cu K radiation) for the fine clay (<0.2 µm), coarse clay (0.2–2 µm), silt (2–50 µm), and fine-sand (50–250 µm) fractions from the 2Bt3 horizons. Mg = Mg-saturated; Mg-G = Mg-saturated, glycerol solvated; Mg-F = Mg-saturated, formamide solvated; K-25/300/550 = K-saturated with heating to 25, 300, and 550°C. Samples were pretreated with H2O2 to remove organic matter, and citrate-dithionite to remove free Fe oxides before dispersion and particle-size fractionation

View larger version (16K): [in a new window]   Fig. 4. Adsorbed K+ (EK) vs. solution K+ compared to the nonpreference line for Ca2+–K+ exchange. Data are for the <2-mm fraction (filled circle) of the nontreated soil and for the 2Bt3 clay fraction (open triangle) that was treated with H2O2 to remove organic matter and citrate-dithionite to remove free Fe oxides before dispersion and particle-size fractionation. Nonpreference line (dashed line) calculated following Sposito et al. (1983) (i.e., KV = 1; see Eq. [2])

View larger version (30K): [in a new window]   Fig. 5. Gapon selectivity coefficient (KG) (upper panels) and natural logarithm of the Vanselow selectivity coefficient (ln Kv) (lower panels) as a function of the equivalent adsorbed fraction of K (EK). Data are for the <2-mm fraction (filled circle) of the nontreated soil, and for the 2Bt3 clay fraction (triangle) that was treated with H2O2 to remove organic matter and citrate-dithionite to remove free Fe oxides before dispersion and particle-size fractionation

View larger version (28K): [in a new window]   Fig. 6. X-ray diffractograms (Co K radiation) of Red Tank 2Bt3 and LeTrab 2Bt3 clay (<2 µm) (lower diffractograms) and silt (2–50 µm) (upper diffractograms) fractions. Mg = Mg-saturated; Mg-EG = Mg-saturated, ethylene glycol vapor treated; K-EG = K-saturated, ethylene glycol vapor treated; K-550 = K-saturated with heating to 550°C

View larger version (15K): [in a new window]   Fig. 7. Aluminum-27 nuclear magnetic resonance (NMR) spectrum of Red Tank 2Bt3 clay. Soil was treated with H2O2 to remove organic matter and citrate-dithionite to remove free Fe oxides before dispersion and particle-size fractionation. The spectra of a well-ordered halloysite (Matauri Bay) and a poorly ordered halloysite (Te Akatea), as reported by Newman et al. (1994), are included for reference. = chemical shift measured relative to the signal from 0.1 M aluminum sulfate