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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)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
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

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
THE CLAY mineralogy of soils formed in volcanic materials varies widely depending on such factors as the composition of the parent material, stage of soil formation, pH, soil moisture regime, and the accumulation of organic matter (Shoji et al., 1993). Poorly ordered materials such as allophane, imogolite, ferrihydrite, and Al– and Fe–humus complexes often dominate the clay-size fraction of volcanic soils. Halloysite, a more ordered phase, is also a common constituent of many volcanic soils. Kaolin minerals can show a wide range of structural disorder (Churchman and Theng, 1984; Soma et al., 1992; Newman et al., 1994) due primarily to Al-vacancy displacements in the octahedral sheet (Plançon and Tchoubar, 1977a, 1977b; Soma et al., 1992). These vacancies may originate from nonstoichiometric substitution of Fe³⁺ for Al³⁺ in the octahedral sheet (Soma et al., 1992). Variable effects of hydration might also add to the degree of disorder in halloysite.

An Andisol Inceptisol Alfisol progression is observed in soil formation from volcaniclastic materials under the xeric soil moisture regime in northern California (Takahashi et al., 1993). In this sequence, the clay mineralogy (<2 µm) is dominated by allophanic materials and/or halloysite, often with minor amounts of gibbsite, 2:1 layer silicates, and Al–humus complexes. The coexistence of allophanic materials and halloysite in these soils may be due to their differential formation. This is possible because the Si concentrations in the soil solution fluctuate widely under the xeric soil moisture regime, and/or short range–ordered products can transform into more ordered minerals upon seasonal desiccation (Takahashi et al., 1993). The clay fraction of the subsurface horizons is almost exclusively composed of halloysite and accumulates large amounts of exchangeable K (Takahashi et al., 1993). The halloysite-rich clays in these horizons also possess a high CEC as well as a high K⁺ selectivity compared with most halloysites reported in the literature.

There have been several reports of soils derived from volcanic materials having a strong K⁺ or NH⁺₄ retention (e.g., Okamura and Wada, 1984; Quantin et al., 1988; Delvaux et al., 1989, 1990a, 1990b, 1992; Fontaine et al., 1989; Espino-Mesa and Hernandez-Moreno, 1994; Escudey and Galindo, 1988; Escudey et al., 1997). High K⁺ (or NH⁺₄) selectivity has been explained in terms of a K⁺- or NH⁺₄-selective halloysite (Wada and Odahara, 1993; Okamura and Wada, 1984), the presence of a halloysite–smectite mixed-layer clay (Delvaux et al., 1990a, 1990b), the possible preferential repulsion of divalent cations by Fe oxide coatings (Escudey and Galindo, 1988; Escudey et al., 1997), whole salt (KCl) retention by halloysite (Wada, 1958, 1959; Thomas, 1960), the possible formation of an alunite phase [KAl₃(OH)₆(SO₄)₂] (Espino-Mesa and Hernandez-Moreno, 1994), and the presence of mica or vermiculite contaminants (Parfitt, 1992). To date, no common mechanism has been identified to account for this high K⁺ selectivity. It is important to note that most of the halloysite samples in these previous studies were not pure specimens, and therefore the results may be affected by 2:1 contaminants. Here we examine the relationship between the clay mineralogy and the surface charge and K⁺ selectivity characteristics associated with soils formed in volcanic materials in the xeric moisture regime of northern California.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Study Area and Soil Samples
The study area is located in the vicinity of Mt. Shasta in northern California (Takahashi et al., 1993). This area has a typical Mediterranean climate resulting in a xeric soil moisture regime. The mean annual precipitation is 1000 mm, almost all of which occurs during the period November to March. The mean annual air temperature is 8 to 10°C. The soils are formed in andesitic lava or andesitic basalt flows and are overlain by rhyolitic ash (25–40 cm) from Holocene eruptions. Soil formation proceeds through an Andisol Inceptisol Alfisol progression.

Out of the five pedons previously studied (Takahashi et al., 1993), we selected two pedons (LeTrab series: fine-loamy, halloysitic, mesic, Andic Haploxeralf; Red Tank series: fine, halloysitic, mesic, Andic Haploxeralf) for this investigation because their subsurface horizons (2Bt1–2Bt3) had high exchangeable K⁺ concentrations and K⁺ saturation (K/ECEC = 0.25–0.39) (Table 1). These horizons have their clay mineralogy dominated by halloysite and display a moderate to high degree of weathering as evidenced by the high clay (32–68%) and free Fe oxide (5–10 wt. % Fe) concentrations.

Semiquantitative XRD methods were used to estimate the content of crystalline minerals in the clay fraction (Whitton and Churchman, 1987), except for total kaolin (halloysite and kaolinite) content, which was measured by DTA. Te Akatea halloysite (Churchman and Theng, 1984) and a well-crystalline kaolinite (Dana) similar to the KGa-1 reference material of the Clay Minerals Society (van Olphen and Fripiat, 1979, p. 13–15) were used as standards. Halloysite was distinguished from kaolinite by the presence of a peak near 1.0 nm before or after intercalation of formamide (Churchman and Theng, 1984; Churchman, 1990) and by calculating the ratio of the intensity of the 001 peaks (i.e., the sum of the intensity of the peak at 0.7 nm and that at 1.0–1.1 nm). We used the differential response of K⁺- and Mg²⁺-saturated clays to ethylene glycol to search for evidence of halloysite–smectite mixed-layer clays as described by Delvaux et al. (1990b). Diffractograms for the treatments described above were performed using Co K radiation generated with a Philips PW 1710 instrument (Philips Analytical, Eindhoven, the Netherlands) operating at 40 kV and 60 mA; the scanning rate was 2° 2 min^-1.

In addition, 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 were isolated for XRD analysis using the methods of Jackson (1975). The bulk soil was treated with H₂O₂/citrate-dithionite and dispersion by ultrasonic vibration at 20 kHz for 5 min before particle-size fractionation. X-ray diffraction was performed on oriented samples for the clay and silt fractions and on random powder mounts for the sand fraction. All samples were saturated with K⁺ (then heated to 25, 300, and 550°C) and Mg²⁺ (nontreated and solvated with glycerol and formamide) for XRD analysis. The latter diffractograms were obtained with a Diano 8000 x-ray diffractometer (Diano Corp., Woburn, MA) using Cu K radiation generated with 50 kV accelerating potential and 15 mA tube current. Samples were step-scanned for 2 s at a 0.04° 2 step. Throughout the manuscript we will use the following definitions for the various kaolin minerals:

1. "(1.0 nm) halloysite" for hydrated halloysite
   
2. "(0.7 nm) halloysite" for dehydrated halloysite that is expandable by formamide
   
3. "tubular kaolinite" (Churchman and Gilkes, 1989) for material with a tubular particle morphology and 0.7-nm basal reflection that does not expand with formamide.
   

Transmission electron microscopy was performed on clay specimens spotted onto C-coated collodion films. Total elemental analysis was performed by fusion with lithium metaborate, dissolution in nitric acid, and quantification by inductively coupled plasma spectroscopy (Ingamells, 1970). X-ray photoelectron spectroscopy was used to determine elemental concentrations in the surface layers of halloysite particles (Soma et al., 1992). Halloysite disorder and tetrahedral Al concentration were assessed by medium-field ²⁷Al-NMR spectroscopy as described by Newman et al. (1994). Specific surface area (total surface area) was measured on Ca²⁺–saturated clays by adsorption of para-nitrophenol (Theng, 1995).

A K⁺–Ca²⁺ exchange equilibrium study was performed using the methods of Wada and Odahara (1993) on nontreated, bulk soil samples (<2 mm) and on the H₂O₂/citrate-dithionite treated, clay fraction (<2 µm) from the 2Bt3 horizons. Exchange equilibrium values were measured across a range of K adsorption ratios ([K⁺]/[Ca²⁺]^1/2) from 0.1 to 4.5 mM^1/2 (final values after equilibrium). Samples were washed five times with 0.5 M CaCl₂ to saturate exchange sites with Ca²⁺, then washed six times to equilibrate the exchange sites with mixed KCl–CaCl₂ solutions having a constant 0.02 M Cl concentration. After the final decantation, adsorbed K⁺ and Ca²⁺ were extracted five times using 1 M NH₄OAc solution. Concentrations of K⁺ and Ca²⁺ in the equilibrium solution and exchangeable-cation extracts were measured by atomic absorption–flame emission spectroscopy. Chloride was measured in the exchangeable-cation extracts using the mercury thiocyanate method (Frankenberger et al., 1996) to determine whether whole salt (KCl) retention contributed to CEC values. The Gapon (K_G) and Vanselow (K_V) selectivity coefficients were calculated using the equations

(1)

(2)

where E_K and E_Ca denote equivalent fractions of adsorbed K⁺ and Ca²⁺, N_K and N_Ca denote molar fractions of adsorbed K⁺ and Ca²⁺, and (Ca²⁺) and (K⁺) denote aqueous ion activities. Aqueous ion activities were calculated using activity coefficients computed from the Davies equation. The overall selectivity values expressed in this study bear the dimensions of (L mol^-1)^-1/2 for K_G and L mol^-1 for K_V.

Cation-exchange capacity originating from clinoptilolite and non-zeolitic materials was estimated by the method of Ming and Dixon (1987) using bulk soil samples (<2 mm) from the halloysite-rich horizons after treatment with H₂O₂ and citrate-dithionite. We substituted 0.5 M tetramethyl-ammonium chloride for the tert-butylammonium chloride used by Ming and Dixon (1987). The amount of variable and permanent charge on the <2-mm fraction from selected horizons, after treatment with H₂O₂/citrate-dithionite, was estimated using the cesium-adsorption method of Anderson and Sposito (1991).

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Clay Mineralogy
The clay-size fraction (<2 µm) of both pedons is dominated by kaolin and allophanic clays (Fig. 1 and Table 2). Allophanic clays are prevalent (9–21%) in the upper 50 cm of both pedons and occur at very low concentrations in the lower horizons (Table 2). The allophanic materials have an Al/Si atomic ratio of 2.0 and a thread-like morphology characteristic of imogolite (Takahashi et al., 1993). The enrichment of allophanic materials in the surface horizons correlates well with the concentrations of volcanic glass in the fine-sand fraction (Takahashi et al., 1993). Thus, the distribution of allophanic clays appears to be related to the recent additions of rhyolitic ash whose volcanic glass component rapidly weathers to allophanic clays. In the lower horizons, 75 to 91% (estimated by DTA) of the clay-size fraction is composed of kaolin minerals (kaolinite and halloysite). Only trace quantities of 2:1 type layer silicates and gibbsite are present in the lower soil horizons.

View larger version (33K): [in this window] [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 this table: [in this window] [in a new window]   Table 2. Semiquantitative mineralogical analysis of the clay-size (<2 µm) fraction of the Red Tank and LeTrab pedons.

View larger version (180K): [in this window] [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 this window] [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

Elemental Analysis of Clay-Size (<2 µm) Fraction
Total elemental analysis of the deferrated, Na-saturated clay fraction (<2 µm) shows a Si/Al atomic ratio of nearly 1.0 for the lower soil horizons, consistent with the dominance of kaolin minerals (Table 3). The Red Tank 2Bt3 is the only horizon having a Si/Al atomic ratio greater than 1.0 because of the presence of some quartz (Table 2). Thus, the elemental analysis supports the finding by semiquantitative mineralogy (Table 2) that 2:1 type layer silicate concentrations are low. Furthermore, the low K concentration (0.08–0.15% K₂O) in the lower two horizons indicates that micaceous minerals are virtually absent. Iron and Mg concentrations are similarly low in the halloysite-dominated lower horizons (0.94–1.32% Fe₂O₃ and 0.06–0.15% MgO), indicating that they are only a minor structural component of the clay minerals. The XPS analysis of clays from the Red Tank 2Bt3 horizon (Table 4) shows an Al/Si ratio of 1.0 and a Fe/Si ratio of 0.02 in the surface layers, which are similar to the values derived from elemental composition of the bulk clay. However, because of the uncertainty in the quantitative XPS analysis, the existence of 1:1–2:1 mixed layer minerals cannot be determined by XPS unless the observed octahedral cation/Si ratio is less than 0.9.

View larger version (16K): [in this window] [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 this window] [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

Cation-Exchange Capacity and Surface Area of Clay-Size Fractions
The CEC (CaOAc, pH 7.0) of the organic matter-free, deferrated clay-size fraction ranges from 19 to 37 cmol_c kg^-1 (Table 6). The lower soil horizons dominated by halloysite show CEC values from 19 to 26 cmol_c kg^-1, which are very high for halloysite. The CEC values for kaolin minerals generally range between 2 and 10 cmol_c kg^-1, although Bailey (1990) reported values as high as 60 cmol_c kg^-1 for halloysite-rich clays in a review of the literature. These uncommonly high CEC values may be due to contamination by 2:1 layer silicates (Lim et al., 1980). However, Norrish (1995) described a hydrated halloysite with high CEC (20 cmol_c kg^-1 clay) and no real evidence of 2:1 contaminants. This halloysite occurred as long laths (up to 20 µm), had a high surface area, and lost one-half of its CEC upon field drying.

Possible Mechanisms for High Cation-Exchange Capacity and K⁺ Selectivity
Only a few instances of high K⁺/NH₄⁺ selectivity in soils dominated by halloysite have previously been reported (Okamura and Wada, 1984; Fontaine et al., 1989; Delvaux et al., 1989, 1990a, 1990b). We tested four hypotheses that could possibly explain the exceptionally high values of K⁺ selectivity, CEC, and surface area of our samples:

1. the presence of halloysite–smectite mixed-layer clays
   
2. the presence of 2:1 layer silicate contaminants
   
3. the presence of trace concentrations of zeolites (e.g., clinoptilolite)
   
4. the existence of a high-charge halloysite with a strong affinity for K⁺
   

Halloysite–smectite mixed-layer clays have been shown to contribute to the high CEC and K⁺ selectivity of soils derived from basaltic volcanic ash, primarily in tropical and subtropical areas (Quantin et al., 1988; Delvaux et al., 1990b, 1992; Delvaux and Herbillon, 1995). Halloysite-smectite mixed-layer clays form in silica- and base-rich environments at the expense of primary minerals through syngenetic formation, which is microenvironment dependent (Delvaux and Herbillon, 1995). The existence of such a halloysite–smectite mixed-layer clay was inferred from several analytical techniques, including differences in XRD patterns between K⁺- and Mg²⁺-saturated clays following ethylene glycol treatment (Delvaux et al., 1990b, 1992; Delvaux and Herbillon, 1995). In the case of the Mg²⁺-saturated clay, the 1.0-nm reflection is shifted to 1.05 nm and exhibits a distinct broadening towards low diffraction angles; simultaneously, a weak peak appears at 1.49 nm that cannot be assigned to halloysite. In contrast, there is no detectable shift for the K⁺-saturated sample. This differential response to ethylene glycol treatment provides strong evidence for the existence of mixed layer clays.

We followed this procedure for our clay (<2 µm) and silt (2–50 µm) fractions. The XRD patterns for the K⁺- and Mg²⁺-saturated samples solvated with ethylene glycol were nearly indistinguishable (Fig. 6) . However, ethylene glycol treatment tends to cause broadening of the 1.0-nm reflection towards low angles for both K⁺- and Mg²⁺-saturated samples, possibly due to the expansion of a poorly ordered 2:1 layer silicate. Otherwise, there was no detectable shift of the 1.0-nm peak for either the K⁺- or the Mg²⁺-saturated samples, nor did we observe a 1.49-nm reflection. Although the existence of 1:1–2:1 mixed-layer clays cannot be ruled out, the data suggest that mixed layer clays, if present, occur in low concentrations and/or that they do not contain smectitic layers. The 1.4-nm peak observed in the silt-size fraction indicates the presence of a vermiculitic mineral. This peak was not observed in the clay-size fraction. However, a broad diffraction band in the 10 to 7° (2) region following heating to 550°C may indicate the presence of some 2:1 layers in the clay fractions.

View larger version (28K): [in this window] [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

The bulk soil (<2 mm) had a greater K⁺ selectivity than the deferrated clay fraction (<2 µm), thus the trace quantities of 2:1 layer silicates must contribute in part to the high CEC and K⁺ selectivity. Although XRD analysis of random powder mounts did not show 2:1 layer silicates in the fine sand fraction (50–250 µm), it is possible that these constituents may be present in the sand fraction. Delvaux and Herbillon (1995) provided evidence to indicate that smectite preferably forms within the clast substrate, whereas halloysite forms in the microenvironments subject to greater leaching. Similarly, Jongmans et al. (1999) showed the presence of halloysite and smectite in pseudomorphs after pyroxenes in andesitic pyroclasts. They showed spherical halloysite transitioning to smectite when moving from the outer margin to the center of the pyroxene crystal. These weathering patterns would result in halloysitic clays dominating the clay-size fraction, and 2:1 layer silicates the coarser-size fractions.

We also investigated the possibility that trace concentrations of zeolites may contribute to the unusual charge characteristics of our soils. Clinoptilolite, for example, commonly occurs in soils derived from hydrothermally altered volcanic parent material (Boettinger and Graham, 1995). This zeolite has a strong affinity for K⁺, a high CEC (220 cmol_c kg^-1), and a very high specific surface area (Ming and Mumpton, 1989). However, XRD analysis of the clay, silt, and fine-sand fractions from the soils used here showed no evidence for the occurrence of zeolites (detection limits of 3 wt. %). Following Ming and Dixon (1987), we used a CEC method based on ion-sieving properties in an attempt to quantify clinoptilolite concentrations in the deferrated bulk soil samples (<2 mm) of the 2Bt3 horizons. However, the zeolitic CEC value could account for no more than 15% of the soil's CEC (Table 7), corresponding with a clinoptilolite concentration of 1 to 2 wt. %, which is close to the detection limit of this method (Ming and Dixon, 1987). The apparent absence or low abundance of zeolites in these pedons is not surprising because these minerals are sensitive to chemical weathering (Ming and Mumpton, 1989). Thus, we conclude that zeolites do not contribute significantly (<15%) to the high CEC values of these soils.

Figure 7 shows the ²⁷Al-NMR spectrum of the halloysite-rich clay from the Red Tank 2Bt3 horizon. For comparison, we have included the spectra of the most ordered halloysite (Matauri Bay) and one of the least ordered halloysites (Te Akatea) from the study of Newman et al. (1994). The spectrum of the Red Tank 2Bt3 clay is dominated by a signal at a chemical shift () of -9 ppm, assigned to octahedral Al. The peak has an asymmetric line shape with a low-frequency tail, indicating that the halloysite has a very high degree of structural disorder, presumably arising from Al-vacancy displacements in the octahedral sheet (Newman et al., 1994). A weak, broad peak at = 56 ppm is due to tetrahedral Al, which accounts for 2% of the Al in the sample. If all of the four-coordinate Al is assigned to the tetrahedral sheet, 50% of the measured CEC can be accounted for by isomorphous substitution of Al³⁺ for Si⁴⁺. However, some or all of the Al(IV) signal in our sample may originate from small amounts of impurities, such as those previously identified in halloysite samples by Churchman and Theng (1984). The remainder of the charge may originate from Al vacancies in the octahedral sheet.

View larger version (15K): [in this window] [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

The relatively high concentrations of free Fe oxides (27–101 g kg^-1) in the form of surface coatings (giving rise to soil colors of 7.5YR 4/6) may also contribute to some K⁺ selectivity relative to Ca²⁺. Escudey and Galindo (1988) have shown that clays with Fe oxides exhibit a higher preference for K⁺ than their counterparts that had been treated with citrate-bicarbonate-dithionite. With an isoelectric point 8.7, Fe oxides have a net positive surface charge at the equilibrium pH of our soils (pH 6–7; Tables 1 and 5). Charge-to-charge repulsion would make access of divalent cations (i.e., Ca²⁺) to the internal negative surface exchange sites more difficult than that of monovalent cations (K⁺). In our study, the deferrated clay-size fraction showed a much lower K⁺ selectivity compared with the nontreated bulk soil (<2 mm). This may be explained in terms of the removal of Fe oxide coatings, the preferential accumulation of 2:1 layer silicates in the silt and sand fractions, and/or modification of mineral charge properties after reduction with dithionite.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Although numerous analytical methods were applied to characterize these halloysite-rich soils, it has not been possible to offer an unambiguous explanation for the high CEC and K⁺ selectivity of these soils. Though the existence of 1:1–2:1 mixed-layer clays was not evident from our analytical data, genuine 2:1 layer silicates were detected by XRD, especially in the silt- and sand-size fractions. This led us to suggest that the high CEC and K⁺ selectivity displayed by the soils cannot be attributed unequivocally to the charge characteristics of the poorly ordered halloysite that dominates the clay fraction, even though this clay species may represent up to 90 wt. % of the clay fraction. For the 2Bt3 horizons, K⁺ selectivity was appreciably higher in the nontreated, bulk soil (<2 mm) as compared with the deferrated clay-size (<2 µm) fraction. It seems probable, therefore, that Fe oxide surface coatings and/or the presence of appreciable 2:1 layer silicates in the silt and sand fractions may contribute to the K⁺ selectivity in these soils. Because basaltic rocks are low in K, this characteristic plays an important role in enhancing K availability and attenuating K losses by leaching.

Received for publication April 12, 1999.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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