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Surface charge characteristics of variable charge soils in Thailand.

Introduction

Oxisols and Ultisols are variable charge soils (Uehara and Gillman 1981; Qafoku et al. 2004) which are widespread in the tropics and comprise an important economic resource for agriculture in many countries (Beinroth et al. 2000). They are generally acidic and infertile with low values of cation exchange capacity (CEC). The mineralogy of the clay fraction of these soils is characterised by the prominence of kaolin, hematite, goethite, gibbsite, and titanium oxides (Fontes and Weed 1991; Singh and Gilkes 1992b; Melo et al. 2001) with the various proportions of these minerals depending on parent material and weathering intensity (Schaefer et al. 2008). Gibbsite, goethite, and hematite have mostly variable charge on their surfaces (Uehara and Gillman 1981; Bleeker and Sageman 1990; Yu 1997; van Ranst et al. 1998; Becquer et al. 2001; Qafoku et al. 2004; Anda et al. 2008). Highly weathered soils also exhibit both permanent negative and permanent positive surface charges (Tessens and Zauyah 1982) but variable charge commonly dominates in these soils (Anderson and Sposito 1991; Chorover and Sposito 1995).

The surface charge of variable charge constituents depends on the pH and ionic strength of soil solution (Van Raij and Peech 1972; Naidu et al. 1994). Variable surface charge is attributed to the ionisation of functional groups on organic matter, hydrous Fe and A1 oxides, and edge sites on kaolin by protonation and deprotonation processes (Phillips and Sheehan 2005). Surface charge can be positive, negative, or zero depending on conditions of the soil solution. Surface sites of these soil constituents generally have net positive charge under acidic conditions (Qafoku et al. 2004). Moreover, adsorbed organic anions (Xu et al. 2003), phosphate (Naidu et al. 1990), and sulfate on surfaces contribute to negative surface charge (Bolan and Barrow 1984; Fahrenhorst et al. 1999).

Kaolin in highly weathered soils is typically poorly ordered with associated with elevated values of specific surface area (SSA) and CEC (Singh and Gilkes 1992b). The very small crystal size of iron oxides in tropical soils may be due to the high degree of A1 substitution (Fitzpatrick and Schwertmann 1982), and the high SSA of iron oxides causes them to exert a major influence on anion retention capacity (Borggaard 1983). However, the specific contributions of soil minerals to the variable charge of soils have not been determined.

The concept of soil charge characteristics as encapsulated in the charge fingerprint was first described by Gillman and Sumpter (1986) and subsequently refined by Gillman (2007). It determines the base cation exchange capacity (CECB) and anion exchange capacity (AEC) as a function of soil pH. The concept has been applied to soils from tropical savanna of Thailand and humid subtropics of Australia in order to quantify the degree of degradation associated with anthropic disturbance (Noble et al. 2000). The contributions of soil constituents to fingerprints of these soils have not been established.

Highly weathered soils in Thailand are mostly Oxisols and Ultisols and cover ~21% of the country's agricultural area (Moncharoen 1992). Kaolin, goethite, and hematite are the dominant clay constituents (Yoothong et al. 1997) which variously contribute to soil variable charge. The surface charge properties of highly weathered Thai soils are not well known and are the subject of this investigation. The main objectives of this study were to determine the charge characteristics of Thai Oxisols and Ultisols, to determine mineralogical and other factors contributing to the charge characteristics, and to relate charge characteristics to P sorption capacity. This information is an important contribution to developing strategies for liming and P fertilisation of these soils.

Materials and methods

Sampling sites

Ninety soil samples were obtained from 32 profiles of Oxisols and Ultisols from North-east Plateau, South-east Coast, and Peninsular Thailand (Fig. 1). The soil classification, parent rock, and general environment of the sites where the soils were sampled are described in Table 1. Pedon analysis in soil pits was carried out at each site, including detailed profile description and sampling of soil genetic soil horizons (Soil Survey Staff 1993; Kheoruenromne 2004). Samples of the prominent genetic horizons of Oxisols (Ap, Bt, Bto, Bo) and Ultisols (Ap, E, Bt, Bv) were used to investigate their charge characteristics.

Physicochemical analysis

Bulk soil samples were air-dried and crushed to pass through a 2-mm sieve before laboratory analysis. Particle size analysis was determined by pipette analysis (Gee and Bauder 1986). Soil pH was determined in water and in 1M KCl using 1:1 soil : liquid, and in 1 M NaF (pH 8.0) using 1 : 50 soil : liquid (Fieldes and Perrott 1966). Organic carbon (OC) was determined by the Walkley and Black wet oxidation procedure (Nelson and Sommers 1996) and was used for calculation of organic matter content (OM) in the relationship OM = OC x 1.724. CEC was determined by N[H.sub.4]OAc buffered at pH 7.0. SSA of whole soils, soil after removal of organic matter, and of kaolin and iron oxide concentrates was measured by the [N.sub.2]-BET method (Aylmore et al. 1970) with a Micromeritics Gemini Ill 2375 surface analyser. Crystalline, non crystalline, and organic forms of Fe, Al, and Mn were extracted by dithionite-citratebicarbonate solution (DCB) ([Fe.sub.d], [Al.sub.d], [Mn.sub.d]), 0.2M ammonium oxalate solution at pH 3.0 ([Fe.sub.o], [Al.sub.o], [Mn.sub.o]), and sodium pyrophosphate solution ([Fe.sub.p], [Al.sub.p], [Mn.sub.p]), respectively (Mehra and Jackson 1960; McKeague and Day 1966; McKeague 1967). Dissolved Fe, Al, and Mn were measured using atomic absorption spectrophotometry. The total element concentration of soils was determined by X-ray fluorescence spectrometry of fused samples (Norrish and Hutton 1969).

[FIGURE 1 OMITTED]

Mineralogical and structural analysis

X-ray diffraction (XRD) patterns of the clay fraction, and kaolin and iron oxide concentrates were obtained using CuK[alpha] radiation with a Philips PW-3020 diffractometer equipped with a graphite diffracted beam monochromator. Oriented clay was prepared on ceramic plates and XRD patterns from 4-35[degrees] 2[theta] were obtained after various pretreatments to aid identification of accessory clay minerals (Brown and Brindley 1980). Random powder patterns of kaolins were obtained over the range of 4-70[degrees] 2[theta] with a step size of 0.02[degrees] 2[theta] and a scan speed of 0.04[degrees]/s to determine the degree of structural order of the kaolin, which is expressed as the HB index (Hughes and Brown 1979). Accurate measurements of d values of kaolin and iron oxides were made using the quartz present in samples as an internal standard. The semi-quantitative proportions of minerals were estimated by comparison of integrated areas of reflections with XRD patterns of standard minerals (Klug and Alexander 1974). Random powder patterns of iron oxide concentrates were recorded for 4-70[degrees] 2[theta] using a step size of 0.01[degrees] and a scan speed of 0.25[degrees] 2[theta]/min. For calculation of A1 substitution in goethite, the c dimension ([Angstrom]) was derived from d(110) and d(111) (Schulze 1984) and was used in the relationship: molar Al/(Al+Fe)=1462-483c (Schwertmann and Carlson 1994). For hematite, the a dimension was derived from d(110) and was used in the relationship: molar Al/(Al+Fe)=3141-623a (Schwertmann 1988). Mean coherently diffracting length (MCD) for kaolin and iron oxides was calculated from the width of reflections at half maximum using the Scherrer formula, after correction for instrument broadening (Schulze 1984).

Surface charge analysis

Charge fingerprints that are curves describing variations in the base CEC ([CEC.sub.B]) and anion exchange capacity (AEC) across a range of pH values were determined using the methodology described by Gillman (2007). [CEC.sub.B] and AEC are measures of the retention of exchangeable basic cations and anions, respectively, at a particular pH. Four sets of charge fingerprint determinations were performed: (i) charge fingerprints for whole soils without any pretreatment; (ii) charge fingerprints for 5 topsoils (Ap) following pretreatment with 6% [H.sub.2][O.sub.2] to remove organic matter; (iii) charge fingerprints for the 5 topsoils after pretreatment with [H.sub.2][O.sub.2] then DCB to remove free iron oxides (Mehra and Jackson 1960) (these deferrated soil samples consisted almost entirely of kaolin group minerals and will be referred to as kaolin in this paper); (iv) charge fingerprints for the 5 topsoils after pretreatment with [H.sub.2][O.sub.2] followed by 5 M NaOH digestion to dissolve kaolin and concentrate iron oxides (Singh and Gilkes 1991a).

To determine charge fingerprints, 8 portions of 2 g of 2-mm air-dried soil samples were weighed into 50-mL centrifuge tubes and 20 mL 0.1 M Ca[Cl.sub.2] was added to saturate exchange sites with Ca, followed by 3 washes with 0.002 M Ca[Cl.sub.2], which is considered to approximate the ionic strength of soil solution for highly weathered soils (Gillman and Bell 1978). Subsequently, the pH of the suspensions was adjusted to values in the range 3.5-7.0 with either 0.1 M HCl or saturated Ca[(OH).sub.2] and the pH was recorded after equilibration. The supernatant solution was retained for Ca and Cl analysis, and the tubes were re-weighed to estimate the volume of entrained solution; 20mL of 1 M N[H.sub.4]N[O.sub.3] was added to displace exchangeable Ca and Cl from the exchange sites. Amounts of adsorbed Ca and Cl were computed by subtracting the amounts present in the entrained solution.

The relationships of ion adsorption capacity ([CEC.sub.B], AEC) with pH were fitted to a linear equation as follows:

[CEC.sub.B] = [A.sub.c] pH + [B.sub.c] (1)

AEG = [A.sub.a]pH + [B.sub.a] (2)

where pH is the pH of the soil suspension and [A.sub.c], [B.sub.c], [A.sub.a] and [B.sub.a], are constants for each soil. [A.sub.c] and [A.sub.a] coefficients are the rate of change in surface charge with pH. [B.sub.c] and [B.sub.a] are constants equivalent to the magnitudes of negative and positive charge at pH zero.

Phosphate sorption analysis

Phosphate sorption was measured following methodology described by Singh and Gilkes (1991b). P sorption data were fitted to the linear form of the Langmuir equation as follows:

c/x = 1/b[X.sub.m] + C/[X.sub.m]

where c is the concentration of P in equilibrium solution ([micro]g/mL), x is the amount of P sorbed ([micro]g P/g soil), [X.sub.m] is the Langrnuir sorption maximum ([micro]g P/g soil), and b is a constant related to bonding energy (mL/[micro]g P) (Singh and Gilkes 1991b). The plot of c/x against c gave a straight line with a slope and intercept equal to 1/[X.sub.m] and 1/b [X.sub.m], respectively.

The data were also fitted to the Freundlich equation as follows:

x = [kc.sup.B]

where x is the amount of P sorbed ([micro]g P/g soil), c is the equilibrium P concentration ([micro]g P/mL), and k and B are empirical coefficients, where k indicates the maximum sorption capacity ([micro]g P/g soil) and B is related to bonding energy.

Results and discussion

Soil characteristics

All soils are highly weathered Oxisols and Ultisols. The Oxisols have developed on colluvium and residuum derived from limestone and basalt, whereas most of the Ultisols have developed on colluvium, residuum, and alluvium derived from granite, sedimentary, and metasedimentary rocks (Table 1). The surrounding landforms are mostly undulating, having 1-8% slope. Horizonation of Oxisols is difficult to identify, since iron oxides are uniformly dispersed throughout the profile, resulting in a uniform colour. However, either kandic or oxic horizons are present. An argillic horizon occurs in Ultisols, since extensive illuviation has taken place, and a kandic horizon also occurs, since the CEC of that horizon is <16 [cmol.sub.c]/kg clay. Therefore, the genetic horizons of the Oxisols are Ap, Bt, Bto, and Bo, whereas genetic horizons of the Ultisols are Ap, E, Bt, Bv. These are very deep soils that are commonly acidic, welldrained, with moderate to rapid permeability and moderate to slow runoff. The colour of the Oxisols is dusky red to dark yellowish brown, whereas the Ultisols are generally red and occasionally very pale brown. The texture is clay, sandy clay to sandy loam throughout the profile. Structure is subangular blocky to semi-angular blocky for Ultisols and subangular blocky to strong granular for Oxisols. Variable amounts and sizes of clay balls occur in all Oxisols. There are a few faint/thin clay coatings generally present in all soils except for Ak2 and Ak3, where there are distinct clay coatings. Quartz fragments are generally present in Ultisols (Fd, Yt1, Yt2, Hp, Sh, Tim1, Tim2, Pk, Kh), in particular for the soils derived from granite. Plinthite occurs in the deeper part of Ultisols profiles (Kc, Tim2, Pga, Pk, Cp).

The pH(1:1 [H.sub.2]O) of Oxisols and Ultisols ranges from extremely acid to slightly alkaline (4.0-7.5) (Table 2). All profiles are acidic except for topsoils of Pc2, Pc3, and Ak2, which are neutral possibly due to liming practiced in these areas. The pH values measured in KCl are consistently lower than those measured in water, indicating that negative charge prevails. Values of pH in NaY solution range from 8.2 to 10.0, with higher values indicating that the soils contain mineral constituents with abundant surface hydroxyl ions that are exchanged by the fluoride ion (Perrott et al. 1976). Organic matter content (0.11-79g/kg), CEC (0.90-34 [cmol.sub.c]/kg), and SSA (0.066-83 [m.sup.2]/g) of the soils vary considerably, with values for Oxisols being higher than for Ultisols. Extractable Fe and Al concentrations for Oxisols are higher than for Ultisols for all 3 extractants. Crystalline iron oxides as estimated by DCB extraction ([Fe.sub.d],) are the dominant form of iron oxide in these soils, with amounts differing greatly between the soils (0.34-137g/kg). Oxisols derived from basalt under more humid conditions (Ti, Nb) have elevated values of [Fe.sub.o]/[Fe.sub.d], indicating that there are substantial amounts of noncrystalline iron oxides in these soils (Fontes et al. 1992).

Clay mineralogy

Kaolin is the dominant mineral of the clay fraction, with moderate amounts of goethite and hematite in the Oxisols and minor amounts in the Ultisols (Table 3). Various amounts of accessory minerals including hematite, goethite, quartz, hydroxyl-Al interlayer vermiculite, smectite, illite, gibbsite, boehmite, maghemite, anatase, and feldspar are present in the soils. Goethite dominates over hematite in the soils under a udic soil moisture regime, except for Ak2 and Ak3 profiles where hematite is more abundant. Boehmite is only present in a soil derived from limestone (Ak1) formed under a high rainfall condition and may be indicative of an advanced stage of weathering. Gibbsite is present in Oxisols derived from limestone (Ak1) and basalt (Ti1, Ti2, Ti3, Nb1, Nb2) under more humid conditions. Maghemite occurs only in Oxisols formed on basalt in a more humid regime. Kaolin crystal size ([MCD.sub.001]) determined by XRD line broadening is 9-32 nm for Oxisols, and 5-41 nm for Ultisols. Kaolins in Ti and Nb have the smallest crystal size (9-14 nm), and kaolin in Ak has the largest crystal size (17-32nm). For kaolin in Ultisols, Re has the smallest crystals (5.3-6 nm), and Kbi has the largest crystals (31-41 nm). The [MCD.sub.110] of both hematite and goethite in Oxisols is smaller than in Ultisols, with the lowest values for Oxisols formed on basalt. The trend of decreasing [MCD.sub.110] for hematite is in the sequence of soils formed on basalt< limestone < local alluvium < sedimentary rocks, whereas for goethite it is in the sequence of soils formed on basalt< limestone < sedimentary rocks < local alluvium.

Soils surface charge characteristics

The surface charge characteristics of Thai Oxisols and Ultisols were determined using the concept of charge fingerprinting as described by Gillman and Sumpter (1986), which provides both the positive and negative charge characteristics of a soil over a range of pH values. It is useful for assessing the current nutrient-holding capacity and can predict changes in CEC and AEC resulting from both the impact of management and continuing weathering (Noble et al. 2000). Some representative charge fingerprints for Thai Oxisols and Ultisols are given in Fig. 2. All soils exhibit variable charge behaviour but the magnitudes of charge and the rate of change in surface charge with pH vary greatly between the soils. Magnitudes of both negative and positive charges for Oxisols are greater than for Ultisols, which is presumably due to the more clayey texture and greater contents of sesquioxides in Oxisols. An increase in pH causes a decrease in the electric potential of oxide surfaces and an increase in the dissociation of bivalent cations, being favourable to cation sorption on oxide surfaces and resulting in a rapid increase in cation sorption with increasing pH (Barrow 1987; Naidu et al. 1997).

[FIGURE 2 OMITTED]

Values of the magnitude and rate of change in negative charge with pH for topsoils are mostly greater than for subsoils, due to the contribution of OM as the mineral constituents of associated topsoils and subsoils are identical (Becquer et al. 2001). The dissociation of carboxyl groups in OM is strongly pH-dependent, increasing progressively with pH (Chorover et al. 2004). The positive charge of topsoils is commonly lower than for subsoils because organic molecules are sorbed onto the surface of iron and aluminum oxides, decreasing the abundance of sites available for anion sorption (Gu et al. 1995; van Ranst et al. 1998) or because the relatively more abundant negatively charged sites repel anions from the double layer (Gillman and Sumpter 1986).

The values of positive charge of soils formed under more humid conditions (Ti, Nb) are typically higher than for the soils occurring under drier conditions (Pc, Ci). This difference is probably due to the Nb and Ti profiles containing more sesquioxides, mostly goethite with a trace of gibbsite, with hematite dominating in Pc and Ci profiles. The SSA of soil goethite may be larger than for hematite (Parfitt 1989)but this is not always the case (Singh and Gilkes 1992a), and the median size of hematite is larger than goethite for the present samples. Curl and Franzmeier (1984) reported that goethitic soils adsorbed more P than hematitic soils in Brazilian Oxisols, which they considered to be a consequence of the larger crystal size of goethite relative to hematite. Fontes and Weed (1996) also reported that the goethite content of iron oxide concentrates of clay samples explained most of the variation in P sorption.

The relatively high amounts of dithionite-extractable Al in Ti and Nb profiles may reflect the small crystal size of iron oxides in these soils as the crystal size of iron oxides commonly decreases with increasing Al substitution (Fitzpatrick and Schwertmann 1982). Appreciable amounts of noncrystalline iron and aluminum oxides as indicated by oxalate-extractable Fe and Al are present in Nb and Ti profiles and may contribute to positive surface charge in these soils (Parfitt 1981; Sposito 1989). This is similar to the report for Portuguese Andisols indicating that [Fe.sub.o] and [Al.sub.o] constituents are predictive of the positive surface charge (Auxtero et al. 2004). The Ak1 profile also has high positive charge, which is due to the aluminum hydroxides (gibbsite, boehmite) present in its clay fraction contributing to anion adsorption (van Ranst et al. 1998). Gibbsite has been identified as a major cause of phosphate sorption in some tropical soils (Fontes and Weed 1996). Working with a pure mineral system, Cabrera et al. (1977) reported that aluminum oxides (gibbsite, boehmite, corundum) provided more reactive surfaces for P sorption than do iron oxides (goethite, hematite, lepidocrocite).

The relationship between clay content and negative surface charge is clear for the Ultisols. Extensive illuviation has taken place in these soils, creating an argillic horizon so that the negative charge of subsoils is systematically higher than for topsoils (Fig. 2e, [florin]). However, the rate of change in variable negative charge with pH for topsoils of Ultisols is higher than for subsoils, reflecting the contribution of OM (Noble et al. 2000).

Surface charge characterisrcs in relation to soil properties

Relationships of [CEC.sub.B] and AEC with pH are well described by the linear equations. Values of equation constants, which will be referred to as charge coefficients ([A.sub.c], [B.sub.c], [A.sub.a], [B.sub.a]), and the coefficient of determination [R.sup.2] for Thai Oxisols and Ultisols are provided in Tables 4 and 5. The values of charge coefficients [A.sub.c] (0.09 to 0.36), [B.sub.c] (-12.7 to 5.33), [A.sub.a] (-0.85 to -0.04), and [B.sub.a] (8.17 to 1.76) vary markedly. For topsoils the [A.sub.c] coefficient, which is the rate of change in negative charge with soil pH, is higher than for subsoils, indicating that OM exerts a substantial influence on variable charge (Becquer et al. 2001). Values of the [A.sub.c] coefficient for Oxisols are higher than for Ultisols, presumably due to the greater clay content of Oxisols providing a greater surface area.

The [B.sub.c] coefficient, which is the extrapolated value of surface charge at pH zero, varies greatly between soils. The magnitude of this coefficient for Oxisols is large compared to Ultisols, and [B.sub.c] has negative and positive values reflecting the various contributions of permanent and variable negative charge.

The charge coefficient [A.sub.a] has negative values indicating that AEC increases with decreasing pH. The magnitude of [A.sub.a] values for Oxisols is smaller than for Ultisols, and values for subsoils are smaller than for topsoils. The magnitude of positive charge at pH zero (i.e. [B.sub.a]) for Oxisols and subsoils is larger than for Ultisols and topsoils, respectively. This is consistent with the greater abundance of sesquioxides in Oxisols and in subsoils than in Ultisols and topsoils, respectively.

All charge coefficients ([A.sub.c], [B.sub.c], [A.sub.a], [B.sub.a]) for subsoils and [CEC.sub.B] charge coefficients ([A.sub.c], [B.sub.c]) for topsoils have highly significant (P < 0.005) relationships with SSA, CEC, pH(NaF), and various forms of extractable Fe, Al, and Mn (Table 6). Some bivariate plots for [A.sub.c] which illustrate these relationships are shown in Fig. 3. These relationships indicate that these soil properties, which are themselves closely related, are major contributors to the variable charge of these soils. The relationship of the values of [A.sub.c] and [A.sub.a] with pH(NaF) is a consequence of pH(NaF) being a measure of the abundance of exposed OH on the surface of soil constituents (Perrott et al. 1976); this OH is the primary site for protonation and deprotonation processes (Gillman and Sumpter 1986). The significant statistical relationships for [A.sub.c] and [B.sub.c] v. OM in topsoils (r = 0.81, -0.74), with the absence of these relationships in subsoils (r = 0.34, -0.21), indicate that OM is the major contributor to negative variable charge. There are no statistically significant relationships between charge coefficients for AEC ([A.sub.a], [B.sub.a]) and OM. This is attributed to the complexes between OM and sesquioxides decreasing the abundance of sites available for anion sorption (Gu et al. 1995) and also indicates that OM is not a consistent factor affecting anion sorption (Sibanda and Young 1986; Gu et al. 1995).

Although there are highly significant statistical relationships between charge coefficients ([A.sub.c], [B.sub.c], [A.sub.a], [B.sub.a]) and soil properties (r = 0.005, Table 6), some of these relationships may be spurious because of the distribution of data. For example, plots of charge coefficients against the values of oxalate-extractable Fe are either bimodal or tadpole shapes so that the regression relationships are not statistically valid. Chorover et al. (2004) suggested that it is not possible nor is it appropriate to identify a single soil constituent as the principal driver for the pH enhancement of variable charge due to covariance and the complex composition of soil constituents. Bivariate plots of interrelationships of charge coefficients ([A.sub.c], [B.sub.c], [A.sub.a], [B.sub.a]) are given in Fig. 4. There is a systematic inverse relationship for all samples between [A.sub.a] and [B.sub.a] but there is no single inverse relationship between [A.sub.c] and [B.sub.c] for all soils. There are no strongly systematic relationships between [A.sub.c] and [A.sub.a] or between [B.sub.c] and [B.sub.a]. The absence of a relationship is attributed to the very small range of values of these coefficients for Ultisols having relatively high contents of sand, which does not contribute to surface charge. The diversity and complex mineralogy of the clay fraction for Oxisols results in a wide range in magnitude and rate of change in variable charge.

Such a large dataset with much variation of soil properties cannot be coherently interpreted by considering simple bivariate relationships. Principal component and classification analysis using the Statistica Program (Version 6.1) were used to determine similarities of surface charge behaviour between soil samples and to group soil samples on the basis of their charge and other characteristics. Factor analysis of standardised raw data was used with charge coefficients being included with soil properties in the dataset. The first 2 factors explain 71% of the variation in the data, which is an acceptable description considering the diverse nature of these soils (Fig. 5). Two affinity groups of properties are recognised. The first group consists of [B.sub.c] and sand which simply relates to Ultisols, which contain relatively low amounts of Fe and Al oxides and more sand, having relatively few negatively charged surface sites. The second group consists of [A.sub.c], -[A.sub.a], [B.sub.a], pH(NaF), Clay, CEC, SSA, OM, [Ti.sub.t], [Fe.sub.t], [Al.sub.t], [Fe.sub.d], [Al.sub.d], [Mn.sub.a], [Fe.sub.o], [Al.sub.o], [Mn.sub.o], [Fe.sub.p] [Al.sub.p], and [Mn.sub.p]. This group of properties relates to the clay, OM, and sesquioxide contents, which are the major sources of variable charge and are more abundant in Oxisols.

When grouping the soil samples in the 2-factor space it is evident that Ultisols, Oxisols formed on limestone under both ustic and udic soil moisture regimes (Pc1, Pc2, Pc3, Ak2, Ak3), and Oxisols formed on basalt under an ustic soil moisture regime (Ci1, Ci2, Ci3) have similar properties. These include surface charge behaviour as is indicated by the tight grouping of these samples in the factor plot. Oxisols developed on basalt (Ti1, Ti2, Ti3) and limestone (Ak1) under more humid conditions are different from the above groups. They show more diversity in surface charge properties and other properties including OM, and dithionite-, oxalate-, and pyrophosphate-extractable Fe, Al, and Mn.

[FIGURE 3 OMITTED]

Factors affecting the soils surface charge characteristics

Five representative topsoils (Ap) of Oxisols were treated with [H.sub.2][O.sub.2], DCB, and 5 M NaOH treatments to remove OM (Gorsuch 1970), sesquioxides (i.e. kaolin concentrate) (Mehra and Jackson 1960), and kaolin (i.e. iron oxide concentrate) (Singh and Gilkes 1991a), respectively. The charge fingerprints of these modified soils were determined to investigate factors contributing to surface charge characteristics.

XRD patterns (Fig. 6) show that the concentrated kaolin samples consist predominantly of kaolin with minor amounts of quartz, maghemite, boehmite, and gibbsite. Maghemite is present in the soils formed on basalt under a high rainfall (Ti3, Nb2) and is not removed by DCB treatment but most was removed by a subsequent magnetic separation (Taylor and Schwertmann 1974). Boehmite and gibbsite occur in a soil formed on limestone under more humid conditions (Ak1). Mean coherently diffracting length for the basal reflection ([MCD.ssub.001] of kaolin indicates that the average size of kaolin crystals ranges from 10 to 31 nm. Kaolin in Pc1 has the smallest crystal size (10nm), and kaolin in Ak1 has the largest crystal size (31 nm); the corresponding values of SSA for these kaolins are 60 and 27 [m.sup.2]/g, which is consistent with the crystal size values.

[FIGURE 4 OMITTED]

The major constituents of iron oxide concentrates of these 5 soils are goethite and hematite (Fig. 7) with minor amounts of quartz, maghemite, and anatase. A moderate amount of quartz is present in iron oxide concentrates of Pc1 and Ci3. Goethite is relatively more abundant in iron oxide concentrates of soils formed under more humid conditions (Ak1, Nb2, Ti3) and is not present in the 2 soils formed under drier conditions (Pc1, Ci3). Considerable amounts of maghemite are present in Nb2 and Ti3. The SSA of iron oxide concentrates ranges from 42 to 94 [m.sup.2]/g (Table 7). The highest SSA for Ti3 and the lowest for Ci3 indicate that goethite has a greater SSA than hematite (Parfitt 1989). Nevertheless, this interpretation should be treated with caution as all iron oxide concentrates are mixture of iron oxides and other minerals. The [MCD.sub.110] of goethite determined by XRD line broadening ranges from 7 to 17 nm, with the smallest values for Ti3 and the largest for Ak1. For hematite, [MCD.sub.110] is larger than [MCD.sub.012] for most samples, indicating a greater crystal size in the a-axis direction than the c-axis dimension, which is consistent with platy nature of soil hematite (Fontes and Weed 1991). MCD of hematite is slightly larger than MCD of associated goethite (Fontes and Weed 1991), which is consistent with the smaller SSA of hematite previously discussed. A1 substitution in goethite ranges from 14 to 19 mol% and in hematite from 5 to 11 mol%, being about half the level in associated goethite as has been reported by others (Singh and Gilkes 1992a).

Charge fingerprints of the whole soils, after removal of OM, and of concentrated kaolin and iron oxides for the 5 representative Thai soils are shown in Fig. 8. Values of charge coefficients ([A.sub.c], [B.sub.e], [A.sub.a], [B.sub.a]) and the coefficient of determination [R.sup.2] for these samples are given in Table 8. Removal of OM reduced negative charge so that the slope of negative charge v. pH line (i.e. [A.sub.c]) decreased after removal of OM. This result indicates that for the 5 topsoils, OM contributes substantially to pH-dependent CEC. The positive charge of the soil increased due to removal of OM, possibly due to adsorbed OM complexes being removed from the surface of Fe and Al oxides and kaolin exposing more variable positive charge sites (van Ranst et al. 1998; Anda et al. 2008).

Charge fingerprints of kaolin concentrates from these soils indicate that kaolins exhibit both permanent and variable negative and positive charge (Fig. 8). Permanent charge might be due to [Al.sup.3+] substitution for [Si.sup.4+] in the tetrahedral sheet or [Fe.sup.2+] or [Mg.sup.2+] substituting in the octahedral sheet of kaolin (Qafoku et al. 2004). Some authors considered that [Ti.sup.4+] substitutes for [Al.sup.3+] in the octahedral sheet of kaolin, causing locations of permanent positive charge (Tessens and Zauyah 1982). Kaolin shows variable charge behaviour because its crystal surface contains hydroxyl groups at both basal and edge sites, which are the prime sites for variable charge surface (Uehara and Gillman 1981). The rate of change of variable negative charge with pH ([A.sub.c]) for kaolins is significantly linearly positively related to SSA ([R.sup.2]=0.98, Fig. 9) and is significantly linearly negatively related to mean crystal size ([MCD.sub.001]) ([R.sup.2]= 0.91, Fig. 10). However, there is no clear relationship of [A.sub.c] and the magnitude of negative charge at pH zero ([B.sub.c]) with the Hughes and Brown (1979) crystallinity index, which is a measure of crystallinity or degree of structural order in soil kaolins. The absence of a distinct relationship is attributed to the very small range (i.e. 6-10) of values of this index for these soils ([R.sup.2]-0.57, Fig. 10).

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

The iron oxide concentrates have strongly pH-dependent charge with high AEC and relatively low CEC values (Fig. 8). The SSA of iron concentrates exerts a strong influence on the magnitude and rate of change in variable negative charge with pH as indicated by the close relationships between SSA of iron concentrates and both [A.sub.c] and [B.sub.c] (Fig. 9). This result is in agreement with the report indicating that the magnitude of the surface charge is associated with the SSA (Naidu et al. 1997). The significant relationships between crystal size of hematite ([MCD.sub.110]) and both [A.sub.a] (-[R.sup.2]=0.83) and [B.sub.a] ([R.sup.2]=0.89) (Fig. 11) indicate that crystal size of hematite is the major contributor to both the magnitude and rate of change in variable positive charge with pH.

Surface charge characteristics in relation to phosphate sorption

Charge coefficients ([A.sub.c], [B.sub.a], [A.sub.a], [A.sub.a]) have highly significant (P<0.005) relationships with phosphate sorption maximum coefficients ([X.sub.m], k) provided by the Langmuir and Freundlich relationships (Table 9). However, the distribution of data points in these graphs is highly skewed (tadpole-like) (Fig. 12). This relationship is attributed to phosphate initially being sorbed weakly by electrostatic forces and this interaction being followed by strong covalent bonding onto variable charge surfaces (Lindsay 1979), whereas chloride is sorbed by electrostatic forces (Essington 2004). The divergence of some points from the regression lines for relationships of charge coefficients ([A.sub.c], [B.sub.c], [A.sub.a], [B.sub.a]), v. [X.sub.m] and k are systematic, in that the same soils deviate in a consistent manner. This deviation may indicate that for these soils some factor(s) other than the charge characteristic variables exert a strong influence on [X.sub.m] and k. The statistical relationships indicate that the surfaces of crystalline, amorphous, and poorly ordered Fe and Al oxides are the prime sites for P sorption and these are the same sites that exhibit variable charge. There are no systematic relationships between charge coefficients ([A.sub.c], [A.sub.c], [A.sub.a], [B.sub.a]) and the Langmuir b and Freundlich B coefficients, which are indicators of P bonding energy. The absence of relationships may be due to the large diversity of adsorption sites including several sesquioxides, kaolin, and OM in these soils which will exhibit diverse bonding energies (Siradz 2000; Wisawapipat et al. 2009).

Chemical depreciation of Thai Oxisols and Ultisols

Soil chemical depreciation for Thai Oxisols and Ultisols has been assessed by the depreciation index as described by Gillman (2007). Soil chemical depreciation is an expression of acidification reducing exchangeable basic cations. In most variable charge acidic soils, a soil pH of 5.5 appears to be near-optimal with respect to cation exchange chemistry, since exchangeable [Al.sup.3+] is eliminated from the exchange complex (Menzies and Gillman 1997). Thus, the degree of reduction in basic cation content can be assessed at pH 5.5. In order to assess the degree of degradation that an individual soil has undergone, Menzies and Gillman (1997) formulated a 'depreciation index' (DI) expressed as:

DI = 100 x ([CEC.sub.5.5] - [SIGMA]basic cations)/[CEC.sub.5.5]

where [CEC.sub.5.5] is the cation exchange capacity estimated from the charge fingerprint at pH 5.5 and [SIGMA]basic cations is the sum of exchangeable [Ca.sup.2+], [Mg.sup.2+], [K.sup.+], and [Na.sup.+] at the actual soil pH. The soils have a wide range of DI values (0-91%), with DI values generally increasing with depth, reflecting differences in parent materials, land use, and agricultural practices (Fig. 13). The high leaching condition of these soils developed under tropical climates also contributes to the low DI values.

Subsoils of Oxisols formed on basalt have the highest values of mean DI, and the trend of increasing values for the subsoils is in the sequence of soils formed from basalt > shale/ limestone ~ granite > old alluvium, clastic sediments, and metasediments reflecting the base cation status and acidity of these parent materials. Most subsoils have higher DI values than do topsoils, reflecting the valuable contribution of OM to topsoils and the development of acidity in subsoils (Gillman 2007). The low values of DI for most topsoils indicate that they are close to an ideal condition and that any minor soil chemical depreciation can be easily ameliorated by conventional agricultural practices such as surface organic matter management and additions of fertiliser and lime.

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

[FIGURE 13 OMITTED]

Conclusions

Highly weathered Thai Oxisols and Ultisols exhibit a wide range of variable charge behaviour. Kaolin, goethite, and hematite are the dominant clay constituents and variously contribute to variable charge in these soils. Oxisols have higher amounts of variable charge than do Ultisols due to difference in clay and sesquioxides contents. Oxisols formed on basalt and limestone under more humid conditions have higher values of AEC due to the contributions of goethite, gibbsite, and boehmite. Differences in SSA, CEC, pH(NaF), and various forms of extractable Fe, Al, and Mn are responsible for differences in rates of change with pH of variable charge. Organic matter exerts a consistent effect on both the magnitude of negative charge and the rate of change with pH. Kaolin contributes to both permanent and variable charge, with SSA and crystal size being major factors responsible for rate of change in kaolin variable negative charge with pH. Iron oxide concentrates exhibit strong pH-dependent positive charge, and the mean coherently diffracting length ([MCD.sub.110]) of hematite is closely related to both the magnitude and rate of change in variable positive charge with pH. The surfaces of amorphous, poorly ordered, and crystalline Fe and Al oxides are prime sites for P sorption and are the same sites that exhibit variable charge. Conventional agricultural practices such as surface organic matter management, fertiliser addition, and liming application can be used to decrease the chemical depreciation and enhance the chemical fertility of these soils.

doi: 10.1071/SR09151

Acknowledgments

The authors are grateful to The Royal Golden Jubilee Ph.D. Program under the Thailand Research Fund for financial support and to the laboratory staff at School of Earth and Environment, UWA, particularly Mr Michael Smirk for his kind assistance with chemical analysis.

Manuscript received 24 August 2009, accepted 8 January 2010

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W. Wisawapipat (A), I. Kheoruenromne (A,C), A. Suddhiprakarn (A), and R. J. Gilkes (B)

(A) Department of Soil Science, Faculty of Agriculture, Kasetsart University, Bangkok 10900, Thailand.

(B) School of Earth and Environment, Faculty of Natural and Agricultural Sciences, University of Western Australia, Crawley, WA 6009, Australia.

(C) Corresponding author. Email: irbs@ku.ac.th
Table 1. Soil series, classification, and environmental settings of
Oxisols and Ultisols investigated in this study

Soil series            Classification           Parent rock

Pathio (Ptu)           Kandiudalfic Eutrudox    Shale/limestone
Ao Luck1 (Ak1)         Typic Kandiudox          Limestone

Ao Luek2 (Ak2)         Typic Kandiudox          Limestone
Ao Luek3 (Ak3)         Rhodic Kandiudox         Limestone
Pak Chong1 (Pc1)       Typic Kandiustox         Limestone
Pak Chong2 (Pc2)       Rhodic Kandiustox        Limestone
Pak Chong3 (Pc3)       Rhodic Kandiustox        Limestone

Tha Mai1 (Ti1)         Rhodic Kandiudox         Basalt

Tha Mai2 (Ti2)         Typic Kandiudox          Basalt
Tha Mai3 (Ti3)         Rhodic Kandiudox         Basalt
Nong Bon1 (Nb1)        Typic Kandiudox          Basalt

Nong Bon2 (Nb2)        Typic Kandiudox          Basalt
Chok Chai1 (Ci1)       Rhodic Kandiustox        Basalt
Chok Chai2 (Ci2)       Typic Kandiustox         Basalt
Click Chai3 (Ci3)      Rhodic Kandiustox        Basalt
Huai Pong (Hp)         Typic Kandiudult         Granite
Sattahip (Sh)          Typic Kandiudult         Granite
Thai Mueang1 (Tim1)    Typic Kandiudult         Granite
Thai Mueang2 (Tim2)    Typic Kandiudult         Granite
Phang-nga (Pga)        Typic Kandiudult         Granite
Phuket (Pk)            Typic Plinthudult        Granite
Fangdaeng (Fd)         Typic Kandiudult         Sedimentary rocks
Krabi (Kbi)            Typic Kandiudult         Sedimentary rocks
Sadao (Sd)             Typic Kandiudult         Sedimentary rocks
Kohong (Kh)            Typic Peleudult          Sedimentary rocks
Roi Et (Re)            Typic Plinthudult        Sedimentary rocks
Satuek1 (Suk1)         Typic Peleustult         Sedimentary rocks
Satuek2 (Suk2)         Typic Kandiudult         Sedimentary rocks
Chumphon (Cp)          Typic Plinthudult        Metasedimentary rocks
Kbhong Chak (Kc)       Typic Plinthudult        Metasedimentary rocks
Yasothon1 (Yt1)        Typic Paleustult         Old local alluvium
Yasothon2 (Yt2)        Typic Kandiustult        Old local alluvium

Soil series            Physiographic position

Pathio (Ptu)           Footslope in karst corrosion plain
Ao Luck1 (Ak1)         Crestal slope of residual hill in karst
                         corrosion plain
Ao Luek2 (Ak2)         Karst corrosion plain
Ao Luek3 (Ak3)         Rise crestal slope in karst corrosion
Pak Chong1 (Pc1)       Karst corrosion flat footslope
Pak Chong2 (Pc2)       Karst corrosion plain
Pak Chong3 (Pc3)       Karst corrosion plain on perimeter of buried
                         lapies
Tha Mai1 (Ti1)         Upper dissected footslope of lava corrosion
                         hill
Tha Mai2 (Ti2)         Top of dissected lava corrosion plain
Tha Mai3 (Ti3)         Upper footslope of lava corrosion hill
Nong Bon1 (Nb1)        Shoulder slope on undulating lava corrosion
                         plain
Nong Bon2 (Nb2)        Shoulder slope on rolling lava corrosion plain
Chok Chai1 (Ci1)       Lava corrosion plain
Chok Chai2 (Ci2)       Lava corrosion plain
Click Chai3 (Ci3)      Top of lava corrosion plain
Huai Pong (Hp)         Lower midslope of residual hill
Sattahip (Sh)          Dissected lower footslope
Thai Mueang1 (Tim1)    Shoulder slope of residual hill
Thai Mueang2 (Tim2)    Upper dissected footslope
Phang-nga (Pga)        Lower coalescing
Phuket (Pk)            Dissected lower footslope
Fangdaeng (Fd)         Crestal slope of low hill
Krabi (Kbi)            Shoulder slope of low hill
Sadao (Sd)             High local alluvial terrace
Kohong (Kh)            Shoulder spur hill slope
Roi Et (Re)            Footslope on lower part of high ten-ace
Satuek1 (Suk1)         Low part of high erosional terrace
Satuek2 (Suk2)         Top of upper middle erosional terrace
Chumphon (Cp)          Erosional terrace
Kbhong Chak (Kc)       Dissected lower residual footslope
Yasothon1 (Yt1)        High terrace
Yasothon2 (Yt2)        High ten-ac

^ Taxonomic names (Soil Survey Staff 2006).

Table 2. Range and median values for some properties of Thai Oxisol
and Ultisol samples used for the determination of charge fingerprints
n.d., Not detected

                                       Oxisols
Properties                    Range            Median

pH([H.sub.2]O)               4.3-7.5             4.9
pH(KCl)                      3.6-0.9             4.1
pH(NaF)                      8.5-10              9.0
OM (g/kg)                    0.11-79             7.2
CEC ([cmol.sub.c]/kg)        3.2-34              10
Sand (g/kg)                  16-545              76
Silt (g/kg)                  16-453              107
Clay (g/kg)                  260-968             792
SSA ([m.sup.2]/g)             15-83              45
[Fe.sub.t] (g/kg)            53-293              150
[Fe.sub.d] (9/kg)            39-137              79
[Fe.sub.o] (g/kg)            1.3-21              2.5
[Fe.sub.p] (g/kg)           0.036-11            0.22
[Al.sub.t] (g/kg)            100-492             296
[Al.sub.d] (9/kg)            2.8-27              6.8
[Al.sub.o] (g/kg)            2.6-18              6.4
[Al.sub.p] (g/kg)            0.68-19             2.7
[Mn.sub.d] (9/kg)           0.29-5.1            0.97
[Mn.sub.o] (g/kg)          0.0077-4.5           0.56
[Mn.sub.p] (g/kg)          0.0044-0.51          0.041
[Fe.sub.o]/[Fe.sub.d]      0.016-0.26           0.040

                                       Ultisols
Properties                    Range            Median

pH([H.sub.2]O)               4.0-6.4             4.8
pH(KCl)                      3.2-5.3             3.8
pH(NaF)                      8.2-9.0             8.5
OM (g/kg)                    0.11-21             4.3
CEC ([cmol.sub.c]/kg)       0.90-0.5             2.7
Sand (g/kg)                  263-838             695
Silt (g/kg)                  21-536              104
Clay (g/kg)                  76-631              160
SSA ([m.sup.2]/g)           0.066-33             6.0
[Fe.sub.t] (g/kg)            2.2-55              12
[Fe.sub.d] (9/kg)            0.34-26             5.9
[Fe.sub.o] (g/kg)           0.24-0.2             1.2
[Fe.sub.p] (g/kg)           0.0034-11           0.25
[Al.sub.t] (g/kg)            11-288              55
[Al.sub.d] (9/kg)           0.60-9.9             2.8
[Al.sub.o] (g/kg)           0.45-7.7             2.0
[Al.sub.p] (g/kg)            0.40-28             1.5
[Mn.sub.d] (9/kg)           n.d.-1.00           0.14
[Mn.sub.o] (g/kg)          0.0013-0.60          0.077
[Mn.sub.p] (g/kg)          0.009-1.00           0.019
[Fe.sub.o]/[Fe.sub.d]       0.037-3.3           0.23

Table 3. Mineralogy of the clay fraction of Thai Oxisols and Ultisols
Kao, Kaolin; goe, goethite; hem, hematite; qtz, quartz; HIV, hydroxyl
A1 interlayered vermiculite; sme, smectite; ill, illite; ant, anatase;
gib, gibbsite; rob, maghemite; fel, feldspar; boe, boehmite

Soil        Abundant     Moderate    Little
series       (>60%)      (20-60%)    (5-20%)

Oxisols

Ptu            kao                   hem, qtz, HIV
Ak1                      kao, gib    goe, hem, boe
Ak2            kao                   qtz
Ak3            kao                   hem
PC1            kao                   hem
Pc2            kao                   hem
Pc3            kao                   hem, qtz, HIV
Ti1            kao                   goe, mh
Ti2            kao                   goe
Ti3            kao                   goe
Nb1            kao                   goe, gib, qtz
Nb2            kao                   goe, gib, HIV
Ci1            kao                   hem
Ci2            kao                   hem
Ci3            kao                   qtz

Ultisols

Hp             kao                   qtz
Sh             kao         qtz       ill, fel
Tim1           kao                   qtz, ill
Tim2           kao
Pga            kao
Pk             kao
Fd             kao
Kbi            kao                   qtz
Sd             kao                   HIV
Kh             kao                   qtz
Re             kao         HIV       qtz
Suk1           kao                   HIV, qtz
Suk2           kao                   qtz
Cp                      kao, ill,    qtz
Kc             kao
Yt1            kao                   qtz, sme
Yt2            kao         qtz       sme

Soil        Trace
series      (<5%)

Oxisols

Ptu         goe, ant
Ak1
Ak2         hem, goe, HIV, ant
Ak3         goe, qtz, HIV, ant
PC1
Pc2         goe, qtz, HIV
Pc3         goe
Ti1         hem, qtz, HIV, ant, gib
Ti2         hem, qtz, HIV, ant, mh
Ti3         hem, qtz, HIV, ant, gib, mh
Nb1         hem, HIV, ill, ant, mh
Nb2         hem, qtz, ant, mh
Ci1         qtz
Ci2         goe, qtz, ant
Ci3         hem, goe, ant

Ultisols

Hp          HIV, ill, ant
Sh          ant, mh
Tim1        goe, hem, ant
Tim2        goe, hem, qtz, HIV, ill, ant
Pga         goe, hem, qtz, HIV, ill, ant
Pk          goe, hem, qtz, HIV, ant, fel
Fd          goe, hem, qtz, HIV, ant
Kbi         goe, hem, HIV, ant
Sd          goe, hem, qtz, ant
Kh          goe, hem, HIV, ill, ant
Re          goe, hem, ant
Suk1        goe, sme, ill
Suk2        goe, hem, fel
Cp          sme, HIV
Kc          goe, hem, qtz, HIV, ant
Yt1         goe, hem, ill
Yt2         goe, hem, ill, ant

Table 4. Charge coefficients for linear relationships between pH and
both [CEC.sub.B] and AEC (i.e. [A.sub.c], [B.sub.c], [A.sub.a],
[B.sub.a],) and coefficient of determination ([R.sup.2]) for Thai
Oxisols

[A.sub.c], Slope ([cmol.sub.c]/kg x pH unit) for [CEC.sub.B];
[B.sub.c], intercept ([cmol.sub.c]/kg) for [CEC.sub.B]; [A.sub.a],
slope for AEC; [B.sub.a], intercept for AEC

                                        [CEC.sub.B]
                        [A.sub.c]        [B.sub.c]        [R.sup.2]

Ptu/Ap                     0.29             0.47             0.96
Ptu/Bto2                   0.93            -0.37             0.93
Ptu/Bto3                   0.82             0.27             0.92
Ak1/Ap                     1.33            -4.03             0.98
Ak1/Bto2                   0.33            -1.07             0.92
Ak1/Bto4                   0.44            -1.68             0.94
Ak2/Ap                     1.50             0.48             0.94
Ak2/Bto2                   0.96             0.99             0.88
Ak2/Bto4                   1.02            -0.45             0.93
Ak3/Ap                     1.39            -2.67             0.85
Ak3/Bto3                   0.49             1.46             0.61
Ak3/Bol                    0.74            -0.47             0.97
Pc1/Ap2                    2.60             1.60             0.97
Pc1/Bt2                    1.93             0.20             0.85
Pc1/Bto1                   0.94             5.05             0.80
Pc2/Ap                     1.16             5.33             0.86
Pc2/Bt2                    1.30             1.93             0.87
Pc2/Bto1                   1.37             0.27             0.96
Pc3/A                      2.44            -2.25             0.89
Pc3/Bt3                    1.51            -1.56             0.87
Pc3/Bto1                   0.99             0.79             0.93
Til/Apl                    2.39            -6.39             0.93
Ti1/Bto1                   1.20            -1.67             0.86
Ti1/Bo1                    1.43            -3.85             0.93
Ti2/Ap                     2.91            -8.51             0.96
Ti2/Bto1                   1.15            -2.23             0.86
Ti2/Bo1                    1.43            -4.01             0.95
Ti3/Ap                     2.39            -6.94             0.98
Ti3/Bo1                    1.91            -6.59             0.99
Ti3/Bto1                   1.61            -5.35             0.99
Nb1/Ap                     2.98            -11.17            0.98
Nb1Bt3                     1.38            -4.83             0.99
Nb1/Bto1                   1.35            -4.56             0.98
Nb2/Ap                     3.36            -12.74            0.97
Nb2/Bt3                    1.71            -6.27             0.99
Nb2/Bto1                   1.74            -6.18             0.93
Cil/Ap2                    0.67             1.19             0.95
Ci1/Bto2                   0.65             0.01             0.94
Ci1/Bo1                    0.86            -0.78             0.85
Ci2/Ap                     0.99             0.74             0.93
Ci2/Bto2                   0.97            -1.61             0.94
Ci2/Bo1                    1.23            -1.66             0.83
Ci3/Ap1                    1.39            -2.65             0.94
Ci3/Bto1                   1.41            -2.84             0.94
Ci3/Bo2                    0.36             1.71             0.82
Mean [+ or -] s.d.    1.38 [+ or -]    -2.06 [+ or -]   0.92 [+ or -]
                           0.72             3.74             007

                                            AEC
                        [A.sub.a]        [B.sub.a]        [R.sup.2]

Ptu/Ap                     -0.1             2.52             0.90
Ptu/Bto2                  -0.26             3.93             0.98
Ptu/Bto3                  -0.28             3.93             0.92
Ak1/Ap                    -0.14             3.12             0.90
Ak1/Bto2                  -0.68             6.73             0.90
Ak1/Bto4                  -0.85             8.17             0.88
Ak2/Ap                    -0.08             2.30             0.76
Ak2/Bto2                  -0.17             3.10             0.67
Ak2/Bto4                  -0.33             4.28             0.92
Ak3/Ap                    -0.16             2.69             0.72
Ak3/Bto3                  -0.23             3.55             0.85
Ak3/Bol                   -0.28             4.17             0.95
Pc1/Ap2                   -0.18             3.35             0.90
Pc1/Bt2                   -0.22             3.93             0.86
Pc1/Bto1                  -0.24             3.92             0.88
Pc2/Ap                    -0.09             2.64             0.74
Pc2/Bt2                   -0.19             3.32             0.96
Pc2/Bto1                  -0.15             3.15             0.95
Pc3/A                     -0.15             2.82             1.00
Pc3/Bt3                   -0.37             4.49             0.92
Pc3/Bto1                  -0.28             4.15             0.85
Til/Apl                   -0.17             3.27             0.81
Ti1/Bto1                  -0.46             5.82             0.87
Ti1/Bo1                   -0.49             6.25             0.96
Ti2/Ap                    -0.21             3.52             0.84
Ti2/Bto1                  -0.45             5.33             0.93
Ti2/Bo1                   -0.54             6.28             0.83
Ti3/Ap                    -0.22             3.65             0.82
Ti3/Bo1                   -0.64             6.38             0.93
Ti3/Bto1                   -0.6             6.60             0.92
Nb1/Ap                    -0.15             2.66             0.89
Nb1Bt3                    -0.65             6.60             0.85
Nb1/Bto1                  -0.65             6.65             0.92
Nb2/Ap                    -0.21             3.19             0.84
Nb2/Bt3                   -0.65             6.29             0.78
Nb2/Bto1                  -0.54             6.01             0.86
Cil/Ap2                   -0.13             2.75             0.92
Ci1/Bto2                  -0.21             3.86             0.95
Ci1/Bo1                   -0.22             3.75             0.81
Ci2/Ap                    -0.11             2.76             0.83
Ci2/Bto2                   0.19             3.34             0.78
Ci2/Bo1                   -0.33             4.64             0.94
Ci3/Ap1                   -0.07             2.40             0.78
Ci3/Bto1                  -0.13             2.85             0.66
Ci3/Bo2                   -0.17             3.96             0.85
Mean [+ or -] s.d.    -0.30 [+ or -]   4.20 [+ or -]    0.87 [+ or -]
                           0.20             1.50             0.08

Table 5. Charge coefficients for linear relationships between pH and
both [CEC.sub.B] and AEC (i.e. [A.sub.c], [B.sub.c], [A.sub.a],
[B.sub.a]) and coefficient of determination ([R.sup.2]) for Thai
Ultisols

[A.sub.c], Slope ([cmol.sub.c]/kg x pH unit) for [CEC.sub.B];
[B.sub.c], intercept ([cmol.sub.c]/kg) for [CEC.sub.B]; [A.sub.a],
slope for AEC; [B.sub.a], intercept for AEC

Soil series/                            [CEC.sub.B]
horizon                 [A.sub.c]        [B.sub.c]        [R.sup.2]

Hp/Ap1                     0.27            -0.62             0.96
Hp/Bt2                     0.17             0.24             0.77
Sh/Ap1                     0.26            -0.64             0.96
Sh/AB                      0.25            -0.66             0.95
Sh/Bt1                     0.18            -0.32             0.94
Tim1/Ap1                   0.27            -0.45             0.98
Tim1/Bt1                   0.27            -0.08             0.84
Tim1/Bt3                   0.19             0.33             0.87
Tim2/Ap                    0.54            -1.43             0.96
Tim2/Bt3                   0.39            -0.36             0.80
Tim2/Bv                    0.39            -0.19             0.83
Pga/Ap                     0.38            -0.75             0.91
Pga/Bt3                    0.42            -0.48             0.66
Pga/Bv                     0.37            -0.14             0.69
Pk/Ap                      0.72             -2.1             0.95
Pk/Bt3                     0.42            -0.62             0.78
Pk/Bv1                     0.38            -0.47             0.75
Fd/Ap                      0.15             0.29             0.97
FdBt4                      0.13             0.40             0.96
Kbi/Ap                     0.28            -0.61             0.97
Kbi/Bt3                    0.15            -0.13             0.95
Sd/Ap                      0.31            -0.78             0.97
Sd/E                       0.17            -0.28             0.96
Sd/Bt3                     0.10             0.36             0.93
Kh/Ap                      0.18            -0.67             0.95
Kh/Bt2                     0.16            -0.62             0.90
Re/Ap                      0.24            -0.33             0.97
ReBt3                      0.15             0.23             0.91
Re/Bv1                     0.18             0.46             0.92
Suk1/Ap1                   0.21            -0.13             0.94
Suk1/Bt1                   0.32             0.92             0.97
Suk1/Bt3                   0.18             1.30             0.97
Suk2/Ap                    0.26            -0.61             0.98
Suk2/Bt1                   0.15            -0.12             0.89
Suk2/Bt3                   0.15            -0.04             0.94
Cp/Ap                      0.17            -0.51             0.89
Cp/Bt                      0.17            -0.51             0.89
Kc/Ap                      0.54            -1.39             0.96
Kc/Bt1                     0.29            -0.16             0.84
Kc/Btc1                    0.24             0.10             0.85
Yt1/Ap                     0.17            -0.14             0.90
Yt1/Bt3                    0.26            -0.27             0.89
Yt2/Ap                     0.17            -0.16             0.93
Yt2/Bt1                    0.09             0.08             0.96
Yt2/Bt2                    0.12             0.24             0.76
Mean [+ or -] s.d.    0.26 [+ or -]    -0.26 [+ or -]   0.90 [+ or -]
                           0.13             0.58             0.08

Soil series/                                AEC
horizon                 [A.sub.a]        [B.sub.a]        [R.sup.2]

Hp/Ap1                    -0.15             2.43             0.85
Hp/Bt2                    -0.11             2.18             0.78
Sh/Ap1                     -0.1             1.98             0.88
Sh/AB                     -0.12             2.29             0.86
Sh/Bt1                    -0.22             2.81             0.94
Tim1/Ap1                  -0.11             2.34             0.76
Tim1/Bt1                  -0.04             1.89             0.88
Tim1/Bt3                  -0.09             2.13             0.91
Tim2/Ap                   -0.16             2.45             0.88
Tim2/Bt3                  -0.13             2.23             0.91
Tim2/Bv                   -0.05             1.90             0.82
Pga/Ap                     -0.1             2.03             0.75
Pga/Bt3                   -0.08             1.97             0.81
Pga/Bv                    -0.07             1.88             0.72
Pk/Ap                     -0.16             1.97             0.84
Pk/Bt3                    -0.13             2.40             0.86
Pk/Bv1                    -0.11             2.23             0.88
Fd/Ap                     -0.24             3.82             0.80
FdBt4                     -0.18             3.66             0.80
Kbi/Ap                    -0.19             2.59             0.83
Kbi/Bt3                   -0.12             2.89             0.92
Sd/Ap                     -0.32             3.56             0.97
Sd/E                      -0.14             2.39             0.90
Sd/Bt3                    -0.05             1.93             0.83
Kh/Ap                     -0.22             2.52             0.81
Kh/Bt2                    -0.07             1.76             0.79
Re/Ap                     -0.18             2.35             0.72
ReBt3                     -0.13             2.63             0.88
Re/Bv1                    -0.09             2.69             0.90
Suk1/Ap1                  -0.33             3.58             0.95
Suk1/Bt1                  -0.12             3.02             0.79
Suk1/Bt3                  -0.18             3.12             0.88
Suk2/Ap                   -0.23             2.87             0.81
Suk2/Bt1                   -0.1             2.67             0.96
Suk2/Bt3                  -0.24             3.53             0.93
Cp/Ap                     -0.09             1.97             0.83
Cp/Bt                      -0.1             1.97             0.72
Kc/Ap                     -0.24             2.79             0.92
Kc/Bt1                    -0.16             2.46             0.86
Kc/Btc1                   -0.21             3.24             0.86
Yt1/Ap                    -0.31             3.64             0.88
Yt1/Bt3                   -0.33             3.32             0.94
Yt2/Ap                    -0.22             3.25             0.83
Yt2/Bt1                   -0.24             3.86             0.84
Yt2/Bt2                   -0.22             3.60             0.77
Mean [+ or -] s.d.    -0.16 [+ or -]   2.64 [+ or -]    0.85 [+ or -]
                           0.08             0.62             0.07

Table 6. Correlation matrix (r) for relationships between charge
coefficients ([A.sub.c], [B.sub.c], [A.sub.a], [B.sub.a]) and
properties of topsoils and subsoils for Thai OxisOls and Ultisols

[A.sub.c], Slope ([cmol.sub.c]/kg x pH unit) for [CEC.sub.B];
[B.sub.c], intercept ([cmol.sub.c]/kg) for [CEC.sub.B]; [A.sub.a],
slope for AEC; [B.sub.a] intercept for AEC. * P < 0.05; ** P < 0.01;
*** P < 0.005; **** P <0.001

                                  Topsoils (n = 30)

Properties                    [A.sub.c]       [B.sub.c]

pH([H.sub.2]O)                0.18            0.30
pH(KCl)                       0.43 *          0.16
pH(NaF)                       0.88 ****       -0.68 ****
OM (g/kg)                     0.81 ****       -0.74 ****
CEC                           0.90 ****       -0.64 ****
Sand (g/kg)                   -0.76 ****      0.29
Silt (g/kg)                   0.56            -0.62 ****
Clay (g/kg)                   0.58 ****       -0.02
SSA ([m.sup.2]/g)             0.93 ****       -0.60 ****
[Fe.sub.t] (g/kg)             0.84 ****       -0.55 ***
[Fe.sub.d] (9/kg)             0.81 ****       -0.44 *
[Fe.sub.o] (g/kg)             0.78 ****       -0.81 ****
[Fe.sub.p] (g/kg)             0.54 ***        -0.70 ****
[Al.sub.t] (g/kg)             0.72 ****       -0.3
[Al.sub.d] (9/kg)             0.75 ****       -0.83 ****
[Al.sub.a] (g/kg)             0.73 ****       -0.65 ****
[Al.sub.p] (g/kg)             0.68 ****       -0.84 ****
[Mn.sub.d] (9/kg)             0.79 ****       -0.57 ***
[Mn.sub.o] (g/kg)             0.71 ****       -0.50 **
[Mn.sub.p] (g/kg)             0.06            0.08
[Ti.sub.t] (g/kg)             0.77 ****       -0.57
[Fe.sub.o]/[Fe.sub.d]         -0.28           0.07
Kao ([MCD.sub.001]) (nm)      -0.25           0.12
[A.sub.c]                     1.00            -0.71 ****
[B.sub.c]                     -0.71 ****      1.00
[A.sub.a]                     0.06            0.19
[B.sub.A]                     0.31            -0.25

                                  Topsoils (n = 30)

Properties                    [A.sub.a]       [B.sub.a]

pH([H.sub.2]O)                0.05            0.34
pH(KCl)                       0.09            0.30
pH(NaF)                       0.12            0.20
OM (g/kg)                     -0.01           0.24
CEC                           0.00            0.37
Sand (g/kg)                   -0.40 *         -0.14
Silt (g/kg)                   0.04            0.21
Clay (g/kg)                   0.43 *          0.05
SSA ([m.sup.2]/g)             0.14            0.34
[Fe.sub.t] (g/kg)             0.16            0.37
[Fe.sub.d] (9/kg)             0.26            0.28
[Fe.sub.o] (g/kg)             -0.11           0.35
[Fe.sub.p] (g/kg)             -0.04           0.01
[Al.sub.t] (g/kg)             0.37 *          0.13
[Al.sub.d] (9/kg)             0.00            0.31
[Al.sub.a] (g/kg)             0.05            0.34
[Al.sub.p] (g/kg)             -0.12           0.16
[Mn.sub.d] (9/kg)             0.05            0.33
[Mn.sub.o] (g/kg)             0.02            0.32
[Mn.sub.p] (g/kg)             0.03            0.00
[Ti.sub.t] (g/kg)             0.12            0.35
[Fe.sub.o]/[Fe.sub.d]         -0.1            -0.23
Kao ([MCD.sub.001]) (nm)      -0.34           0.16
[A.sub.c]                     0.06            0.31
[B.sub.c]                     0.19            -0.25
[A.sub.a]                     1.00            -0.73 ****
[B.sub.A]                     -0.70 ****      1.00

                                  Subsoils (n=60)

Properties                    [A.sub.c]       [B.sub.c]

pH([H.sub.2]O)                -0.05           0.12
pH(KCl)                       0.29 *          -0.32 *
pH(NaF)                       0.73 ****       -0.69 ****
OM (g/kg)                     0.34 *          -0.21
CEC                           0.83 ****       -0.34
Sand (g/kg)                   -0.80 ****      0.35 **
Silt (g/kg)                   0.27 *          -0.63
Clay (g/kg)                   0.71 ****       -0.19
SSA ([m.sup.2]/g)             0.90 ****       -0.58
[Fe.sub.t] (g/kg)             0.80 ****       -0.57
[Fe.sub.d] (9/kg)             0.72 ****       -0.45
[Fe.sub.o] (g/kg)             0.68 ****       -0.79
[Fe.sub.p] (g/kg)             0.26            -0.35 **
[Al.sub.t] (g/kg)             0.63 ****       -0.22
[Al.sub.d] (9/kg)             0.55 ****       -0.72
[Al.sub.a] (g/kg)             0.42 ***        -0.33
[Al.sub.p] (g/kg)             0.04            0.07
[Mn.sub.d] (9/kg)             0.61 ****       -0.63
[Mn.sub.o] (g/kg)             0.47 ****       -0.55 ****
[Mn.sub.p] (g/kg)             -0.30 *         0.2
[Ti.sub.t] (g/kg)             0.74 ****       -0.69 ****
[Fe.sub.o]/[Fe.sub.d]         -0.31 *         0.06
Kao ([MCD.sub.001]) (nm)      -0.21           0.12
[A.sub.c]                     1.00            -0.61 ****
[B.sub.c]                     -0.61 ****      1.00
[A.sub.a]                     -0.58 ****      0.70 ****
[B.sub.A]                     0.62 ****       -0.65 ****

                                  Subsoils (n=60)

Properties                    [A.sub.a]       [B.sub.a]

pH([H.sub.2]O)                -0.22           0.26
pH(KCl)                       -0.61 ****      0.61 ****
pH(NaF)                       -0.78 ****      0.77
OM (g/kg)                     -0.22           0.23
CEC                           -0.46 ****      0.53 ****
Sand (g/kg)                   0.58 ****       -0.64
Silt (g/kg)                   -0.40 ***       0.37 ***
Clay (g/kg)                   -0.46 ****      0.53 ****
SSA ([m.sup.2]/g)             -0.73 ****      0.79 ****
[Fe.sub.t] (g/kg)             -0.77 ****      0.83 ****
[Fe.sub.d] (9/kg)             -0.71 ****      0.78 ****
[Fe.sub.o] (g/kg)             -0.68 ****      0.70 ****
[Fe.sub.p] (g/kg)             -0.26           0.24
[Al.sub.t] (g/kg)             -0.55           0.59 ****
[Al.sub.d] (9/kg)             -0.89 ****      0.89 ****
[Al.sub.a] (g/kg)             -0.66 ****      0.72 ****
[Al.sub.p] (g/kg)             -0.01           0.05
[Mn.sub.d] (9/kg)             -0.67 ****      0.71 ****
[Mn.sub.o] (g/kg)             -0.55 ****      0.58 ****
[Mn.sub.p] (g/kg)             0.21            -0.18
[Ti.sub.t] (g/kg)             -0.67 ****      0.73 ****
[Fe.sub.o]/[Fe.sub.d]         0.22            -0.24
Kao ([MCD.sub.001]) (nm)      0.08            -0.09
[A.sub.c]                     -0.58 ****      0.62 ****
[B.sub.c]                     0.70 ****       -0.65 ****
[A.sub.a]                     1.00            -0.97 ****
[B.sub.A]                     -0.97 ****      1.00

Table 7. Properties of iron oxide concentrates

SSA, Specific surface area; G/(G+H), goethite/(goethite+hematite);
MCD, mean coherently diffracting length (nm); n.d., not detected

Soil series/         SSA         G/(G+H)      Mol        Goethite
horizon         ([m.sup.2]/g)               Al (%)    [MCD.sub.110]

Pc1/Ap2               62           n.d.      n.d.          n.d.
Ci3/Ap                42           n.d.      n.d.          n.d.
Ak1/Ap                76           0.52       14            17
Ti3/Ap                94           0.46       16            7
Nb2/Ap                79           0.48       19            15

Soil series/                       Mol        Hematite
horizon         [MCD.sub.110]    Al (%)    [MCD.sub.110]

Pc1/Ap2              n.d.          11            31
Ci3/Ap               n.d.           7            38
Ak1/Ap                16            8            45
Ti3/Ap                8             5            15
Nb2/Ap                9             5            26

Soil series/
horizon         [MCD.sub.112]

Pc1/Ap2               20
Ci3/Ap                18
Ak1/Ap                27
Ti3/Ap                14
Nb2/Ap                27

Table 8. Charge coefficients ([A.sub.c], [B.sub.c], [A.sub.a],
[B.sub.a]) and coefficient of determination ([R.sup.2]) for linear
relationships of [CEC.sub.B] and AEC with pH for whole soil, soil
after removal of organic matter, kaolin concentrate, and iron oxide
concentrate for five Thai soil samples

[A.sub.c], Slope ([cmol.sub.c]/kg x pH unit) for [CEC.sub.B];
[B.sub.c], intercept ([cmol.sub.c]/kg) for [CEC.sub.B]; [A.sub.a]
slope for AEC; [B.sub.a], intercept for AEC

Soil series/                           [CEC.sub.B]
horizon               [A.sub.c]         [B.sub.c]         [R.sup.2]

Whole soil

Pc1/Ap2                 2.60              1.60              0.97
Ci3/Ap1                 1.39              -2.65             0.94
Ak1/Ap                  1.33              -4.03             0.98
Ti3/Ap                  2.39              -6.94             0.98
Nb2/Ap                  3.36             -12.74             0.97

Mean [+ or -]       2.21 [+ or -]    -4.95 [+ or -]     0.97 [+ or -]
s.d.                    0.86              5.33              0.02

Removed OM

Pc1/Ap2                 1.43              -0.5              0.91
Ci3/Ap1                 0.69              -1.22             0.74
Ak1/Ap                  0.67              -2.18             0.99
Ti3/Ap                  1.98              -5.67             0.98
Nb2/Ap                  2.06              -6.82             0.97

Mean [+ or -]       1.37 [+ or -]    -3.28 [+ or -]     0.92 [+ or -]
s.d.                    0.67              2.80              0.10

Kaolin concentrate

Pc1/Ap2                 1.12              2.44              0.91
Ci3/Ap1                 0.72              -0.15             0.99
Ak1/Ap                  0.48              -1.09             1.00
Ti3/Ap                  1.05              -0.26             0.98
Nb2/Ap                  1.09              -2.82             0.99

Mean [+ or -]       0.89 [+ or -]    -0.38 [+ or -]     0.97 [+ or -]
s.d.                    0.28              1.90              0.04

Iron oxide concentrate

Pc1/Ap2                 1.19              -2.92             0.95
Ci3/Ap1                 2.45              -8.64             0.99
Ak1/Ap                  3.31             -11.84             1.00
Ti3/Ap                  3.99             -16.05             0.89
Nb2/Ap                  3.74             -13.02             1.00

Mean [+ or -]       2.94 [+ or -]    -10.49 [+ or -]    0.97 [+ or -]
s.d.                    1.14              5.00              0.05

Soil series/                               AEC
horizon               [A.sub.c]         [B.sub.c]         [R.sup.2]

Whole soil

Pc1/Ap2                 -0.18             3.35              0.90
Ci3/Ap1                 -0.07             2.40              0.78
Ak1/Ap                  -0.14             3.12              0.90
Ti3/Ap                  -0.22             3.65              0.82
Nb2/Ap                  -0.21             3.19              0.84

Mean [+ or -]      -0.16 [+ or -]     3.14 [+ or -]     0.85 [+ or -]
s.d.                    0.06              0.46              0.05

Removed OM

Pc1/Ap2                 -0.21             4.19              0.82
Ci3/Ap1                 -0.16             4.00              0.99
Ak1/Ap                  -0.35             5.50              0.81
Ti3/Ap                  -0.52             6.58              0.90
Nb2/Ap                  -0.41             5.55              0.88

Mean [+ or -]      -0.33 [+ or -]     5.16 [+ or -]     0.88 [+ or -]
s.d.                    0.15              1.07              0.07

Kaolin concentrate

Pc1/Ap2                 -0.1              3.74              0.95
Ci3/Ap1                 -0.09             3.66              0.90
Ak1/Ap                  -0.32             4.95              0.93
Ti3/Ap                  -0.6              6.68              0.77
Nb2/Ap                  -0.44             6.06              0.93

Mean [+ or -]      -0.31 [+ or -]     5.02 [+ or -]     0.90 [+ or -]
s.d.                    0.22              1.35              0.07

Iron oxide concentrate

Pc1/Ap2                 -0.85             9.37              0.87
Ci3/Ap1                 -1.36             12.48             0.69
Ak1/Ap                  -1.9              16.12             0.83
Ti3/Ap                  -0.55             6.60              0.74
Nb2/Ap                  -1.19             10.95             0.99

Mean [+ or -]      -1.17 [+ or -]    11.10 [+ or -]     0.82 [+ or -]
s.d.                    0.51              3.55              0.12

Table 9. Correlation matrix (r) for charge coefficients ([A.sub.c],
[B.sub.c], [A.sub.a], [B.sub.a]) and P sorption coefficients
([X.sub.m], b, k, B) for Thai Oxisols and Ultisols (n = 64)

[X.sub.m], P sorption maximum capacity; b, bonding energy constant; k,
sorption surface constant; B, energy of sorption constant; [A.sub.c],
slope ([cmol.sub.c]/kg x pH unit)of [CEC.sub.B] v. pH; [B.sub.c],
intercept ([cmol.sub.c]/kg) of [CEC.sub.B] v. pH; [A.sub.a], slope of
AEC v. pH; [B.sub.a], intercept of AEC v. pH. ** P < 0.01; *** P <
0.005

P sorption                       [CEC.sub.B]
coefficients              [A.sub.c]     [B.sub.c]

[X.sub.m] ([micro]g/g)      0.72 ***      -0.68 ***
b (mL/[micro]g)             0.2           -0.32 **
k ([micro]g/g)              0.64 ***      -0.67 ***
B                           0.24          0.08

P sorption                            AEC
coefficients              [A.sub.a]     [B.sub.a]

[X.sub.m] ([micro]g/g)      -0.52 ***     0.63 ***
b (mL/[micro]g)             -0.36 **      0.32 **
k ([micro]g/g)              -0.67 ***     0.72 ***
B                           0.22          -0.15
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Article Details
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Author:Wisawapipat, W.; Kheoruenromne, I.; Suddhiprakarn, A.; Gilkes, R.J.
Publication:Australian Journal of Soil Research
Article Type:Report
Geographic Code:9THAI
Date:Jul 1, 2010
Words:13340
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