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The geochemistry of soils on a catena on sedimentary rock at Nam Phong, north-east Thailand.


Element concentrations in soils are mainly related to the chemical composition of parent materials and to changes due to weathering which differ for various climatic environments (Duddy 1980; Topp et al. 1984). As soil profiles develop there is a redistribution of elements within the soil fabric, then between profile horizons, and finally between soils within the landscape (Jenkins and Jones 1980). The spatial redistribution of elements during weathering is particularly complicated because elements are affected by various pedogenic processes including dissolution of primary minerals, formation of secondary minerals, redox processes, transport of material, and ion exchange (Chesworth et al. 1981; Fritz and Mohr 1984; Middelburg et al. 1988). Topography may also exert substantial control over elemental distribution in soils (Gallez et al. 1976). Under acid oxidising conditions, Fe and Al remain, whereas the Ca, Mg, Mn, and Na move from upslope to the lower parts of the slope where alkaline conditions may prevail (Tardy et al. 1973). Birkeland (1999) proposed that elements released by weathering may or may not be redistributed along the slope as a function of their mobility in the constant or changing geochemical environments along the slope.

These multiple interaction processes result in complex patterns of element distribution in soil catenae which are not readily comprehended if data are presented in tables or as multiple depth and landscape graphical representations. Multivariate statistical analyses are especially useful for assessing and depicting multiple chemical and physical variables of this type. Factor analysis has been widely successful for the interpretation of geochemical and lithogeochemical affinities for elements and materials in sediments and soils (Balakrishnan and Bhaumik 1994; Evans et al. 1996; Bakac and Kumru 2001; Kumru and Bakac 2003). This procedure has been used in this study of relationships between the various components (elements) for the Nam Phong catena, north-east Thailand. The representative catena developed on sedimentary rock, as are most soils in this region. The hypothesis is that there are elemental affinity groups for the soils that can provide a basis for describing the geochemical evolution of soils on the catena.

Materials and methods

Geology setting and site study

The regionally representative Nam Phong catena in Khon Kaen province, north-east Thailand, was investigated (Fig. 1). The Nam Phong catena can be divided into 6 geomorphie positions: summit, shoulder, upper midslope, lower midslope, footslope, and toeslope. The soils were classified according to Soil Taxonomy (Soil Survey Staff 1999) as Typic Kandiustult, Kandic Paleustalf, Typic Haplustalf, Oxic Dystrustept, and Aeric Endoaqualf for the soils on summit, shoulder, midslope, footslope, and toeslope positions, respectively. Altitudes are 209, 206, 202, 199, 191, and 180 m MSL from the highest position to the lowest position on the landscape. Their coordinates (UTM) range from 48267414E to 482678346 E and 18 55617N to 1855419N from the highest position to the lowest position on the eatena. The catena length is about 1 km. The Nam Phong catena has developed on sedimentary rocks of the upper Triassic to Cretaceous Khorat group and lower Tertiary to lower Pleistocene Krabi group. Soils from the summit to footslope positions are on the Khok Kruat formation of the Khorat group. The soil on the toeslope position is on quaternary deposits of the Krabi group. The Khok Kruat formation consists of red to purplish grey bedded sandstone, siltstone, and shale with interbedded calcareous conglomeratic sandstone, sandy limestone, and calcareous siltstone which is mottled with purplish grey and greenish grey colours. The quaternary Khok Kruat floodplain deposit comprises unconsolidated silt, clay, sand, and gravel derived in part from the Khok Kruat group (Suwanasing 1972). Soils on the summit to footslope positions are occupied by cultivated cassava fields but the main land use at the toeslope position of this catena is paddy rice field. The Nam Phong catena is under a tropical savannah climate with an average mean annual rainfall of 1300 mm/year and average temperature of 27[degrees]C.


Laboratory work

Representative profiles of each position on the Nam Phong catena were described and sampled by genetic horizon according to standard field study methods (Kheoruenronme 1999). Disturbed soil samples were air-dried and crushed to pass through a 2-mm mesh sieve. Particle size distribution was determined by the pipette method (Kilmer and Alexander 1949; Day 1965). Soil pH was determined using a 1:1 soil:water mixture and measured by pH meter (National Soil Survey Center 1995). Organic carbon was measured by the Walkley-Black titration method (Walkley and Black 1934). Extractable Na, K, Ca, and Mg were leached from soil with N[H.sub.4]Oak pH 7.0 and element concentrations in the extract measured by atomic absorption spectrometry (AAS) (Thomas 1987). Soils were extracted with dithionite-citrate-bicarbonate (DCB) (Mehra and Jackson 1960), ammonium oxalate (McKeague and Day 1966), and sodium pyrophosphate (Loveland and Digby 1984) to determine the quantities of crystalline, amorphous, and organic associated iron ([Fe.sub.d], [Fe.sub.o], [Fe.sub.p]), respectively, Fe in digests being measured by AAS.

The removal of organic matter in soils before particle size separation used [H.sub.2][O.sub.2] and removal of carbonate used NaOAc at pH 5.0; it is accepted that this process may modify the forms and amounts of elements to a certain extent. The sand fraction was separated from the silt and clay fractions by wet sieving, oven-dried, and then fractionated by dry sieving into the 5 classes: very coarse sand (1-2 mm), coarse sand (0.5-1 mm), medium sand (0.25-0.5 mm), fine sand (0.1-0.25 mm), very fine sand (0.05-0.1 mm). Clay (<0.002 mm), silt (0.002-0.05 mm), and fine sand fractions were finely ground by hand in an agate mortar for further analysis. Major and minor elements in soils and their size fractions were determined by a combination of X-ray fluorescence spectrometry (XRF) analysis (Norrish and Hutton 1969) and aqua regia digestion (HCl and [H.sub.2]C[O.sub.3]) followed by inductively coupled plasma-mass spectrometry (ICP-MS) (Soltanpour et al. 1996; Lynch 1999). Aluminum, Si, Ti, Fe, Ca, Mg, K, Mn, and Zr were determined by XRF on fused lithium tetraborate discs and P, Li, Cs, As, Cr, V, Ni, Rb, Cu, Zn, Ga, Co, Pb, Mo, Zr, U, and Sr were analysed by ICP-MS on a strong acid digest of finely ground soil, fine sand, and silt. For the clay fraction, Al, Si, Ti, Fe, Ca, Mg, K, Mn, P, Ni, Zr, Rb, St, Zn, Cu, Co, Cr, Ba, and V were determined by XRE

Semi-quantitative determination of minerals was by X-ray diffraction (XRD) analysis using CuK[alpha] radiation with a Philips diffractometer equipped with a graphite monochromator and operating at 50kV and 20 mA. Soil, sand, silt, and clay were analysed as random powders and clays were also analysed as oriented aggregates. The XRD scans extended from 4-70[degrees] 2[theta] with a step size of 0.02[degrees] and scan speed of 1.2[degrees] per min.

Undisturbed soil samples of selected horizons were impregnated with a 70:30 resin:monostryrene mixture and 5 g of the catalyst benzoyl peroxide ([C.sub.14][H.sub.10][O.sub.4]). The resin mixture was poured onto samples at atmospheric pressure, followed by a period in a vacuum oven at 60[degrees]C while the pressure was slowly increased from 10 to 65 cm Hg, and then the samples were left for 4 h. Impregnated samples were cut into 0.5-cm-thick slabs using a diamond saw and polished with corundum abrasives from 250 mesh down to 600 mesh. After ultrasonic cleaning, the polished samples were mounted onto glass slides followed by polishing and hand-finishing to produce thin sections of 30 nm thickness. The thin sections were analysed under a polarising microscope using standard micromorphological techniques as described by Bullock et al. (1985). Selected areas described under the polarising microscope were examined by scanning electron microscopy with energy dispersive X-my analysis (SEM/EDS) to determine the composition of the soil plasma.

Statistical analysis

Factor analysis, a widely used multivariate statistical method, was employed to interpret data. This technique reveals the correlation structure of the geochemical variables allowing the identification of affinity groups of elements and samples. The statistical analyses of the geochemical data were performed using the STATISTICA program (version 6.1).

Results and discussion

General soil properties

Table 1 summarises the properties of soils along the Nam Phong catena. All soils have the highest organic matter concentration in the surface, decreasing uniformly with depth in the profile. The differences in the amount of organic matter in the soils are probably due to the differences in plant production and litter decomposition rate, both of which are affected by soil moisture content. The soils on all positions on the catena are deep. They have a high sand content except for the soil on the toeslope position. Hence, their texture ranges from sandy to sandy clay for the soil on the upper slope, whereas the texture of horizons of the toeslope soil ranges from clay loam to silty clay. The soils on the summit to the footslope positions are acidic but the soil on the toeslope is alkaline. This alkalinity is associated with the high extractable base content of the toeslope soil, especially Ca and the presence of CaC[O.sub.3] nodules. The large difference in CEC between soils in upslope positions and that on the toeslope is due to the greater clay content of the toeslope soil, which may be due to in situ formation of clay and to a difference in parent material.

The [Fe.sub.d] values decrease downslope as the red colour gradually changes to a grey in the lowest position on the catena (Weitkamp et al. 1996). In general, the soils on the upper part of a catena, where well-drained conditions prevail, contain more free iron than the poorly drained soils at the base of a catena (Boonsompoppunth 1984). For this catena, most Fe in the toeslope soil is [Fe.sub.d], indicating that crystalline iron oxides are present and that prolonged reducing conditions do not occur. The relatively low [Fe.sub.o] values for this and all upslope soils indicate that amorphous iron compounds that are characteristic of seasonally reducing soil environment are only minor constituents of these soils (Schwertmann and Taylor 1989). The higher [Fe.sub.d] in soil on the toeslope position may be at least partly attributed to the influence of the fine-grained sedimentary parent rock, which is the probable parent material. Amounts of [Fe.sub.p] are minor and most abundant in the surface horizons of the profiles where most organic matter occurs. The soil on the toeslope position has the highest [Fe.sub.p] values, which are associated with the increase of organic matter downslope as organic matter forms complex active iron (Schwertmann and Taylor 1989).

Soil mineralogy

The minerals in fine sand, silt, and clay fractions of soils on the Nam Phong catena are given in Table 2. The soil on the summit position contains much kaolin, and minor goethite and quartz, and traces of illite, vermiculite, and hematite. Soil on the shoulder position is similar to soil on the summit but vermiculite and hematite are absent. Soils on the midslope position contain large amount of kaolin, moderate quartz, and traces of illite and goethite. Quartz content decreases whereas illite increases with depth. Traces of vermiculite are present in the deeper horizons of the soil on the lower midslope position. Kaolin is a major constituent of soil on the footslope position, and both quartz and illite are moderate constituents of this soil; a little vermiculite and goethite are also present. For the toeslope soil on the lowest position on the Nam Phong catena, kaolin and illite are major constituents. Decreases in kaolin and increases in illite with depth are observed. A little vermiculite is also present and it increases in abundance with depth. Soils on upper positions on the catena experience a leaching, base-depleting environment due to the high rainfall resulting in the formation of kaolin (Vijarnsorn and Fehrenbacher 1973; Gallez et al. 1976; Boonsompoppunth 1984; Suddhiprakarn et al. 1985). The relative abundance of illite in soils on the footslope and toeslope positions reflect a lower intensity of weathering and some of the illite may be derived from the sedimentary parent rock. In the toeslope soil, ions leached from upslope accumulate providing a higher pH and a geochemical environment that favours retention of illite (Fanning et al. 1989). Additionally the toeslope soil has formed upon a fine-grained illitic sediment that will have contributed illite to the profile. In addition the alkaline environment of the toeslope soil may be conducive to the authigenesis of illite.

All soils on this catena contain only quartz in both the silt and fine sand fractions due to the parent materials of this catena being colluvium derived from sandstone for the soil on the summit, shoulder, midslope and footslope positions. The sandstone contains low amounts of primary minerals apart from quartz. Quartz is a highly resistant mineral in soils (Sudom and Arnaud 1971; Cornu et al. 1999; Stiles et al. 2003). The soil on the toeslope position has developed at least in part from residuum of a fine-grained sedimentary rock.

Geochemistry of soils

The complete chemical analysis of soils and size fraction are not presented here as they comprise a very large dataset. Silicon is the major component in whole soils (439-362 g/kg) of all soils in this catena, as would be expected on the basis of the sandy soil texture. Aluminum and Fe are minor constituents of soils at all positions, with mean concentrations ranging from 2.9 to 65.2 and 2.6 to 38.1 g/kg, respectively. Although different positions on the catena experience a different intensity of weathering there are few differences in element concentration in soils from summit to footslope but there is a substantial difference for the toeslope soil.

Factor analysis is one of the most widely used statistical treatments for evaluating multi-element geochemical data of this type (Weber and Davis 1990; Bellehumeur et al. 1994). In this study, factor analysis of standardised data was used to identify relationships between the elements and materials on the Nam Phong catena.

Figure 2 shows that 2 factors represent 83% of the total data variability for analyses of whole soil. There are four groups of elements of similar behaviour in the Nam Phong catena (Fig. 2a). The first group (Fe group) is composed of Fe, As, Cr, Mo, Cu, V, Pb, P, and U; the second group (Al group) is Al, Mn, Co, Ca, Mg, K, Sr, Cs, Rb, Ga, Zn, Ni, Li, and Ti; the third group is Zr; and the last group is Si. The metals in the iron group can substitute in Fe oxides as they have ionic properties that are compatible with Fe oxide structures (Farrah and Pickering 1979). Arsenic and P occur in this group as they exist in soils as oxyanions that are strongly adsorbed by Fe oxides. The Al-Mn group represents clay minerals and manganese oxides together with elements that are compatible with these minerals (Dolcater et al. 1970; Kinniburgh et al. 1976; Kabata-Pendias and Pendias 2001). Soils from the summit to the footslope are uniform in composition except for the 2C horizon of the shoulder and midslope soils (Fig. 2b), which is attributed to a discontinuity of parent material within the soil profile, possibly indicating the presence of a veneer of colluvium. The soil on the toeslope is distinctly different from the other soils as it exhibits considerably diversity between horizons.


Geochemistry of the fine sand fraction

Factor analysis of the element concentrations in the fine sand fraction (Fig. 3a) shows that 2 factors describe 89% of the variation in the total dataset. Two distinct groups with similar elemental behaviours are recognised. The first group is solely Si, the second (Al group) with an opposite relationship to Si comprises all the remaining elements, with Cr, Zr, and Mn falling slightly outside this group. Thus, the groupings represent Si in quartz sand grains and the other elements in various silicate and oxide grains including sand-size soil concretions. Soils from the summit to footslope overlap closely in this diagram (Fig. 3b). Sand in the toeslope soil shows a large variation in composition and is distinctly different from the other soils.


Geochemistry of the silt fraction

Three groups of elements of similar behaviour occur in the silt fraction (Fig. 4a) and describe 84% of the variation in the dataset; these groups are the same groups as for the fine sand fraction. The first group is Si, representing quartz, and the second is Zr, representing zircon; the last group (Al group) includes most other elements (Al, Cs, Sr, Rb, As, Ga, Cu, Ni, Co, Cr, V, Li, P, Mn, Mg, K, Ca, Fe, Ti, Pb, U, and Zn), which are quite widely distributed. The soils from the summit to footslope positions show little variation in composition (Fig. 4b). The toeslope soil shows a large variation in chemical composition and is distinctly different from the other soils.


Geochemistry of the clay fraction

Figure 5a shows that 2 factors represent 65% of the total data variability. Three groups of elements with different behaviours are indicated. The first group is Si, Ni, Mn, Co, Mg, K, Ba, and Pb; the second group is Al, Fe, Zr, Ti, and V; the last group is Ca, Zn, Cu, Sr, Cr, and E There are no simple mineralogical explanations for these groupings, which is probably a consequence of the diverse and complex mineralogy of the clay fraction that contains several clay minerals and sesquioxides with diverse associations of elements; for example, Mn, Co, Ni, Ba, Pb, and K may be associated in manganese oxide minerals (Kabata-Pendias and Pendias 2001). All soils on this catena show moderate within-profile variation in element composition and are quite widely dispersed in figure. The footslope soil is distinctly different, as is the toeslope profile, with much Ni, K, and Mg and less Pb, Co, and Ba. The compositions of clay in summit, shoulder, and midslope soils are diverse and overlap to some extent.


Distribution of elements between size fractions

The relative contributions of fine sand, silt, and clay fractions to the chemical composition of the soils on the Nam Phong catena are shown in triangle graphs (Fig. 6). Note that coarser sand fractions are not included within this diagram and for most elements will contribute elements in a similar manner to fine sand but generally at lower concentrations (Darmody 1985; Chatupa and Direng 2000). Aluminum, Fe, K, Mg, Ca, Co, Rb, Zn, and Sr are associated with the clay fraction for most of soils except that Al, Zn, and Fe are contributed by silt and sand for the footslope soil. The contributions of minor elements are generally high for the clay fraction as has been reported by other workers (Song et al. 1999). Silicon and P are strongly associated with the sand fraction, reflecting the abundance of quartz and possibly the presence of apatite grains (Ca, P) in the silt and sand. In the toeslope soil, Si and Ca are contributed by all 3 fractions. Chromium, Cu, Ni, and V are contributed mostly by the sand and clay. Titanium is contributed by all size fractions; however, it tends to mostly be present in the silt and clay fractions. Silicon, Zr, and Ti exist in soils in sand-size grains mainly as quartz for Si, zircon for Zr, and anatase, futile, or ilmenite for Ti, all of which are highly resistant to weathering (Cornu et al. 1999; Stiles et al. 2003).


The spatial distribution of major elements in soil materials

The SEM images of the thin sections of the soils on the upslope position on the catena are quite similar and show various sizes and shapes of quartz grains mixed with little matrix in a dominantly grain support matrix. The element maps (Figs 7 and 8) show that the major constituent in the soils in the high landscape positions is Si with minor amounts of Al and Fe, which is consistent with the bulk mineralogical and chemical compositions discussed earlier. The matrix of the soil at the summit position has the most uniform chemical composition, being a mixture of kaolin with iron oxide as indicated by the normalised element composition of the matrix, which mostly falls on the 'kaolin line' (Fig. 7). In addition, this soil exhibits relatively little evidence of clay illuviation as is also the case for other soils. The soil matrix on the shoulder, midslope, and footslope positions varies more widely in chemical composition. Data points are displaced from the 'kaolin composition line' (Fig. 8) towards the Si[O.sub.2] apex, which may indicate that some very fine-grained quartz is present within the soil matrix. Displacement of the data points from the 'kaolin line' may also be attributed to the very thin coatings of soil matrix on quartz grains, so that some of the Si X-rays originated from the adjacent sand grains.


The Al, Si, Fe, and Ca maps (Fig. 9) of soil from the toeslope position reveal that this soil also contains much Si; however, it contains relative more Al than the other soils. Also, most of data points are located away from the 'kaolin line' towards to the Si[O.sub.2] apex. This corresponds to the presence of illite in the matrix, as very little fine-grained quartz occurs in this soil. Authigenesis of illite probably occurs in the toeslope soil.



Soils at all positions on Nam Phong catena are dominated by quartz and thus contain much Si, together with minor Al and Fe. The major elements are closely related to minerals present in soils, with kaolin being the major clay mineral along with smaller amounts of iron oxide in the clay fraction in soils at all positions. Illite and vermiculite are present in the footslope and toeslope positions. Only quartz was detected by XRD in the silt and fine sand fractions, so <~1% of other minerals will be present. Factor analysis did not supported the hypothesis that there would be systematic differences in element composition in the whole soil, fine sand, and silt samples between the several soils in the catena. The element compositions of upslope soil show little variation and the chemical compositions of upslope profiles overlap closely. However, the chemical composition of the toeslope soil is distinctly different from the soils on higher positions as indicated by Al group (Al, Co, Ca, Mg, K, Sr, Cs, Rb, Ga, Zn, Ni, Li, Mn, Ti) and there is a wide variation in chemical composition of horizons of the toeslope soil. For the clay fraction, the different concentrations of elements in the Si group (Si, Ni, Mn, Co, Mg, K, Ba, Pb) and Ca group (Ca, Zn, Cu, Sr, Cr, P) result in the soils on toeslope and footslope being distinctly different from other soils. The factor based on the Al group (Al, Fe, Zr, Ti, V) causes the soils on summit, shoulder, and midslope to overlap to some extent but these soils are different from other soils and show moderate variation with depth. The small variations in the chemical composition of upslope soils are probably due to different degrees of weathering of the same parent rock, whereas soil on the toeslope position has a quite different elemental composition, which is possibly due to both a difference in composition of the parent rock and formation of authigenic minerals.


This work was supported by the Royal Golden Jubilee Program under The Thailand Research Fund. The authors would like to thank the Centre for Microscopy and Microanalysis, The University of Western Australia for providing access to SEM/EDS analysis, and Michael Smirk for assisting with chemical analysis.

Manuscript received 1 March 2005, accepted 5 December 2005


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Suphicha Thanachit (A), Anchalee Suddhiprakarn (A,C), Irb Kheoruenromne (A), and Robert J. Gilkes (B)

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

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

(C) Corresponding author. Email:
Table 1. Summarised properties of soils on the Nam Phong catena

Depth Horizon pH OM
(m) [H.sub.2] (g/kg)
 O (A)

Summit: Typic Kandiustult

0-0.20 Ap 6.3 4.85
0.20-0.40 E 6.4 1.64
0.40-0.59 Btl 6.3 1.37
0.59-0.73 Bt2 5.3 1.65
0.73-1.05 Bt3 4.7 1.68
1.05-1.42 Bt4 4.7 1.68
1.42-1.72 Bt5 4.7 1.68
1.72-2.05+ Bt6 4.7 1.27

Shoulder: Kandic Paleustalf

0-0.23 Ap 6.6 3.71
0.23-0.35 E 6.9 1.31
0.35-0.58 Btl 6.1 1.83
0.58-0.82 Bt2 5.0 1.78
0.82-1.10 Bt3 4.6 1.91
1.10-1.40 Bt4 4.7 2.35
1.40-1.90+ 2C 5.0 2.71

Upper midslope: Typic Haplustalf

0-0.20/0.30 Ap 6.0 3.73
0.30-0.45 E 6.3 0.59
0.45-0.67 Btl 7.0 1.28
0.67-1.00 Bt2 6.6 1.19
1.00-1.30 Bt3 6.6 0.97
1.30-1.55 Bt4 4.9 0.87
1.55-1.82 Bt5 5.0 1.01
1.82-1.98 Bt6 5.3 0.88
1.98-2.10+ 2C 5.3 1.99

Lower midslope: Typic Haplustalf

0-0.27/0.35 Ap 4.8 2.83
0.35-0.46 Btl 5.2 0.94
0.46-0.63 Bt2 5.2 0.63
0.63-0.80 BO 5.2 0.61
0.80-1.00 213C 5.0 0.90
1.00-1.30 2C1 5.0 1.57
1.30-1.80+ 2C2 4.8 1.72

Footslope: Oxic Dystrustept

0-0.10/0.20 Ap1 5.4 6.21
0.20-0.20/0.30 Ap2 5.3 4.89
0.30-0.45 Bw 5.4 1.17
0.45-0.80 Bt 5.5 0.33
0.80-1.10 Btg1 5.4 0.31
1.10-1.30+ Btg2 5.6 0.62

Toeslope: Aeric Endoaqualf

0-0.05 Apg 6.0 5.50
0.05-0.23/0.30 Btcg 7.0 3.30
0.30-0.48 Btg1 8.5 1.24
0.48-0.80 Btg2 8.9 1.03
0.80-1.13 Btg3 9.2 0.93
1.13-1.43 Btg4 9.1 1.00
1.43-1.62 BCg1 9.2 1.07
1.62-1.95+ BCg2 9.1 0.69

Depth Extractable cations CEC
(m) Ca Mg K Na

Summit: Typic Kandiustult

0-0.20 0.97 0.17 0.05 0.16 1.48
0.20-0.40 0.74 0.15 0.05 0.19 0.90
0.40-0.59 1.18 0.19 0.03 0.11 3.31
0.59-0.73 1.21 0.28 0.05 0.04 2.26
0.73-1.05 0.16 0.42 0.05 0.06 1.56
1.05-1.42 0.81 0.93 0.05 0.05 2.32
1.42-1.72 0.08 0.56 0.06 0.07 1.41
1.72-2.05+ 0.14 0.46 0.05 0.04 1.97

Shoulder: Kandic Paleustalf

0-0.23 0.34 0.26 0.32 0.28 1.10
0.23-0.35 0.13 0.08 0.46 0.10 1.15
0.35-0.58 0.20 0.08 0.59 0.37 0.70
0.58-0.82 0.87 0.40 0.31 0.31 2.31
0.82-1.10 1.07 0.90 0.07 0.11 2.37
1.10-1.40 1.57 0.80 0.07 0.16 5.54
1.40-1.90+ 2.81 1.39 0.14 0.11 3.92

Upper midslope: Typic Haplustalf

0-0.20/0.30 0.95 0.37 0.09 0.03 1.15
0.30-0.45 0.16 0.13 0.11 0.18 1.10
0.45-0.67 0.14 0.11 0.22 0.43 1.35
0.67-1.00 0.12 0.07 0.28 0.27 0.50
1.00-1.30 0.14 0.06 0.26 0.41 0.60
1.30-1.55 0.15 0.10 0.18 0.37 1.45
1.55-1.82 0.19 0.13 0.08 0.10 0.65
1.82-1.98 0.46 0.35 0.05 0.12 2.16
1.98-2.10+ 2.14 1.36 0.14 0.08 7.82

Lower midslope: Typic Haplustalf

0-0.27/0.35 0.15 0.05 0.03 0.10 0.75
0.35-0.46 0.11 0.04 0.02 0.37 0.80
0.46-0.63 0.10 0.04 0.03 0.11 0.65
0.63-0.80 0.07 0.03 0.02 0.07 0.70
0.80-1.00 0.07 0.05 0.03 0.09 0.65
1.00-1.30 0.05 0.61 0.05 0.22 5.71
1.30-1.80+ 1.65 1.94 0.15 0.09 5.81

Footslope: Oxic Dystrustept

0-0.10/0.20 1.23 0.17 0.09 0.11 1.50
0.20-0.20/0.30 0.74 0.11 0.10 0.23 1.40
0.30-0.45 0.10 0.03 0.03 0.38 0.60
0.45-0.80 0.10 0.02 0.04 0.12 0.55
0.80-1.10 0.20 0.05 0.04 0.17 0.60
1.10-1.30+ 0.18 0.08 0.03 0.18 0.60

Toeslope: Aeric Endoagualf

0-0.05 3.39 0.76 0.19 0.46 19.98
0.05-0.23/0.30 13.60 2.79 0.15 2.25 16.71
0.30-0.48 18.62 3.90 0.19 0.08 20.36
0.48-0.80 27.03 4.81 0.19 4.71 22.23
0.80-1.13 18.91 5.01 0.22 0.22 24.48
1.13-1.43 17.67 5.66 0.25 0.28 27.80
1.43-1.62 21.79 6.35 0.27 1.61 27.38
1.62-1.95+ 22.79 6.23 0.28 0.35 28.58

Depth Extractable Fe (B)
(m) [Fe.sub.d] [Fe.sub.o] [Fe.sub.p]

Summit: Typic Kandiustult

0-0.20 0.38 0.23 0.07
0.20-0.40 1.06 0.21 0.04
0.40-0.59 5.12 0.27 0.03
0.59-0.73 7.75 0.29 0.03
0.73-1.05 7.06 0.21 0.02
1.05-1.42 12.19 0.23 0.01
1.42-1.72 11.08 0.23 0.02
1.72-2.05+ 11.24 0.21 0.02

Shoulder: Kandic Paleustalf

0-0.23 1.43 0.20 0.08
0.23-0.35 1.57 0.16 0.05
0.35-0.58 2.41 0.26 0.05
0.58-0.82 5.05 0.32 0.05
0.82-1.10 6.95 0.25 0.04
1.10-1.40 7.18 0.25 0.05
1.40-1.90+ 12.19 0.21 0.03

Upper midslope: Typic Haplustalf

0-0.20/0.30 0.90 0.14 0.08
0.30-0.45 1.10 0.21 0.06
0.45-0.67 1.55 0.20 0.06
0.67-1.00 1.75 0.22 0.05
1.00-1.30 1.72 0.24 0.05
1.30-1.55 2.37 0.25 0.04
1.55-1.82 2.40 0.24 0.04
1.82-1.98 3.07 0.27 0.05
1.98-2.10+ 14.47 0.35 0.05

Lower midslope: Typic Haplustalf

0-0.27/0.35 1.39 0.33 0.13
0.35-0.46 1.61 0.41 0.09
0.46-0.63 1.57 0.35 0.07
0.63-0.80 2.00 0.37 0.07
0.80-1.00 2.62 0.26 0.07
1.00-1.30 27.29 0.32 0.05
1.30-1.80+ -- -- --

Footslope: Oxic Dystrustept

0-0.10/0.20 1.14 0.33 0.25
0.20-0.20/0.30 1.46 0.60 0.28
0.30-0.45 0.72 0.31 0.09
0.45-0.80 0.69 0.36 0.07
0.80-1.10 1.17 0.33 0.06
1.10-1.30+ 1.30 0.33 0.06

Toeslope: Aeric Endoaqualf

0-0.05 2.69 1.15 0.22
0.05-0.23/0.30 15.34 1.33 0.13
0.30-0.48 9.98 0.78 0.08
0.48-0.80 10.43 0.62 0.06
0.80-1.13 10.68 1.03 0.07
1.13-1.43 10.05 0.85 0.08
1.43-1.62 11.37 0.86 0.09
1.62-1.95+ 10.93 0.73 0.07

Depth Sand Silt Clay
(m) (g/kg)

Summit: Typic Kandiustult

0-0.20 851 104 46
0.20-0.40 830 114 56
0.40-0.59 759 110 131
0.59-0.73 724 99 177
0.73-1.05 745 99 157
1.05-1.42 675 98 227
1.42-1.72 668 105 227
1.72-2.05+ 672 95 233

Shoulder: Kandic Paleustalf

0-0.23 832 118 51
0.23-0.35 804 141 56
0.35-0.58 786 144 71
0.58-0.82 727 131 142
0.82-1.10 688 129 182
1.10-1.40 697 115 188
1.40-1.90+ 540 159 301

Upper midslope: Typic Haplustalf

0-0.20/0.30 909 61 30
0.30-0.45 888 87 25
0.45-0.67 864 106 30
0.67-1.00 867 88 45
1.00-1.30 862 88 50
1.30-1.55 862 78 60
1.55-1.82 851 94 55
1.82-1.98 826 119 55
1.98-2.10+ 678 101 221

Lower midslope: Typic Haplustalf

0-0.27/0.35 914 61 25
0.35-0.46 876 84 40
0.46-0.63 874 86 40
0.63-0.80 867 98 35
0.80-1.00 870 75 55
1.00-1.30 784 76 140
1.30-1.80+ 575 79 346

Footslope: Oxic Dystrustept

0-0.10/0.20 895 100 5
0.20-0.20/0.30 894 86 20
0.30-0.45 894 101 5
0.45-0.80 878 117 5
0.80-1.10 892 83 25
1.10-1.30+ 883 112 5

Toeslope: Aeric Endoaqualf

0-0.05 723 191 86
0.05-0.23/0.30 401 245 354
0.30-0.48 334 294 372
0.48-0.80 294 303 403
0.80-1.13 192 400 408
1.13-1.43 92 489 419
1.43-1.62 79 477 444
1.62-1.95+ 57 467 476

(A) (1:1).

(B) [Fe.sub.d], [Fe.sub.o], [Fe.sub.p] extracted
by dithionite citrate bicarbonate (DCB),
ammonium oxalate, and pyrophosphate, respectively.

Table 2. Mineralogy of the fine sand, silt and clay fractions for the
Nam Phong catena Kao, Kaolin; 111, illite; Ver, vermiculite; He,
hematite; Goe, goethite; Q, quartz; -, not detected; tr, <5%; x,
5-20%; xx, 20-40%; xxx, 40-60%; xxxx, >60%Depth(m)

Dept Horizon Clay
(m) Kao Ill Ver

Summit: Typic Kandiustult

0-0.20 Ap xxx -- tr
0.20-0.40 E xxx tr tr
0.59-0.73 Bt2 xxx tr tr
1.42-1.72+ Bt5 xxx tr tr

Shoulder: Kandic Paleustalf

0-0.23 Ap xxxx -- --
0.23-0.35 E xxx tr --
0.58-0.82+ Bt2 xxx tr --

Upper midslope: Typic Haplustalf

0-0.20/0.30 Ap xxx tr --
0.30-0.45 E xxx tr --
0.67-1.00 Bt2 xxx x --
1.55-1.82+ Bt5 xxx x --

Lower midslope: Typic Haplustalf

0-0.27/0.35 Ap xxx x --
0.46-0.63 Bt2 xxx x x
0.80-1.00+ 2BC xxx x x

Footslope: Oxic Dystrustept

0-0.10/0.20 Ap1 xxx x tr
0.30-0.45 Bw xxx xx tr
0.45-0.80 Bt xxx xx tr
1.10-1.30+ Btg2 xxx xx tr

Toeslope: Aeric Endoaqualf

0-0.05 Apg xx xx x
0.05-0.23/0.30 Btcg xx xxx x
0.30-0.48 Btgl xx xxx x
0.80-1.13 Btg3 x xxx xx
1.62-1.95+ BCg2 x xxx xx

Dept Silt Fine sand
(m) He Goe Q Q Q

Summit: Typic Kandiustult

0-0.20 tr x x xxxx xxxx
0.20-0.40 tr x x xxxx xxxx
0.59-0.73 tr x x xxxx xxxx
1.42-1.72+ tr x x xxxx xxxx

Shoulder: Kandic Paleustalf

0-0.23 -- x x xxxx xxxx
0.23-0.35 -- x x xxxx xxxx
0.58-0.82+ -- x x xxxx xxxx

Upper midslope: Typic Haplustalf

0-0.20/0.30 -- tr xx xxxx xxxx
0.30-0.45 -- tr xx xxxx xxxx
0.67-1.00 -- tr x xxxx xxxx
1.55-1.82+ -- tr x xxxx xxxx

Lower midslope: Typic Haplustalf

0-0.27/0.35 -- tr xx xxxx xxxx
0.46-0.63 -- tr x xxxx xxxx
0.80-1.00+ -- tr x xxxx xxxx

Footslope: Oxic Dystrustept

0-0.10/0.20 -- tr xx xxxx xxxx
0.30-0.45 -- tr xx xxxx xxxx
0.45-0.80 -- tr xx xxxx xxxx
1.10-1.30+ -- tr xx xxxx xxxx

Toeslope: Aeric Endoaqualf

0-0.05 -- tr -- xxxx xxxx
0.05-0.23/0.30 -- tr -- xxxx xxxx
0.30-0.48 -- tr -- xxxx xxxx
0.80-1.13 -- -- -- xxxx xxxx
1.62-1.95+ -- -- -- xxxx xxxx
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Article Details
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Author:Thanachit, Suphicha; Suddhiprakarn, Anchalee; Kheoruenromne, Irb; Gilkes. Robert J.
Publication:Australian Journal of Soil Research
Geographic Code:9THAI
Date:Mar 15, 2006
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