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Microstructure and water retention of Oxisols in Thailand.

Introduction

Oxisols generally have a fine texture, an open macrostructure, and stable aggregates which combine to produce favourable water retention and internal drainage characteristics (El-Swaify 1980). These soils display an essentially uniform microstructure with varying degrees of roundness (Schaefer 2001). Well-developed microgranular structure is often found in the subhorizon of Oxisols (Muggier and Buurrnan 2000; Schaefer et al. 2002). They show a unique water retention behaviour. They release water rapidly because of the presence of abundant micropeds (Muller 1983), thus resembling coarse-textured soils. The water content remains high at high pF (log matric potential) due to high intraped microporosity. This behaviour relates to the highly developed structure of these soils and the stability of aggregates, which promotes the formation and preservation of interaggregate pores, and ultramicroscopic intraaggregate pores (Sharma and Uehara 1968; Tsuji et al. 1975).

Thai Oxisols occupy areas of calcareous sedimentary rocks and basalt under ustic and udic soil moisture regimes. Mostly they have clayey textural class and have red or brownish red color. Their properties are generally similar, as described in Soil Taxonomy (Soil Survey Staff 1999). The objective of this study was to determine the moisture characteristics and related pore size distribution of Thai Oxisols and to determine whether Thai Oxisols resemble those investigated elsewhere. These properties are important in determining crop production potential and leaching of agrochemicals and the research will be of value to land users.

Material and methods

Sampling

The 4 study areas with Red Oxisols on calcareous sedimentary rock and basalt are located in the North-east Plateau, South-east Coast, and Peninsular regions of Thailand between 8 and 16[degrees]N and 98 and 104[degrees]E (Fig. 1). The sites in the North-east Plateau (Pc-1, Pc-2, Pc-3) are on a group of grey to dark grey, massive to bedded, limestones of the Permian period. The sites in Peninsular Thailand (Ak-1, Ak-2, Ak-3) are on a group of massive to thick bedded limestone, shale, slaty shale, and bedded chert of the same period. The sites on the North-east Plateau (Ci-1, Ci-2, Ci-3) and South-east Coast (Ti-1 and Ti-2) are on Tertiary to upper Pleistocene basalt (Chonglakmani et al. 1983). The climate of the North-east Plateau is tropical savanna with an annual average temperature of 26-30[degrees]C, and an annual rainfall of 1000-1500mm. Peninsular Thailand and South-east Coast have a tropical monsoonal climate with an annual average temperature of 28-30[degrees]C, and an annual rainfall of 2000-4000 mm.

[FIGURE 1 OMITTED]

Based on the extent and geographic distribution of Oxisols in Thailand (Soil Survey and Classification Division 1999a, 1999b), 11 representative pedons were selected for this study. Pedon analysis in soil profile pits was carried out at each site including detailed profile description and sampling of soil from each genetic horizon using standard field study methods (Soil Survey Staff 1993; Kheoruenromne 1999). Disturbed bulk samples from genetic horizons, soil cores, and Kubiena samples were collected for laboratory analysis. Bulk samples were air-dried, crushed, and then passed through a 2-mm sieve. The resultant <2 mm samples were used for laboratory analysis. The soil core samples were used for physical analysis.

Kubiena samples were transferred to impregnation mould containers, air-dried at 50-60[degrees]C, and impregnated with resin before slicing to prepare uncovered thin sections on glass slides (Cady et al. 1986) for optical microscopy and scanning electron microscopy.

Micromorphology analyses

The micromorphology of selected horizons was investigated with a polarising microscope on resin-impregnated thin sections prepared for optical microscopic examination at high magnification but without coverslips. Pedological features analysed included structural units, porosity, coatings, and related attributes (Bullock et al. 1985) to identify characteristics that are not visible at a field scale. In addition, thin sections were investigated under a JEOL 6400 scanning electron microscope operated at 15 kV electron beam accelerating voltage and 0.45 mA beam current. Materials were analysed to determine their pore size distribution by using the image analysis software Scion Image for Windows (Hein 2001).

Chemical analyses

Soil pH in [H.sub.2]O and in 1 M KCl (soil: solution, 1 : 1) was measured with a standardised pH meter (National Soil Survey Center 1996). Organic carbon was determined according to the Walkley and Black wet oxidation procedure and the organic matter concentration of soils was taken to be the organic carbon concentration multiplied by 1.724 (Nelson and Sommers 1996). Total nitrogen was determined by the Kjeldahl method (Jackson 1965). Cation exchange capacity (CEC) was determined by saturating the exchange sites with an index cation (N[H.sub.4.sup.+]) using 1 M N[H.sub.4]OAc at pH 7.0 and measuring the amount of the index cation (N[H.sub.4.sup.+]) displaced (Chapman 1965).

Physical analyses

Particle size analysis of bulk soil samples was by the pipette method (Gee and Bauder 1986). Bulk density was determined by a core method (Blake and Hartge 1986). The pore size distribution of the bulk soil samples was determined by the measurement of soil water retention curves and was also measured using multipoint BET nitrogen sorption (Aylmore et al. 1970) with a Micromeritics Gemini III 2375 surface area analyser. Soil water retention measurements were made at potentials of -0.1, - 10, -33 (field capacity), and - 100, -500, -1500 kPa (permanent wilting point) on ceramic pressure plates. Constant humidity chambers containing saturated solutions of sodium chloride, calcium nitrate, and zinc chloride provided water vapour pressures equivalent to potentials of -39, -98, and -316MPa, respectively (O'Brien 1948). Plant-available water was calculated as the difference between the water retained at field capacity and permanent wilting point (Klute 1986).

Results and discussion

General soil properties

All Oxisols have developed on mixed colluvium and residuum (Table 1). Their surrounding landforms are mostly undulating, having 1-8% slope. The soils are well drained with high permeability. Land use is mostly field crops under the tropical savanna climate. Para rubber (Hevea Brasilliensis) and fruit trees are the major crops under the tropical monsoonal climate. Colours of Oxisols range from dark reddish brown (5YR 3/4) to dark red (10R 3/6) (Table 2) and these colours generally depend on the Fe content of the parent material. Red Oxisols on limestone have hues of 2.5YR-10R, while the soils on basalt have hues of 5YR-2.5YR. The soils developed on basalt under a tropical monsoonal climate have low chroma and value, probably due to the presence of manganese oxides (Bigham et al. 1978; Paramananthan and Eswaran 1980; Schwertmann 1993; Kampf et al. 2000). The texture of the Oxisols is clay, as is common for Oxisols developed on limestone and basalt (Paramananthan and Eswaran 1980). Oxisols under a tropical savanna climate have moderate to strong angular, semi-angular, and subangular blocky structures trending downward to moderate to strong granular structure with mostly slightly firm to very firm consistence. Oxisols under a tropical monsoonal climate have weak to moderate angular, semi-angular, and subangular blocky structures trending with depth to moderate granular structure with mostly friable to slightly firm consistence in Bto and Bo horizons. These observations are consistent with results of previous studies of the morphology of Oxisols (Paramananthan and Eswaran 1980; Imhoff et al. 2002).

The Oxisols have a kandic horizon underlying a surface horizon with [greater than or equal to] 40% clay, which meets the weatherable mineral properties of an oxic horizon (Soil Survey Staff 1999). Thus, the classification of these soils is Rhodic Kandiustox and Typic Kandiustox for the soils under a tropical savanna climate and Rhodic Kandiudox and Typic Kandiudox for the soils under a tropical monsoonal climate.

Physico-chemical properties

The pH([H.sub.2]0) values of the Oxisols are generally low especially in their subsoils (Table 3). The pH(KCl) values show the same trend but with values about 1 unit less than the pH([H.sub.2]0), reflecting a net negative surface charge and the predominance of acidic cations ([Al.sup.3+] and [H.sup.+]) at exchange sites. The organic matter concentrations in the Oxisols are highest in the surface soil and decrease sharply with depth. The cation exchange capacity (CEC) of the Oxisols is low (Table 3), which is consistent with presence of low activity clay (kaolin, sesquioxides). CEC is not related to soil organic matter concentration, as indicated by Fig. 2, because much of the CEC in these soils is due to kaolin (Hart et al. 2003) and many exchange sites on the organic matter are occupied by [Al.sup.3+], which is not readily exchanged (Schnitzer 1986). The particle size distributions for these soils are shown on a texture triangle (Fig. 3) with the texture of all samples being clay. The basaltic Oxisols have a larger silt-sized fraction than those on calcareous sedimentary rocks, which is consistent with field observations of texture. Bulk density values range from 0.77 to 1.36 Mg/[m.sup.3] and are thus similar to published values for other Oxisols (0.7-1.7 Mg/[m.sup.3], El-Swaify 1980; 0.94-1.36 Mg/[m.sup.3], Larson and Padilla 1990; 0.98-1.68 Mg/[m.sup.3], Neves et al. 2003). The low bulk density values are due to the high porosity arising from the well-structured condition of these soils (El-Swaify 1980).

[FIGURES 2-3 OMITTED]

Micromorphology

Thin sections of soils developed from limestone and basalt (Fig. 4) show complex microstructures (granular and subangular blocky structure) with very small to medium (10-1500 gm) compound packing voids between aggregates. Soils developed on basalt show better microaggregation, with well rounded micropeds (Fig. 4c, d), whereas the Oxisols on limestone are normally less well microaggregated and the granular particles are less rounded and more blocky (Fig. 4a, b) (Bennema et al. 1970; Muggler and Buurman 2000; Schaefer 2001; Schaefer et al. 2002). A thin lining of ferriargillans occurs on ped faces and pore walls as shown in Fig. 4a-c. Oxisols formed under a tropical monsoonal climate consist of randomly oriented combinations of clay particles, organic matter, Fe oxides minerals, and quartz grains (Fig. 4d) (Buol and Eswaran 1978; Santos et al. 1989; Beinroth et al. 2000).

[FIGURE 4 OMITTED]

For the Kandiustox, the uniform iron oxide-kaolinite composition of soil aggregates is clearly shown by the optical, scanning electron, and X-ray images in Fig. 5a, b. Aggregates are mainly composed of Al, Si, and Fe organised into a rather uniform mixture of kaolin and Fe oxide minerals, as all data for the approximately 1-[micro][m.sup.3] volumes analysed by EDS display little variation and fall on the line in the triangular diagram that denotes mixtures of kaolin and iron oxides. The rather uniform kaolin-gibbsite-iron oxide composition of the soil aggregates in Kandiudox (Ak) is illustrated in Fig. 5c where the Al concentration is more than required for kaolin so that data points for the 1-[micro][m.sup.3] analysed volumes plot away from the kaolin line towards the [Al.sub.2][O.sub.3] apex. The iron oxide concentration in the microaggregates is somewhat variable, possibly indicating a more complex pedogenesis than for the Kandiustox. The composition of Kandiudox (Ti) matrix is a quite uniform mixture of iron oxides and kaolin as was the case for the Kandiustox but with a greater variation in [Fe.sub.2][O.sub.3] concentration.

[FIGURE 5 OMITTED]

Water retention characteristics

Moisture retention curves for individual horizons of all profiles are quite similar (Fig. 6), and the curves, particularly those for Bto and Bo horizons, generally display a marked inflection at about pF 3. This reflects the removal of water from the soil at suctions between pF 0 and 2.5, representing drainage of interaggregate pore space in a manner very similar to drainage from sandy-textured soils (Tsuji et al. 1975; Bui et al. 1989). The major difference between curves for sand and Oxisols is that the water content at the end of the inflection remains high with some of this residual water being retained up to pF 6. The water released at pF values >3 is ascribed to intra-aggregate water and this large amount of water is due to the highly developed microstructure of Oxisols. Amounts of water retained at 33 kPa (field capacity) ranged from 22 to 43 (%weight) and at 1500 kPa (permanent wilting point) from 17 to 34 (%weight) and both are high, as is a usual feature of Oxisols (Macedo and Bryant 1987). Water available to plants, which is the difference between these amounts, is only 3-12% for soils wet to field capacity (Table 3). The N2-BET curves for these soils have been plotted on an equivalent scale to the water retention curves (Fig. 6) and are quite consistent, although they plot below water retention curves and have a different modal pore size value. This difference presumably relates to the different reaction of [H.sub.2]O and [N.sub.2] with surfaces of kaolin, iron oxides, and other soil constituents. Most of the BET curves of Ap horizons are lower than the curves for Bto and Bo horizons.

[FIGURE 6 OMITTED]

The water retained at -33 kPa and -1500kPa by these Oxisols is not simply related to clay content (Fig. 7) and the relationships between clay content and water content are quite different to those reported in the literature for other clayey soil types. This difference is presumably due to the particular soil structure of Oxisols. There are no simple relationships between clay content and water retention for these Thai Oxisols. These observations are consistent with data reported by Macedo and Bryant (1987), Larson and Padilla (1990), and Beinroth et al. (2000), who concluded that the available water in clay rich Oxisols is comparable to that of sands despite the large amount of water retained in the soil at field capacity. Thus, these Thai Oxisols resemble some other strongly weathered tropical soils in having water retention properties typical of sand at low matric potentials yet they resemble clay at high matric potentials (Sharma and Uehara 1968; Sanchez 1976; Holzhey and Kimble 1988).

[FIGURE 7 OMITTED]

The water retention curves for Oxisols can be compared to other clay-rich soils, as indicated in Fig. 6 for USA Udoll (43% clay) (Larson and Padilla 1990) and Brazil Humic Ferralsol (a Ferralsol is the FAO equivalent of an Oxisol) (82% clay) (van den Berg et al. 1997). Both soils retain about half as much water at field capacity as do the Oxisols. The Udoll does retain much of its water at pF 4 and so resembles the Oxisols although without the marked inflection in the characteristic curve. The Humic Ferralsol has released much of its water at pF values <4 with the curve being parallel to the Oxisol curves over this region, even to the extent of showing an inflection at pF ~2. A water retention curve for a Brazil Ustox (Bui et al. 1989) is also shown in Fig. 6 and this exhibits the loss of most water at pF ~2, and the curve is approximately parallel to the Oxisol curves.

Porosity

Pore size frequency distributions (Fig. 8) estimated by manually differentiating the water retention curves (Childs and Collis-George 1950) show a distinct bimodal pore size distribution for all soils with one mode at about 20 [micro]m, especially in Bto and Bo horizons, corresponding to interaggregate pores and another mode at about 0.02 gm corresponding to intraaggregate pores. The Kandiustox (Ci) samples have slightly more interaggregate pores than for the other soils (Fig. 9a) and it has been suggested that micropeds are better developed in soils under ustic relative to udic moisture regimes (Buol and Eswaran 1978). The data from the literature included in Fig. 8 have much smaller amounts of interaggregate pores than do the Oxisols [USA Udoll; (Larson and Padilla 1990), Brazil Ferralsol (van den Berg et al. 1997)]. The pore size distributions derived by the BET method shows abundant pores in the size range 0.02-0.002 [micro]m with a maximum (i.e. the modal value) at about 0.01 [micro]m. Pore volume is less than is estimated from the water retention data and the modal size is smaller.

[FIGURES 8-9 OMITTED]

Cryptopores are <0.1[micro]m in size (Soil Science Society of America 1997) and correspond to those intraaggregate pores (Fig. 8) that are formed by the arrangement of clay size particles, which in the case of these Oxisols has been investigated by TEM and consist predominantly of 60 x 10nm platy kaolinite crystals and ~25nm almost equant iron oxide crystals (Hart et al. 2003) (Fig. 10).

[FIGURE 10 OMITTED]

The very low bulk density of these soils (0.77-1.36 Mg/[m.sup.3]) means that the kaolin crystals (2.5Mg/[m.sup.3]), which comprise ~80% of the matrix (Fig. 5), and iron oxide crystals (5.3Mg/[m.sup.3]), which comprise most of the remainder of the matrix, are organised into an open 'card house' arrangement (Fig. 11). In such an arrangement the pore sizes are similar to the crystal sizes (i.e. ~10-50nm), which coincides with the peak sizes of cryptopores determined from the differential water retention (~20 nm) and BET (~10nm) plots in Fig. 8.

[FIGURE 11 OMITTED]

Cryptopores contribute 23-35% of the total porosity of these Oxisols and these pores, which are <5 [micro]m cannot be imaged by SEM on thin sections. The relative abundance of macropores, mesopores and micropores was estimated using image analysis of SEM (Fig. 12). The Oxisols have a total volume of pores >5[micro]m of ~20-50%, which is much lower than the total pore volume estimated from water retention measurements (40~0%) due to the high abundance of cryptopores. The relative proportions of micro- and meso-pores remain constant with macropores varying in abundance from 30-90% of the measured porosity. There are no systematic differences in the proportions of the four pore types between the four soils (Fig. 9b).

[FIGURE 12 OMITTED]

Conclusions

These Thai Oxisols show differences of microstructure that reflect the limestone and basalt parent materials and different soil moisture regimes. Kandiustox developed on basalt (Ci) has better microaggregation. The matrix of the soils is a uniform mixture of iron oxide and kaolin except for a Kandiudox derived from limestone (Ak), which has a kaolin-gibbsite-iron oxide composition. Water retention curves for individual horizons of all profiles are similar, showing an inflection at suctions pF 0-2.5 and a second inflection at values >pF 3. The amounts of water retained at field capacity and at permanent wilting point are high, in the range 22-43 and 17-34 (%weight), respectively. The amount of water available to plants is only 3-12%. There are no consistent relationships between water retained at either -33 or -1500kPa and clay content. The pore size distribution estimated by the water retention curves show a distinct bimodal pore size distribution with one mode at about 20 [micro]m corresponding to interaggregate pores and another mode at about 0.02 [micro]m corresponding to intraaggregate pores. The Typic Kandiustox (Ci) derived from basalt has a larger amount of interaggregate pores than the other studied soils as a result of better microaggregation. The pore volume derived from the BET method is less than estimated from water retention and the pore size distribution indicates the presence of abundant pores in the size range 0.02-0.002 [micro]m. Cryptopores in these Oxisols contribute 23-35% of the total porosity. The Typic Kandiustox (Ci, basalt) has the lowest proportion of cryptopores compared with the other studied soils.

These data have provided a clear definition of the structure (fabric) of Thai Oxisols at resolutions from nanometer to millimetre. The unusual water retention characteristics of these soils can be readily understood by reference to the existence of stable soil structure at several orders of size. In particular, the large amount of water released at high pF values is simply due to the stable, card-house arrangement of nanometric kaolin and iron oxide crystals.

Acknowledgments

This research was supported by the Royal Golden Jubilee PhD Program of The Thailand Research Fund. We would like to thank Centre of Microscopy and Microanalysis, UWA for laboratory facilities and Mr Michael Smirk of UWA for assistance.

Manuscript received 29 March 2005, accepted 15 August 2005

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S. Tawornpruek (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 Geographical Sciences, Faculty of Natural and Agricultural Science, University of Western Australia, Crawley, WA 6009, Australia.

(C) Corresponding author. Email: irbs@ku.ac.th
Table 1. Environmental setting of Oxisols on limestone and basalt
in Thailand

Profile Physiographic Probable Surrounding
 position parent material land form

 Kandiustox (B)
Typic Kandiustox
Pc-1 Karst corrosion Residuum derived Gently
 flat footslope from limestone undulating
Rhodic Kandiustox
Pc-2 Karst corrosion Residuum derived Gently
 plain from limestone undulating
Typic Kandiustox
Pc-3 Karst corrosion Residuum derived Slightly
 plain on from limestone undulating
 perimeter of
 buried lapies

 Kandiudox
Typic Kandiudox
Ak-1 Crestal slope of Residuum derived Rolling
 residual hill in from mainly
 karst corrosion limestone some
 plain marl
Typic Kandiudox
Ak-2 Karst corrosion Residuum derived Slightly
 plain from limestone undulating
Rhodic Kandiudox
Ak-3 Rise crestal slope Residuum derived Undulating
 in karst corrosion from limestone
 plain

 Kandiustox
Rhodic Kandiustox
Ci-1 Lava corrosion Residuum derived Gently
 plain from weathered undulating
 basalt
Typic Kandiustox
Ci-2 Lava corrosion Colluvium and Gently
 plain residuum derived undulating
 from weathered
 basalt
Rhodic Kandiustox
Ci-3 Top of lava Residuum derived Gently
 corrosion plain from weathered undulating
 basalt
 Kandiudox
Rhodic Kandiudox
Ti-1 Upper dissected Residuum derived Slightly
 footslope of from weathered undulating
 lava corrosion basalt
 hill
Typic Kandiudox
Ti-2 Top of dissected Residuum derived Undulating
 lava corrosion from weathered
 plain basalt

Profile Slope Permeability (A) Mean annual
 (%) rainfall/mean
 evaporation

 Kandiustox (B)
Typic Kandiustox
Pc-1 2 Moderate 1035 mm/1817 mm
Rhodic Kandiustox
Pc-2 2 Moderate 1035 mm/1817 mm
Typic Kandiustox
Pc-3 2 Moderate 1035 mm/1817 mm

 Kandiudox
Typic Kandiudox
Ak-1 8 Moderate 2171 mm/ 1482 mm
Typic Kandiudox
Ak-2 3 Moderate 1883 mm/1381 mm
Rhodic Kandiudox
Ak-3 2 Moderate 1883 mm/1381 mm

 Kandiustox
Rhodic Kandiustox
Ci-1 1 Rapid 1300mm/1793 mm
Typic Kandiustox
Ci-2 1.5 Rapid 1300 mm/ 1793 mm
Rhodic Kandiustox
Ci-3 1 Rapid 1300 mm/1793 mm
Rhodic Kandiudox
Ti-1 3 Rapid 3030 mm/1525 mm
Typic Kandiudox
Ti-2 3 Rapid 3030 mm/1525 mm

Profile Land use

 Kandiustox (B)

Typic Kandiustox
Pc-1 Fast growing trees,
 olive, corn, mango
Rhodic Kandiustox
Pc-2 Corn production
 experimental plot
Typic Kandiustox
Pc-3 Left idle under
 grasses and
 bamboo

 Kandiudox
Typic Kandiudox
Ak-1 Tropical rainforest
 species, para
 rubber, coconut
 and banana
Typic Kandiudox
Ak-2 Tropical rainforest
 species and
 tropical orchards
Rhodic Kandiudox
Ak-3 Tropical rainforest
 species and
 tropical orchards

 Kandiustox
Rhodic Kandiustox
Ci-1 Cassava field
Typic Kandiustox
Ci-2 Cassava field
Rhodic Kandiustox
Ci-3 Cassava field
Rhodic Kandiudox
Ti-1 Tropical orchards
Typic Kandiudox
Ti-2 Tropical orchards

(A) Field estimation (Kheoruenromne 1999; Schoeneberger et al. 2002).

(B) Taxonomic (Soil Survey Staff 1999) equivalent to Ferralsols
(FAO 1998).

Table 2. Morphology of Oxisols from Thailand

Genetic Depth Color
horizon (m)

 Typic Kandiustox (Pc-1)

Ap 0-0.35 5YR 4/4 to 3/4
Bt 0.35-1.15 2.5YR 4/6 to 5/8
Bto 1.15-2.10+ 2.5YR 4/6 to 3/6

 Rhodic Kandiustox (Pc-2)

Ap 0-0.20 2.5YR 3/4
Bt 0.20-0.60 2.5YR 3/3 to 3/4
Bto 0.60-2.10+ 2.5YR 2.5/4

 Rhodic Kandiustox (Pc-3)

Ap (A) 0-0.30 2.5YR 2.5/4
Bt 0.30-1.40 2.5YR 3/6 to 10R 4/8
Bto 1.40-2.10+ 2.5YR 3/6 to 2.5YR 2.5/4 2

 Typic Kandiudox (Ak-1)

Ap 0-0.10 5YR 4/6
Bto 0.10-2.00+ 2.5YR 4/6

 Typic Kandiudox (Ak-2)

Ap 0-0.17 2.5YR 4/6
Bto 0.17-2.00+ 2.5YR 4/6

 Rhodic Kandiudox (Ak-3)

Ap 0-0.18 2.5YR 3/4
Bto 0.18-1.23 2.5YR 3/4
Bo 1.23-1.90+ 2.5YR 3/6

 Rhodic Kandiustox (Ci-1)

Ap 0-0.27 2.5YR 3/4 to 2.5/4
Bto 0.27-1.10 2.5YR 2.5/4
Bo 1.10-2.00+ 2.5YR 2.5/3

 Typic Kandiustox (Ci-2)

Ap 0-0.10 2.5YR 3/6
Bto 0.10-0.76 2.5YR 4/6
Bo 0.76-1.67+ 2.5YR 4/6 to 3/6

 Rhodic Kandiustox (Ci-3)

Ap 0-0.30 10R 3/3
Bto 0.30-1.30 10R 3/4
Bo 0.30-1.85+ 10R 3/4

 Rhodic Kandiudox (Ti-1)

Ap 0-0.27 5YR 3/4
Bto 0.27-0.96 5YR 3/4 to 2.5YR 3/4
Bo 96-200+ 2.5YR 3/4

 Typic Kandiudox (Ti-2)

Ap 0-0.14/0.16 5YR 3/4
Bto 0.16-0.95 5YR 3/4
Bo 0.95-2.00+ 2.5YR 3/4

Genetic Structure A Coatings
horizon

 Typic Kandiustox (Pc-1)

Ap 3 SBK [+ or -] 3G to 3 semi-ABK Very few to common
Bt 3 semi-ABK Common to many
Bto 3 semi-ABK Common to many

 Rhodic Kandiustox (Pc-2)

Ap 3 SBK [+ or -] 3G Very few
Bt 3 semi-ABK Common to many
Bto 3 semi-ABK Common

 Rhodic Kandiustox (Pc-3)

Ap (A) 3 SBK [+ or -] 3G Few
Bt 3 semi-ABK Common
Bto SBK [+ or -] 2G Common

 Typic Kandiudox (Ak-1)

Ap 3G --
Bto 1-2 ABK with 1-2 SBK [+ or -] 2-3G Common

 Typic Kandiudox (Ak-2)

Ap 3 SBK [+ or -] 3G and 2 SBK Common
Bto 1-2 SBK [+ or -] 3G Common

 Rhodic Kandiudox (Ak-3)

Ap 3 SBK Many
Bto 2 semi-ABK to 2 SBK [+ or -] 2G Many
Bo 1-2 semi-ABK [+ or -] 2G Common

 Rhodic Kandiustox (Ci-1)

Ap 3 SBK to semi ABK --
Bto 3 SBK [+ or -] G Few
Bo 2 SBK [+ or -] 3G --

 Typic Kandiustox (Ci-2)

Ap 3 SBK [+ or -] 3G --
Bto 2-3 SBK [+ or -] 3G Common
Bo 2-3 semi-ABK [+ or -] 3G Very few

 Rhodic Kandiustox (Ci-3)

Ap 2 SBK [+ or -] 3G --
Bto 3 ABK Common
Bo 2-3 SBK [+ or -] 2G Common

 Rhodic Kandiudox (Ti-1)

Ap 3 SBK[+ or -]3G Few
Bto 3 semi-ABK [+ or -] 3G Common
Bo 2 semi-ABK [+ or -] 2G Common

 Typic Kandiudox (Ti-2)

Ap 3SBK [+ or -] 3G Few
Bto 2-3 semi-ABK [+ or -] 2G Few to common
Bo 2semi-ABK [+ or -] 2G Common

Genetic Pores
horizon

 Typic Kandiustox (Pc-1)

Ap Many
Bt Many
Bto Common to many

 Rhodic Kandiustox (Pc-2)

Ap Common
Bt Common
Bto Many

 Rhodic Kandiustox (Pc-3)

Ap (A) Many
Bt Many
Bto Many

 Typic Kandiudox (Ak-1)

Ap Many
Bto Common to many

 Typic Kandiudox (Ak-2)

Ap Many
Bto Many

 Rhodic Kandiudox (Ak-3)

Ap Many
Bto Common
Bo Many

 Rhodic Kandiustox (Ci-1)

Ap Many
Bto Many
Bo Many

 Typic Kandiustox (Ci-2)

Ap Many
Bto Many
Bo Many

 Rhodic Kandiustox (Ci-3)

Ap Many
Bto Many
Bo Many

 Rhodic Kandiudox (Ti-1)

Ap Many
Bto Many
Bo Many

 Typic Kandiudox (Ti-2)

Ap Many
Bto Many
Bo Many

(A) 1 = Weak, 2 = moderate, 3 = strong, ABK = angular blocky,
SBK = subangular blocky, G = granular; [+ or -] indicates primary
structure that parts to secondary structure when ruptured.

Table 3. Chemical and physical properties of Oxisols from Thailand

 CEC
Genetic Depth pH (1: 1) OM ([cmol.sub.
horizon (m) [H.sub.O] KCl (g/kg) c]./kg)

 Typic Kandiustox (Pc-1)

Ap 0-0.35 6.3 6.0 34.3 20
Bt 0.35-1.15 5.2 4.5 4.3 13
Bto 1.15-2.10+ 4.7 3.8 2.6 15

 Rhodic Kandiustox Pc-2)

Ap 0-0.20 7.0 6.3 35.9 15
Bt 0.20-0.60 6.7 5.8 13.2 14
Bto 0.60-2.10+ 4.7 3.9 4.1 11

 Rhodic Kandiustox (Pc-3

Ap (A) 0-0.30 7.6 7.0 26.1 13
Bt 0.30-1.40 5.2 4.3 6.6 11
Bto 1.40-2.10+ 4.9 3.7 5.2 10

 Typic Kandiudox (Ak-1)

Ap 0-0.10 6.0 4.6 24.9 13
Bto 0.10-2.00+ 5.8 5.2 6.8 5

 Typic Kandiudox (Ak-2)

Ap 0-0.17 6.5 5.6 17.7 9
Bto 0.17-2.00+ 5.4 3.7 5.5 8

 Rhodic Kandiudox (Ak-3)

Ap 0-0.18 5.3 3.8 11.6 11
Bto 0.18-1.23 5.4 4.0 3.0 10
Bo 1.23-1.90+ 5.4 4.0 1.3 6

 Rhodic Kandiustox (Ci-1)

Ap 0-0.27 4.4 3.6 18.7 8
Bto 0.27-1.10 4.6 3.7 6.7 8
Bo 1.10-2.00+ 4.6 3.7 4.1 7

 Typic Kandiustox (Ci-2)

Ap 0-0.10 5.5 4.5 15.7 10
Bto 0.10-0.76 4.8 3.8 5.9 8
Bo 0.76-1.67+ 4.7 3.9 3.9 7

 Rhodic Kandiustox (Ci-3)

Ap 0-0.30 4.4 3.8 22.1 8
Bto 0.30-1.30 4.5 3.7 10.6 8
Bo 0.30-1.85+ 4.9 3.9 4.0 6

 Rhodic Kandiudox (Ti-1)

Ap 0-0.27 5.0 4.4 36.7 13
Bto 0.27-0.96 5.1 4.7 7.3 11
Bo 0.96-2.00+ 4.8 4.4 3.8 10

 Typic Kandiudox (Ti-2)

Ap 0-0.14/0.16 5.3 4.6 34.2 16
Bto 0.16-0.95 4.6 4.2 12.9 12
Bo 0.95-2.00+ 4.5 4.5 3.3 8

Genetic Bulk density Water content (%weight) at: AWC (A)
horizon (Mg/[m.sup.3]) -33kPa -1500kPa (%)

 Typic Kandiustox (Pc-1)

Ap 1.15 32.9 26.6 6.3
Bt 1.21 37.6 28.3 9.3
Bto 1.17 36.7 28.2 8.5

 Rhodic Kandiustox Pc-2)

Ap 1.13 32.9 25.0 8.0
Bt 1.25 29.6 24.2 5.4
Bto 1.16 34.7 26.6 8.1

 Rhodic Kandiustox (Pc-3)

Ap (A) 1.27 30.3 23.0 7.3
Bt 1.13 33.5 25.4 8.1
Bto 1.11 34.2 24.8 9.3

 Typic Kandiudox (Ak-1)

Ap 0.77 29.5 23.1 6.3
Bto 1.10 33.4 26.6 6.7

 Typic Kandiudox (Ak-2)

Ap 1.25 28.6 21.9 6.7
Bto 1.15 33.6 27.7 5.7

 Rhodic Kandiudox (Ak-3)

Ap 1.36 22.1 17.1 5.0
Bto 1.22 29.2 23.4 5.8
Bo 1.09 29.1 21.7 7.4

 Rhodic Kandiustox (Ci-1)

Ap 1.12 28.3 19.1 9.2
Bto 1.01 29.5 22.9 6.5
Bo 1.02 29.1 22.2 6.9

 Typic Kandiustox (Ci-2)

Ap 0.95 25.3 22.4 3.0
Bto 1.05 28.7 19.4 9.3
Bo 1.08 27.7 19.3 8.4

 Rhodic Kandiustox (Ci-3)

Ap 1.11 28.7 22.2 6.5
Bto 1.15 29.4 20.4 9.1
Bo 1.08 29.2 22.3 6.9

 Rhodic Kandiudox (Ti-1)

Ap 0.81 43.2 32.8 10.3
Bto 0.80 41.8 33.9 7.9
Bo 0.81 40.6 31.3 9.3

 Typic Kandiudox (Ti-2)

Ap 0.79 38.9 32.6 6.3
Bto 0.82 38.7 26.8 11.9
Bo 0.85 39.8 29.1 10.7

(A) AWC, Available water content.
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Author:Tawornpruek, S.; Kheoruenromne, I.; Suddhiprakarn, A.; Gilkes, R.J.
Publication:Australian Journal of Soil Research
Geographic Code:9THAI
Date:Mar 1, 2006
Words:6558
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