Aggregate stability of salt-affected kaolinitic soils on the North-east Plateau, Thailand.
Soil aggregate stability is an important property of clay-rich soil and affects sustainability and crop production. Soil aggregates are held together by various organic and inorganic materials and through several mechanisms (Ben-Hur et al. 1985; Chan and Mullins 1994; Denef and Six 2005). Consequently, many factors affect soil aggregate stability including both internal and external factors. Internal factors include electrolyte concentration, exchange cation species, clay content, clay mineralogy, and contents of carbonate, organic matter, and iron- and aluminum-oxides. External factors include climate, biological activity, and agricultural management (Goldberg et al. 1988; Amezketa 1999; Levy 2000).
Clay flocculation is a pre-requisite for the formation and preservation of soil aggregates (Dexter 1988). Factors that strongly affect clay flocculation are clay mineralogy, electrolyte concentration, pH, sodium adsorption ratio (SAR), and exchangeable sodium percentage (ESP) (Abu-Sharar et al. 1978; Rengasamy and Olsson 1991; Levy 2000).
In mildly salt-affected soils where halite is the major salt species, aggregates are generally unstable due to the presence of high amounts of dissolved sodium, which causes sodieity (Tisdall and Oades 1982) and associated swelling and dispersion of clay particles (Crescimanno et al. 1995). Exchangeable sodium weakens covalent associations between organic materials and soil minerals and increases the osmotic forces that cause particle repulsion during wetting. Even partial sodium saturation greatly increases dispersion of macroaggregates and microaggregates (Rengasamy and Olsson 1991; Levy 2000).
Soil mineralogy has a substantial effect on aggregate stability and dispersion. Smectitic soils are the most dispersive, followed by illitic and kaolinitic soils (Singer 1994). Generally kaolinitic soils have the greatest aggregate stability (Wakindiki and Ben-Hur 2002) and highly smectitic soils are more erodible than the soils that contain only minor amounts of smectite (Stem et al. 1991). Aggregate stability is also influenced by clay content, and aggregate stability is closely related to sodicity only for clayey soils (Frenkel et al. 1978; Levy and Mamedov 2002). As natural wetting of soil aggregates is the major process that affects soil structural stability, a commonly used approach to measuring aggregate stability involves sieving wet soils and estimating the dispersion of clay from aggregates (Kemper and Rosenau 1986). This procedure was followed in this research.
Salt-affected soils with halite as the major salt occupy 3.61 Mha of Thailand with 2.85 Mha occurring in the Northeast Plateau of Thailand (Arunin 1992). The underlying salt-bearing rocks, saline groundwater, strongly seasonal climate, topographic conditions, and land utilisation for paddy rice have variously contributed to salinisation of soils in this region over a long period of time (Takai et al. 1987). In contrast to the clear spatial distribution of salt-affected soils in Thailand, the extent and severity of associated sodicity are poorly understood. The salinity and sodicity of both surface and subsoils are increasing, with ~75% of the salt-affected area being under lowland rice cultivation (Ghassemi et al. 1995). The impact of sodicity on management of rice paddies is significant as extensive manipulation of wet soil profiles is involved (Adachi 1990; So and Ringrose-Voase 2000). There is much information on the stability of salt-affected soil elsewhere in the world, especially under semi-arid and Mediterranean climates, but there is little known of the effects of soil salinity on the structural stability of soils under a tropical savanna climate. Furthermore, there is scant literature on the impact of sodicity on kaolinitic soils which predominate in this region of Thailand (Kanket et al. 2005). Thus, the main objectives of this study were to determine the properties of representative salt -affected soils from Thailand and to establish if relationships between aggregate stability and soil properties are similar to those existing elsewhere.
Materials and methods
The study area is in the North-east Plateau, Thailand (Fig. 1), under a tropical savanna climate with a mean annual rainfall of 900-1300 mm and mean annual temperature of 27°C. Highly and moderately salt-affected soils mostly occur as small areas in the western fringe of this region, whereas slightly salt-affected and sodic soils are more widely distributed. The main bedrocks underlying the area are siltstone, claystone, and sandstone with frequent occurrences of rock salt, potash, gypsum, and anhydrite. The salt-beating beds belong mainly to the upper part of the Maha Sarakham Formation and consist of clay-rich sedimentary rocks of Cretaceous-Tertiary age (Geological Survey Division Staff 1985; Dheeradilok et al. 1992).
Seven pedons of salt-affected soils which are representative of the clayey kaolinitic soils present in this region were used for this study. The soils have developed from alluvium or wash over residuum derived from clastic sedimentary rocks on either floodplain or erosional plain landforms. Land use at the time of sampling was paddy rice or former paddy left idle due to salinity, with local grasses, salt-tolerant grasses and Eucalyptus species. Detailed information on the soils (Table 1) includes sampling locations, parent material, physiographic setting, and taxonomic names (Soil Survey Staff 2006).
Pedon analysis was carried out in soil pits at each site including detailed profile description and sampling of soil from each genetic horizon using standard field study methods (Soil Survey Division Staff 1993; Kheoruenromne 1999). Subsoils were also sampled for this study of soil structural stability as rice growing and construction of paddies involve manipulation and cultivation of both subsoil and topsoil materials. Soil samples were air-dried then sieved after light crushing through a 2-mm sieve for physical, chemical, and mineralogical analysis. Soil aggregates of 1-2 mm, which are the most abundant aggregate size class, were separated by sieving for analysis of aggregate stability.
Particle size distribution was determined by a combination of sieve and pipette analysis (Gee and Bauder 1986). Soil pH was determined for a saturated paste, 1 : 1 soil : water mixture and 1 : 1 soil : KC1 mixture with a pH meter (National Soil Survey Center 1996). Organic carbon was determined by the Walkley Black wet oxidation procedure (Nelson and Sommers 1996). Cation exchange capacity (CEC-7) was measured by first washing the soil free of excess salt, 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, displacing the adsorbed index cation (N[H.sub.4.sup.+]) with NaC1, and measuring the amount of the index cation (N[H.sub.4.sup.+]) displaced (Rhoades 1982). Exchangeable Na, K, Ca, and Mg were determined by subtracting the water-soluble amounts from the extractable amounts. The extractable amounts were determined during the CEC procedure; the cations were measured by atomic absorption spectrometry (Rhoades 1982; National Soil Survey Center 1996). The ESP was calculated as the exchangeable sodium (ES) divided by the CEC-7 value and multiplied by 100. The SAR was calculated by dividing the molar concentration of [Na.sup.+] in the saturation extract by the square-root of the sum of the molar concentrations of [Ca.sup.2+] plus [Mg.sup.2+] ([cmol.sub.c]/kg) (United States Salinity Laboratory Staff 1954). Electrical conductivity (EC) of a saturation extract at 25°C was measured with an electrical-conductivity bridge. The threshold levels of ESP, SAR, and EC for salt affected soils are 15%, 13, and 4 dS/m, respectively (Soil Survey Staff 2006).
X-ray diffraction analysis (XRD) was used to identify and to make semi-quantitative measurements of the crystalline mineral components of the clay fraction. The clay fraction for mineralogical analysis was separated by using a sedimentation procedure and clay minerals were identified using XRD of oriented clay separately pretreated with [Mg.sup.2+] and [K.sup.+] on ceramic plates, 50% glycerol solution of the [Mg.sup.2+]-saturated plate, and the [K.sup.+]-saturated plate heated to 550°C. XRD analysis of the clay samples was carried out using monochromatised CuKa radiation with a Philips PW-3020 diffractometer (50kV, 20 mA). Clay fractions were scanned from 4 to 45° 2?, using a step size of 0.02° 2? and a scan speed of 0.04° 2?/s. Relative proportions of various minerals were calculated by comparing the XRD peak intensities with the intensities for standard minerals (Brown and Brindley 1980).
Wet aggregate stability analysis
Aggregate stability was determined following the principle that unstable aggregates will break down more easily than stable aggregates when dry aggregates are immersed in water. For this study, the wet-sieving method described by Kemper and Rosenau (1986) was used to determine aggregate stability. Aggregates of 1-2 mm size have been found by these and other workers (Seybold and Herrick 2001; Ruiz-Vera and Wu 2006) to be best suited to the screen size and sieving speed employed in this procedure.
Four grams of 1-2-mm air-dried soil aggregates in a sieve were slowly pre-moistened with distilled water to prevent slaking of the aggregates, then the filled sieve was placed into a can filled with distilled water. To determine aggregate stability, the sieve (60 mesh) was immersed for 5 min then lowered and raised for 3 min with a stroke of 1.3 cm, at ~34 strokes/min. Unstable aggregates slaked and/or dispersed and resultant fine materials passed through the sieve and were collected in the water-filled can underneath the sieve. After this fixed time, the liquid was removed and water containing 2 g/L of sodium hexametaphosphate was added to the can for soils with pH >7, or 2 g/L of NaOH for soils with pH <7, and the remaining soil was wet sieved to disperse remaining aggregates and separate the sand fraction from the silt and clay fractions. The aggregates that remained stable after 3 min of sieving were gently rubbed across the screen with a rubber-tipped rod so that all aggregates were destroyed, and the fine residue on the sieve was washed into the collecting can. Sand grains and plant roots remained on the sieve so that the mass of aggregates could be calculated by difference. After drying the suspensions with the disaggregated material from the aggregates, the weight of both stable and unstable aggregates was determined. Dividing the weight of stable aggregates by the total aggregate weight gives an index of aggregate stability. Statistical analysis of data was limited to regression analysis and utilised Statview software (StatSoft, Inc. 2003).
Results and discussion
Profile development features of the soils (Table 1) are Apng-Btg/Btng-2Btng, except for Pedon 4 where they are Apg-Btg2Btg. All soils in this study are deep and several profiles exhibit clay accumulation in their subsoils relative to topsoil due to clay illuviation. Most soils except for Pedon 4 show evidence of sodicity in both A and B horizons. The soils commonly have a blocky and semi-massive structure in some horizons. These features are clearly seen in thin section (Fig. 2). The soil shows a common channel structure with moderate subangular blocky peds. Subsoils have clay coatings on the walls of voids and on quartz and bridged grains. Halite occurs in voids as a common crystalline pedofeature and is generally impure. Soil matrix colour is mostly grey and brown with low chroma (<2) and all soils are mottled indicating poor drainage and profile development under the periodic water-saturated conditions (Buol et al. 2003) that are imposed by paddy management.
The particle size distribution data (Figs 3 and 4, Table 2) indicate that the soils are fine-textured, being mostly clays and clay loams with clay contents ranging from 313 to 849 g/kg. However, some horizons are sandy clay loam in the lower part of Pedons 1, 2, and 3 due to variations in composition of the underlying residual layers.
Based on their chemical data (Fig. 5, Table 2) and criteria of salt-affected soils classification of the National Soil Survey Center (1996) and Soil Survey Staff (2006), these soils are salt-affected, and on the basis of EC and ESP values, many of the soil samples are saline-sodic (Fig. 5). However, some horizons of some soil profiles may be classified as saline, sodic or normal. Sodicity ranges from low to very high (ESP 5-31%). Salinity (EC) ranges from 0.6 to 16.2 dS/m with many samples having values >4 dS/m.
Most of the soil samples are acid (pH 4.9-6.5) (Table 2), although slightly alkaline soils (pH 7.4-7.7) occur in Pedon 7 and some horizons of Pedon 6, and several horizons are neutral (pH 6.6-7.3) (Soil Survey Division Staff 1993). The pH values measured in KC1 are smaller than those measured in water, indicating that the minerals have a net negative charge (Beery and Wilding 1971; Soil Survey Staff 2006). Soil organic matter contents are low and decreasing with depth. The soils have moderately low to high CEC values, ranging from 7.1 to 31.4 cmolc/kg and the variation in CEC closely reflects the clay content of the soils. Furthermore, higher CEC values in some of these predominantly kaolinitic soils reflect the presence of minor amounts of 2 : 1 type clay minerals. XRD shows that the dominant mineral in the clay fraction of all soils is kaolinite with sometimes minor smectite and traces of illite and a little quartz. Total exchangeable bases increase with increasing clay content. Sodium and calcium are the major exchangeable bases in these soils, with sodium having very high values in some instances. The extractable acidity (EA) of these soils is medium to high except for some samples from Pedon 9. EA is linearly related to CEC by sum (r=+0.72) and also increases with increasing organic matter (r=+0.81). All soils have base saturation values >35%.
Aggregate stability and its relationship with soil properties
The aggregate stability percentage for these soils ranges from 3 to 91% (Fig. 6) with subsoil aggregates mostly being more stable than topsoil aggregates. More stable aggregates in the subsoils are due to clay acting as a cementing material as is commonly the case when the soil contains >20% clay (Ben-Hur et al. 1985). The lower aggregate stability in some horizons may be due to the presence of minor amounts of smectite and illite (Wakindiki and Ben-Hur 2002). The relatively low stability of topsoil aggregates may be due to the soil being regularly disturbed by ploughing and puddling during rice cultivation.
Based on the bivarate relationships between aggregate stability and some soil properties (Fig. 7), the soil property that is most strongly correlated with aggregate stability is pH (r=-0.84) followed by exchangeable K (r=+0.75), CEC by sum (r-+0.73), clay content (r=+0.72), extractable acidity (r=-+0.66), and organic matter (r=+0.66). Aggregate stability is not significantly related to ESP, SAR, and EC for these soils. This result contrasts with the findings of many studies that have reported highly significant negative relationships between both ESP and SAR and aggregate stability (e.g. Rengasamy and Olsson 1991; Levy et al. 1993; Levy 2000; Garcia-Orenes et al. 2005).
However, some studies that have included a wide range of soil textures as is the case with these Thai soils have also observed that there was no relationship between aggregate stability and ESP (Coughlan and Loch 1984; Goldberg et al. 1988). For example, aggregate stability in fine kaolinitic, Typic Haploxerults was not significantly affected by increasing SAR (Ruiz-Vera and Wu 2006) and there was no relationship between the EC and aggregate stability for tropical Ultisols from Malaysian Borneo (Chappell et al. 1999). The published data for tropical soils are thus consistent with the present results. These results are also consistent with the known influence of clay mineralogy on soil dispersivity (Oades and Waters 1991; Levy et al. 1993). The structural stability of kaolin-dominated soils is high, which is attributed to the binding capacity of the minerals. Kaolinite is usually in a flocculated state because at normal soil pH values the attraction of the positively charged edges to the negatively charged planar surfaces of crystals gives rise to an edge-to-face flocculation mode. However, Ruiz-Vera and Wu (2006) stated that because kaolin has a much lower charge density than smectite, bonds are weak in kaolinitic soil aggregates.
For these Thai soils aggregate stability shows a strong negative relationship with soil pH as has been reported by some other workers (Suarez et al. 1984; Keren et al. 1988; Chorom et al. 1994; Lieffering and McLay 1996), who suggested that with increasing soil pH, clay dispersion occurs as a result of increased repulsion of negatively charged clay particles. Additionally, Levy and Miller (1997) found that salinity does not affect dispersion where soil pH is <6.5.
Aggregate stability increases with increasing clay content for these Thai soils and many other studies have also demonstrated a positive relationship between clay content and aggregate stability so that clay may be considered to be a dominant cementing agent (Gollany et al. 1991; Shainberg et al. 1992; Curtin et al. 1994; Levy et al. 2003). However, the cementing effect of clay depends inter alia on its mineralogy. At high SAR values and low electrolyte concentrations, soils with high contents of kaolinite and sesquioxides are relatively stable (McNeal et al. 1966) with little swelling and dispersion (Ruiz-Vera and Wu 2006). Aggregate stability generally increases with increasing organic matter but for the Thai soils the relationship is weak ([R.sup.2]=0.44) and is not systematic (Fig. 7) possibly because organic matter does not greatly influence aggregate stability when the organic matter content of the soil is low (Nwadialo and Mbagwu 1991).
Multivariate stepwise regression analysis of original and transformed data has been investigated to develop an equation to predict aggregate stability incorporating several soil properties. A combination of only 2 soil properties is highly predictive of aggregate stability:
%aggregate stability = 149-22pH ([H.sub.2]O)
+ 0.078Clay(g/kg) ([R.sup.2] = 0.80)
Inclusion of extractable acidity, CEC, exchangeable K, and organic matter in multivariate equations and consideration of minor clay mineral associations did not significantly improve the prediction of aggregate stability. Aggregate stability estimated from this equation is compared with the measured aggregate stability (Fig. 8). The result shows a close and systematic relationship between predicted and measured aggregate stability with few significantly outlying data points.
The stability of soil aggregates in these salt-affected soils under a tropical savanna climate is apparently most strongly influenced by the effect of soil pH on clay surface reactions. As the soil pH decreases below 6, the soil solution contains elevated concentrations of Fe and A1 complexes, which are adsorbed and may tend to flocculate clay (Rengasamy and Olsson 1991). In addition, in predominantly kaolinitic soils, pH affects surface charge due to the variable charge nature of clay crystal edges. Increasing pH leads to a charge reversal on the edges of the kaolinitic clay particles, from positive to negative (Schofield and Samson 1954; Tombacz and Szekeres 2006). This, in turn, weakens the bonds between positive and negatively charged surface sites on adjacent particles and thus has an adverse effect on the stability of the aggregates (Chang et al. 1978).
It is clear that aggregates in these salt-affected kaolinitic soils are more stable for subsoils than for surface soils. This is not unexpected since the surface soils are continually disturbed by the ploughing and puddling practices employed for rice cultivation (Adachi 1990). These actions affect both A and upper part of B horizons and induce instability of soil aggregates (So and Ringrose-Voase 2000). In the deeper horizons where the pH decreases and with the accumulation of illuviated clay, the aggregates are more stable and persist. The accumulation of sodium that has caused sodicity in these soils occurred at a late stage of soil development under the influence of shallow saline groundwater and interflow (Arunin 1992; Ghassemi et al. 1995) but the acid, predominantly kaolinitic nature of the soils has helped preserve soil structure.
The authors are grateful to The Royal Golden Jubilee Ph.D. Program of The Thailand Research Fund for financial support, Soil Survey Group, Department of Soil Science, Kasetsart University for the help on preparation of samples and laboratory work, and Soil Mineralogy Group of The University of Western Australia for laboratory facilities.
Manuscript received 7 November 2008, accepted 9 July 2009
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C. Kaewmano (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 Science, University of Western Australia, Crawley 6009, Australia.
(C) Corresponding author. Email: firstname.lastname@example.org
Table 1. Environmental description, classification and morphology of the soils studied 1, Weak; 2, moderate; 3, strong; ABK, angular blocky; SBK, subangular blocky; semi-ABK, semi-angular blocky; G, granular Genetic Depth Colour matrix horizon (m) Location 1: Nakhon Ratchasima, Alluvium over residuum derived from elastic sedimentary rock on floodplain Pedon 1. Typic Natraqualf; fine, kaolinitic, isohyperthermic Apng 0-0.20 7.5YR3/3, 5/4 Btng 0.20-0.88 10YR 5/2, 4/1, 6/1, 7/1 2Btng 0.88-1.90 10YR 7/1, 7/2, 6/3 Pedon 2: Typic Natraqualf; very fine, kaolinitic, isohyperthermic Apg 0-0.30 7.5YR 3/2, 10YR 4/1 Btg 0.30-0.42 10YR 5/3 Btng 0.42-1.51 10YR 4/1, 5/1, 6/1 2 2Btng 1.51-2.00 10YR 6/1, 7/2 Pedon 3: Typic Natraqualf; fine, kaolinitic, isohyperthermic Apg 0-0.11 7.5YR 4/2 Btg 0.11 0.56 10YR 5/3, 2.5Y 4/2 Btng 0.56 1.24 2.5Y 5/1, 5/3, 10YR 6/1 2Btng 1.24-1.80 2.5Y 7/1 Pedon 4: Typic Endoaqualf; fine, kaolinitic, isohyperthermic Apg 0-0.22 10YR 4/2-3 Btg 0.22-1.02 10YR 5/3, 6/1, 7.5YR 4/3, 2.5Y 6/3 2Btg 1.02-1.90 2.5Y 6/2, 7/1, 10YR 6/1, 7/1 Pedon 5: Typic Natraqualf; fine, kaolinitic, isohyperthermic Apg 0-0.16 10YR 5/3 Btg 0.16-0.95 10YR 4/2, 5/2 2Btng 0.95-2.10 2.5Y 5/2, 6/2, 7/1, 7/4 Location 2: Roi Et, Wash over residuum derived from elastic sedimentary rock on erosional plain Pedon 6: Typic Natraqualf; fine, kaolinitic, isohyperthermic Apg 0.00-0.15/0.23 10YR 6/2 Beg 0.23-0.46 10YR 7/1 Btg 0.46-1.[3 10YR 7/1-2, 2.5Y 7/1 2Btng 1.13-1.40 10YR 7/2 Pedon 7: Typic Natraqualf; fine, kaolinitic, isohyperthermic Apg 0.00-0.20 10YR 5/1, 7.5YR 5/3 Apng 0.20-0.27/0.32 7.5YR 7/4, 5/1 Beg 0.32-0.54/0.63 10YR 7/1-2, 7.5YR 6/2 Btg 0.63-1.11/1.14 10YR 7/2, 7.5YR 7/1 2 2Btg 1.14-1.55 10YR 7/2, 7/1 2Btng 1.55-2.07 10YR 7/2 Genetic Mottle Structure horizon Location 1: Nakhon Ratchasima, Alluvium over residuum derived from elastic sedimentary rock on floodplain Pedon 1. Typic Natraqualf; fine, kaolinitic, isohyperthermic Apng 10YR 4/4, 2.5YR 4/8 3 SBK, 3 semi-ABK Btng 10YR 5/8, 6/8 2.5Y 6/4 2 semi-ABK, 2 ABK, 1 ABK to semi-massive 2Btng 10YR 6/8, 5/6 1 ABK to semi-massive, 2 ABK Pedon 2: Typic Natraqualf; very fine, kaolinitic, isohyperthermic Apg 5YR 4/6 3 ABK to semi-ABK Btg 5YR 4/6 2 semi-ABK Btng 7.5YR 5/6, 10YR 5/1-2, 6/2 2 ABK to semi-ABK, 2 SBK 2Btng 10RY 5/8, 2.5Y 4/1 1 ABK to semi-massive Pedon 3: Typic Natraqualf; fine, kaolinitic, isohyperthermic Apg 7.5YR 4/6 1 ABK to semi-massive Btg 10YR 5/6, 2.5Y 5/6 2 semi-ABK Btng 2.5Y 6/4, 6/6 2 semi-ABK parting to 2G, 2 ABK 2Btng 2.5Y 6/6, 5Y 4/6 1 ABK, semi-massive Pedon 4: Typic Endoaqualf; fine, kaolinitic, isohyperthermic Apg 2.5YR 4/8, 7.5YR 5/8 2 SBK, 3 ABK Btg 10YR 4/6, 5/8, 7/8, 2.5Y 7/6 2 ABK 2Btg 7.5Yr 4/6, 10YR 4/6, 6/8 2 ABK Pedon 5: Typic Natraqualf; fine, kaolinitic, isohyperthermic Apg 5YR 5/6 2 SBK Btg 5YR 4/6, 2.5YR 5/8, 7.5YR 5/8 2 semi-ABK, 2-3 ABK 2Btng 2.5Y 6/6 2 semi-ABK, 1 ABK, semi-massive Location 2: Roi Et, Wash over residuum derived from elastic sedimentary rock on erosional plain Pedon 6: Typic Natraqualf; fine, kaolinitic, isohyperthermic Apg 10YR 5/6 1 SBK Beg 10Yr 2/1, 6/8 Massive Btg 10YR 6/8, 2.5Y 7/8, 10R3/6 1-2 ABK, 2 semi-ABK 2Btng 10YR 6/8, 10R 3/6 2 semi-ABK Pedon 7: Typic Natraqualf; fine, kaolinitic, isohyperthermic Apg 10YR 6/8 1-2 semi-ABK Apng 7.5YR 6/8 1-2 semi-ABK Beg 10YR 5/1, 2/1 Massive Btg 2.5YR 6/6, 10YR 6/8 1 ABK (semi-massive) 2Btg 10YR 5/8, 10YR 5/8 2 semi-ABK 2Btng 10YR 5/8, 6/8, 10R 5/8 2 semi-ABK Table 2. Physical, chemical and mineralogical properties of the soils Ex., Exchangeable bases; Kao, kaolinite; Smec, smectite; Ill, illite; VM, very much (>60%); M, much (40-60%); Mo, moderate (20-40%); L, little (5-20%); Tr, trace (<5%); n.d., not detected Horizon Particle size distribution Sand Silt Clay (g/kg) Pedon 1 Apng 320 207 474 Btng 386 197 418 2Btng 568 169 264 Pedon 2 Apg 213 276 512 Btg 234 267 499 Brag 166 239 595 2Btng 496 173 332 Pedon 3 Apg 213 264 523 Btg 148 217 635 Btng 176 243 581 2Btng 127 129 745 Pedon 4 Apg 414 232 355 Btg 262 240 498 2Btg 455 249 296 Pedon 5 Apg 363 290 348 Btg 197 271 532 2Btng 215 301 484 Pedon 6 Apg 642 248 110 Beg 338 246 416 Btg 275 311 413 2Btng 304 380 416 Pedon 7 Apg 667 269 63 Beg 450 214 336 Btg 335 251 414 2Btg 330 322 348 2Btng 362 336 330 Horizon Chemical properties pH [pH. OM EC ESP SAR sub.KCI] (g/kg) (dS/m) (%) Pedon 1 Apng 5.5 4.8 10.1 11.3 20 7.9 Btng 6.6 5.9 2.8 8.6 27 8.1 2Btng 7.2 0.5 0.5 12.8 29 10.2 Pedon 2 Apg 5.1 4.4 18.8 9.5 11 5.0 Btg 4.9 4.1 10.3 7.2 10 5.1 Brag 6.0 5.2 4.7 7.0 22 6.8 2Btng 6.8 5.8 0.4 10.9 27 8.7 Pedon 3 Apg 5.1 4.5 21.5 13.4 9 4.2 Btg 5.7 4.8 8.1 5.4 10 4.1 Btng 6.4 5.8 2.7 5.2 17 4.9 2Btng 6.7 5.9 0.7 8.0 21 6.0 Pedon 4 Apg 5.6 4.9 15.0 7.3 9 3.6 Btg 5.6 5.0 5.1 3.7 9 3.2 2Btg 6.7 6.0 11.1 4.4 9 3.4 Pedon 5 Apg 5.1 4.4 15.6 9.0 5 3.9 Btg 5.1 4.2 7.8 3.3 8 3.5 2Btng 6.6 5.7 1.4 3.6 16 4.8 Pedon 6 Apg 5.2 3.6 6.5 0.4 2 1.8 Beg 7.0 5.6 2.3 0.4 3 3.0 Btg 7.1 6.3 0.8 2.5 9 4.3 2Btng 6.8 6.1 0.5 3.3 15 5.1 Pedon 7 Apg 5.5 4.0 4.5 0.4 13 2.6 Beg 8.0 6.2 1.74 0.4 11 3.3 Btg 7.5 6.1 0.8 0.7 6 5.1 2Btg 7.5 6.3 0.7 1.2 10 5.6 2Btng 7.1 6.3 0.5 1.4 19 4.9 Horizon Chemical properties CEC EA Ex.Ca Ex.Mg Ex.Na Ex.K ([cmol.sub.c] /kg) Pedon 1 Apng 18.3 8.8 9.5 2.0 3.7 0.14 Btng 15.1 3.8 7.6 1.4 4.2 0.08 2Btng 10.7 2.5 5.5 1.0 3.0 0.07 Pedon 2 Apg 20.7 13.0 10.4 1.8 2.3 0.13 Btg 19.6 12.5 9.0 1.6 2.0 0.12 Brag 22.6 8.3 11.2 1.9 4.9 0.11 2Btng 12.3 3.1 6.5 1.2 3.3 0.08 Pedon 3 Apg 21.5 10.1 11.6 2.3 2.0 0.21 Btg 22.5 8.6 12.2 2.0 2.3 0.21 Btng 20.5 2.9 12.7 2.0 3.6 0.11 2Btng 27.8 5.0 16.1 2.8 5.7 0.14 Pedon 4 Apg 12.8 6.6 6.9 1.3 1.1 0.14 Btg 18.4 6.9 10.8 1.7 1.6 0.09 2Btg 15.0 2.4 9.5 1.5 1.4 0.06 Pedon 5 Apg 13.7 8.6 6.5 1.5 0.7 0.13 Btg 21.1 11.4 10.0 1.2 1.7 0.11 2Btng 18.3 4.0 11.0 1.2 3.0 0.09 Pedon 6 Apg 2.2 5.7 0.8 0.1 0.0 0.03 Beg 26.1 6.4 8.3 1.6 0.9 0.06 Btg 22.3 5.3 9.3 1.4 1.6 0.05 2Btng 8.0 4.6 3.8 0.7 1.2 0.04 Pedon 7 Apg 2.9 4.2 0.2 0.0 0.1 0.01 Beg 9.65 5.1 8.29 0.75 1.0 0.04 Btg 25.5 4.9 10.3 1.1 1.6 0.05 2Btg 15.7 5.0 8.0 4.6 1.6 0.04 2Btng 7.1 5.1 3.1 0.6 1.4 0.04 Horizon Clay fraction BSsum Kao Smec Ill (%) Pedon 1 Apng 64 VM Mo Tr Btng 78 VM L Tr 2Btng 79 VM Mo Tr Pedon 2 Apg 53 VM L Tr Btg 50 VM L Tr Brag 69 VM L Tr 2Btng 78 VM L Tr Pedon 3 Apg 62 VM L Tr Btg 66 VM L Yr Btng 86 VM L n.d. 2Btng 83 VM L Tr Pedon 4 Apg 59 VM L Tr Btg 67 VM L Tr 2Btg 85 VM Mo Tr Pedon 5 Apg 50 VM L Tr Btg 53 VM n.d. Tr 2Btng 79 VM L n.d. Pedon 6 Apg 14 VM n.d. n.d. Beg 63 VM L n.d. Btg 70 VM n.d. n.d. 2Btng 56 VM Tr n.d. Pedon 7 Apg 8 VM n.d. n.d. Beg 66 VM n.d. n.d. Btg 73 VM L n.d. 2Btg 72 VM L n.d. 2Btng 58 VM L n.d.
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|Author:||Kaewmano, C.; Kheoruenromne, I.; Suddhiprakarn, A.; Gilkes, R.J.|
|Publication:||Australian Journal of Soil Research|
|Date:||Nov 1, 2009|
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