Clay mineralogy of Thai soils

Applied Clay Science 11 (1997) 357-371

Clay mineralogy of Thai soils Kannica Yoothong av* , Lek Moncharoen a, Pisuth Vijarnson a, Hari Eswaran b d Department of Land Development, Bangkok, Thailand b Soil Conservation Seruice, USDA, Washington, DC, USA

Accepted 28 August 1996

Abstract The wide variety of soils in Thailand results of the interaction between climate, parent rocks, and physiographyof the country. This is reflected in the wide variety of clay minerals.The completecountry can be divided into five regions:the northern,central, north-east,south-east coastandpeninsularregion.The soil mapof Thailandhasbeenusedto define the clay mineral provinces.Froma mineralogicalpoint of view eightclay mineralandthreeother assemblages can be distinguished. The clay mineralassemblages concern:kaolinite; kaolin&eand illite; kaolinite and smectite;kaolinite, smectiteand illite; kaolinite, illite and smectite;smectiteand kaolinite; smectite,kaolinite and illite; kaolinite and gibbsite.The three other land types compriseclayfree/quartz-dominatedsoils,organicsoilsandslopecomplexes.Kaoliniteis presentin all soils.In oxisols,kaoliniteis associated with gibbsite,goethiteandhematite.In areaswherethe parentrock is mica-rich,ill&e is the dominantclay mineral.Smectiteis the dominantclay mineralin areas where brackishwater conditionsprevailed during soil formation and where high amountsof dissolvedionswerepresent. Keywords:

Thai soils; kaolinite; smectite; illite; clay mineralogy

1. Introduction Soil formation processesare complex and largely dependent on the parent rock material, and specific climatic and site conditions. Primary mineral transformations in well-drained soils in tropical areashave extensively been studied (e.g. by Zauyah et al. (1990). The end-product of physico-chemical processesis generally kaolinite with a variable amount of gibbsite and iron hydroxide (Eswaran and Wong, 1978). In humid tropical regions without any distinct dry season,processesare fast and produce kaolinite * Corresponding author 0169.1317/97/$17.00 PII SO169-1317(96)00033-6

Copyright 0 1997 Elsevier Science B.V. All rights reserved.

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and gibbsite (Eswaran et al., 1990). In regions with a dry season, however, processes are slower; due to a less severe weathering, kaolinite is accompanied by various amounts of chlorite, vermiculite and illite. In organic soils, due to acidic conditions, primary micas alter to an Al-interlayered chlorite. The material becomes enriched in this chlorite in the upper part of the soil, and decreases with depth (Tavemier and Eswaran, 1972). In situ genesis of smectite occurs under brackish conditions in coastal plains (Dabbakula et al., 1992) because, of a high pH, the soil solution is enriched in dissolved silica and in magnesium and calcium. The first soil map of Thailand was published by Moorman and Rojanasoonthon (1968). The most recent soil map was produced by Vijamsom et al. (1994). This map and the associated data base are particularly useful for agricultural and engineering purposes. In this perspective, the concept of domains has been developed by Beimoth et al. (1994). A domain is-in terms of resource management-a land entity which has similar use potentials or similar properties for specific purposes. The purpose of this paper is to give an overview about the distribution of clay mineral assemblages identified in Thailand.

2. Soil formation

in thailand

The total area of Thailand is about 550.000 km’. The topography consists of 25% lowlands, 46% uplands and 29% highlands (500 m above sea level) with slopes being higher than 35% (Changprai, 1987). The country can be divided into five main regions (Fig. 1): the central, north-east, south-east coast, northern and peninsular region. 2.1. Climate of Thailand Thailand has three main seasons: a rainy, a cool and dry, and a hot and dry season. The eastern part of the south-east coast and most parts of the peninsular region are submitted to the tropical monsoon. The northern mountainous region is characterised by a humid subtropical climate. The rest of the country is characterised by a tropical savannah climate, i.e. A w type (Koppen, 1931). Temperature varies between 24-26°C in the north, between 28-30°C in the central region and between 26-28°C in the rest of the country. In general, April is the hottest month while January is the coolest. The average annual rainfall ranges between 1000 and 4000 mm (Meteorological Department, 1994). In the southern and south-east regions, however, rainfall is generally the highest compared to the rest of the country. As a consequence, these latter regions do not have a pronounced dry season. 2.2. Soil climatic regime Moncharoen (1983) distinguished five soil moisture regimes: aquic, peraquic, udic, ustic and perudic. The different regimes occur in the following regions: . aquic: in low-lying paddy lands throughout the country; . peraquic: in tidal flats and swamp areas, especially along the Gulf of Thailand;

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Fig. 1. Physiographic

regions of Thailand.

. udic: in the peninsular region, with an annual rainfall higher than 2000 mm, and with more than 160 days of rain, as well as in the northern region where hills, between 1000 and 1600 m in altitude, are covered with an ever green vegetation; . perudic: in areas above 1600 m; . ustic: in the remaining areas of the country.

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2.3. Physiography

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and geology

Details on the geology of Thailand are contained in the geological map published by the Geological Survey Division (Department of Mineral Resources, 1987). 2.3. I. Central region The central region is subdivided into 19 provinces. Its borders are hilly in the east and west, with the Gulf of Thailand in the south, and the Nakhon Sawan Province in the north. The large plain of the central region consists of unconsolidated Quaternary sediments. These alluvial deposits reach to a thickness of 300 m. The broad delta of the Chao Phraya river system is located in the south (at about l-3 m above sea level), and is known as the Bangkok Plain. In the Chao Phraya river, but also in the Mae Klong and Pa Sak deltas, relatively recent fresh water sediments dominate. Marine sediments are found in the southern part of the Bangkok plain, while in the middle of the plain brackish water sediments dominate (Takaya, 197 1; Rua and Nutalaya, 1980; Takaya and Thiramongkol, 1982). In the east, limestone and volcanic rocks (andesite, basalt and rhyolite) occur. The highlands and mountainous areas in the west-which are part of the Tanaosi Mountain-consist of many different types of rock: metamorphic rocks (phyllite and schist); sedimentary rocks from Ordovician-Silurian up to Permian; igneous rocks (basalt, granite and diorite). 2.3.2. North-east region The north-east region-known as the Korat plateau-is subdivided in 19 provinces and occupies approximately one third of Thailand. Its borders are the Phetchabun and Dong Phaya Yen Ranges in the west, the Mekong river in the north and east, and the Phanom Dongrak scarps in the south (along the Thai-Cambodia border). The elevation, above sea level, of the plateau ranges between about 250 m in the north-west and about 100 m in the south-east. The central north-east region consists of two basins which are mutually separated by the NW-SE trending Phu Phan Range: the Sakon Nakhon Basin in the north, and the larger Korat Basin in the south. Briefly, the geology is as follows: Jurassic-Cretaceous arenaceous rocks (e.g. siltstones, sandstones, conglomerates and shales)-known as the Red Beds-overlain by Tertiary semi-consolidated rocks (e.g. mudstones and sandstones) and containing oil and gas. During the Cretaceous, salts belonging to the Korat Group were accumulated. River alluvial deposits are of Quaternary age. 2.3.3. South-east coast region This region is subdivided into 7 provinces. Its borders are the hills in the north, the Gulf of Thailand in the south and west, and the Banthat Range in the east (ThaiCambodia border). In the areas dominated by terrace formations, numerous separate hills and ranges with a NW-SE strike are found. Along the coast, small interconnected marine and brackish water alluvial plains developed. Granites of different ages also occur. Carboniferous rocks are overlain by Permian sediments. The volcanic plateau of this region dates from the Tertiary period and consists of corundum-bearing basalt. 2.3.4. Northern region This region is subdivided into 17 provinces. Its borders are Burma in the west and north, and Laos in the north and east. The most northern region mainly consists of hilly

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to mountainous areas with wide incised plateaus and narrow inter-mountainous valleys. The rocks are of igneous or of sedimentary origin more or less locally metamorphosed. Limestone ridges and volcanic plateaus locally occur. The age of the different rocks varies from Precambrian to Quaternary. Metamorphic rocks mainly comprise gneiss and micaschists. As in the south-east coastal region, granites of different ages crop out. Basalt and andesite are observed scattered in relatively small areas. In the lower part of the northern region, the land is rather flat and dates back to the Quaternary. Andesite, basalt and Permian limestone are found in the east.

2.3.5. Southern region The southern region or peninsular Thailand is subdivided in 14 provinces. Its borders are the Chumphon-Prachuap Kiri Khan Province in the north, the Pacific ocean in the east, the Andaman sea in the west and Malaysia in the south. The southern region is characterised by a number of hilly and mountainous ranges. The most important rocks dated from late Triassic/middle Tertiary &ranite) (Jam&i et al., 19911, and from Ordovician/Carboniferous-Permian/Permian (limestone). These rocks were locally metamorphosed into slate, schist and gneiss. The coastal terraces and plains on the west coast are narrow, but are much wider on the east coast. 2.4. Natural vegetation According to the Royal Forestry Department, the total area of forestrial Thailand decreases from about 70% to about 30% during the last forty years. This deforestation-especially during the last two decades-is due to an increase of the population combined with the need for arable land and economic incentives. The different forest types reflect different climatic regimes. The tropical ruin forest is the most important and occurs in areas with the highest rainfall, i.e. in the south-east coast and the southern peninsular regions. The dry euer green forest dominates in hilly areas of the northern region and in the north-east plateau. The dry dipterocarp forest is the main type of terrace formations and the low sandstone ridges in the north-east plateau; it also occurs extensively on terraces of the northern region. The mangrove swamp forest is found in most coastal areas, but mainly along the west coast of peninsular Thailand. The coniferous pine forest is found in patches on plateaus higher than 6OC- 1300 m. The peat swamp forest occurs to a lesser extent and is limited to peninsular Thailand close to me Malaysian border.

3. Materials

and methods

Vacharotayan et al. (1962) were among the first to study the clay minerals from Thai soils by X-ray diffraction and chemical analyses of clays and silts. In 1963, the first mineralogical laboratory of the Soil Survey Division was set up at the Department of Land Development. Since, a systematic study-mainly as a support for the National Soil Survey Program-was carried out. The methods used were described e.g. by Wilson (1987). The depths of the studied soil horizons are indicated in Figs. 3-10. It is clearly out of the scope of this paper to go into detail of the different standard

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analytical methods which have been used. The results here focus on the clay mineralogy only. Eight different clay mineral and three other assemblages are indicated on the map of Thailand (Fig. 2).

SOClAlIST UNION

REPUBLIC OF OF BURMA

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DOMAINS

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Ksolinite-gibbsite clay free, quaru &minaM orgaIlicsoils Slope complexes,, Clays undiffwenttated

n wat.erbcdles

MALAYSIA Fig. 2. Clay mineral

map of Thailand.

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4. Results and discussion 4.1. Central region The clay mineral assemblage in the marine sediments of the tidal swamp areas in the south is smectite-kaolinite-illite in composition (Fig. 3). In some of these soils, smectite content of the clay fraction can reach 60-70%. Soils of the flood plain are composed of kaolinite, smectite and illite (Fig. 4). Vermiculite, chlorite, interstratified clay minerals, goethite and quartz are also present in minor quantities in some soil series (Yoothong et al., 1986). S01‘1s in the eastern part of the central plain which derive from marls and volcanic rocks, are mainly composed of smectite clays. They belong to the smectite-kaolinite assemblage (Fig. 5). The smectite content occasionally reaches more than 90%. Jackson (1965) reported that smectite in soils might occur in one of the four so-called B’s: (1) in basins, (2) in B-horizons, (3) in upper B-horizons (e.g. vertisols) and (4) beneath basic rocks. These four environments have in common soil solutions with a high silica content and with basic cations which are required for smectite formation. Also in Thailand, the requirements for smectite preservation are met: minor leaching in a poorly drained basin. Soils in the western part vary because of their different physiography and geology. The clay mineral associations are kaolinite, kaolinite and illite with small amounts of smectite (Fig. 61 and kaolinite with smectite (Fig. 7). Kheoruenromne and Suddhiprakam (1984) studied the genetic factors for alfisols in the Mae Klong basins (Western Thailand), and concluded that the type of parent material and the topographic variation of the landforms in the past are the main factors causing genetic differences between soils. Vegetation plays only a role in the subsequent modification of present soils whereas time and climate are rather constant parameters and have only a minor influence. 4.2. North-east

region

Kaolinite is the most important clay mineral in the red and yellow ultisols and oxisols of the north-east plateau (Fig. 8) (Suddhiprakam et al., 1985). Vermiculite, smectite and

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(C)

illite are only present in minor amounts. Hematite, goethite and magnetite content varies according to the nature of the parent materials and the soil pedogenesis. Goethite is associated with a udic soil moisture regime or occurs in deeper layers of the soil; hematite is characteristic of intense weathering. Yoothong et al. (1985) found that yellow soils derived from alluvial deposits mostly contain less than 1% of free iron oxides while soils derived from granite and metamorphic equivalents contain l-4%. In red soils, free iron oxide content varies between less than 1% to more than 10%

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Fig. 6. Typic Haplustalfs (48-l 14 cm). (A) Mg-Sat”, air dried, (B) Mg-Sat”, ethylene glycol solvated, K-Sat”, air dried, (D) K-Sat”, 550°C. K = kaolinite, 1 = illite, S = smectite, G = gibbsite and Q = quartz.

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depending on the parent rock. Soils with kaolinite and illite are found in areas along the Mekong river, with minor smectite, vermiculite and chlorite. The alluvium of the Korat basin is dominated by a kaolinite-smectite-illite assemblage. Soils in some lowland areas, however, may be dominated by smectite. The alluvium in the Sakon Nakorn basin is dominated by the kaolinite-illite assemblage with minor amounts of smectite (Fig. 9). 4.3. South-east coastal region Kaolinite is the major clay mineral in most of the soils of the south-east coast. In soils derived from granite, traces of gibbsite are found in udic soil. Kaolinite is dominant

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Fig. 7. Ultic Haplustalfs (30-50/60 cm). (A) Mg-Sat”, air dried, (B) Mg-Sat”, ethylene K-Sat”, air dried, (D) K-Sat”, 550°C. K = kaolinite, I = illite, S = smectite, G = gibbsite

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in the small basaltic area where it occurs with up to 15% of free iron oxides due to an intense weathering. The smectite-kaolinite-illite assemblage is present in soils in the west, in former tidal flat areas where marine and brackish water sediments accumulated. In small interconnected marine and brackish water alluvial plains, kaolinite-smectiteillite assemblage is the dominant clay material, 4.4. Northern

region

In the northern part of this region, kaolinite is the major clay mineral in most of the soils. Small quantities of illite and iron and aluminium hydroxides are also present in

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Fig. 9. Aeric Tropaquepts (14-67 cm). (A) Mg-Sat”, air dried, (B) Mg-Sat”, ethylene K-Satn, air dried, (D) K-Sat”, 550°C. K = kaolinite, I = illite, S = smectite, G = gibbsite

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cm). (A) Mg-Sat”, air dried, (B) Mg-Sat”, ethylene K = kaolinite, I = illite, S = smectite, G = gibbsite

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many ultisols and oxisols. The dominant clay mineral association in flood plain areas covered by recent alluvium is kaolinite-smectite-illite. Small areas with kaolinite-gibbsite (Fig. 10) are present in elevated areas with a udic moisture regime. This is in accordance with Kunaporn and Moncharoen (1984) who studied mountainous soils in the West-Continental Highlands. The kaolinite-illite groups is found in mica rich areas where the parent material is mica schist. In some areas, higher illite contents were observed. In the southern part of the northern region, kaolinite, kaolinite-illite and smectite-kaolinite are the major groups and are derived from limestone, andesite and basaltic parent rocks. 4.5. Southern region Kaolinite is the dominant clay mineral in most soils of this region. Traces of illite are also mostly present. Because of the similar climate between this region and the eastern part of the south-east region, clay mineralogy is almost identical. Other clay mineral associations are kaolinite-illite and kaolinite-smectite-illite (in the lowland areas near the coast). Beach and dunes along the eastern coast mainly consist of quartz.

5. Clay mineral

domains

As shown in the clay mineral map of Thailand (Fig. 2), eight domains of clay minerals can be distinguished. The most extensive is dominated by kaolinite. The remaining seven clay mineral domains are related to the physiographic position of the soils. Each clay domain can be briefly described in terms of soil, agricultural and engineering properties.

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5. I. Kaolinite domain The kaolinite domain is most common for the upland soils in the north-east, lower north and peninsular Thailand covering about 47% of the total area of the country. Soils containing kaolinite can be classified as kandiustults, paleustults, kandiudults and paleudults (Soil Survey Staff, 1992). These soils have generally a relatively low nutrient status, poor water holding capacity and are prone to erosion if they occur on sloping land. The clay content in the soil itself is one of the key factors determining agricultural potential. Most soils in this region have been exclusively used for producing various crops depending upon climatic and soil moisture conditions. During the dry season in the north-east, casava, ground nut, maize, jute, kenaf, sugarcane, sorghum, mulberry, mango and jack fruit are cultivated. In peninsular Thailand, rubber and fruit trees like rambutan, durian and mangosteen are commonly cultivated. The yield of these crops varies and is dependent on their management. 5.2. Kaolinite-illite

domain

Soils containing kaolinite and illite are found along the Mekong River in the northern rim of the north-east plateau. In the other regions, except along the south-east coast, kaolinite and illite occur in smaller amounts and are found in mica-rich areas. They occupy about 5% of the total area of the country. Soils containing kaolinite and illite are commonly classified as ustifluvents, udifluvents and epiaquults. Depending on their physiographic position, the land use pattern varies. However, due to their high agricultural potentials, this large group of soils are generally intensively managed. The only severe limitation is flash flooding in the rainy season but such damage can be avoided by constructing a dike along the river. On the higher part of the flood plain, fruit trees, various vegetables and cash crops are cultivated. In the low-lying terrain, paddy rice is commonly cultivated. After its harvest and if water is available, non-irrigated crops or vegetables are cultivated as a second crop. With proper management, most crops give a reasonable yield. 5.3. Kaolinite-smectite

domain

Soil containing kaolinite and smectite only occupy 1% of the total area of the country. In most cases, these soils fall into the haplustalfs group (Soil Survey Staff, 1992). Their agricultural potential is generally limited by the effective soil depth or depth to the underlying bedrock. In addition, erosion may occur on steep sloping land. 5.4. Kaolinite-smectite-illite

domain

Soils with kaolinite, smectite and illite extensively occur in the northern part of the central plain as well as in the valleys of the northern region. Its total area is about 9% of the country. They are generally classified as epiaquults and haplustalfs (Soil Survey Staff, 1992). These soils are basically used for cultivation of paddy rice in the rainy season if the soils are poorly drained. Part of the uplands is exclusively used for growing crops like sugar cane, vegetables and fruit trees. These soils are in general highly productive because of their high agricultural potential.

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domain

These soils are not widely spread and cover max. 1% of the total area of the country. The land use pattern and agricultural potential are similar to the previous domain. 5.6. Smectite-kaolinite

domain

Soils containing smectite and kaolinite mainly occur in the Pa Sak valley of the Central Highland. Most of these soils are derived form basic igneous rocks, limestones, marls and other fine-grained elastic rocks. They occupy about 2% of the total area of the country. Most of these soils are classified as vertisols, and to a lesser extent, as ustropepts, eutropepts and calciustolls. The agricultural potential is relatively high. They are very prone to swelling and shrinking, and are very sticky when wetted. However, with proper management, these soils can produce a high yield of maize, cotton, custard apple, mango, coconut and irrigated paddy rice. 5.7. Smectite-kaolinite-illite

domain

These soils mostly occur in deltaic plains and/or in former tidal flats. The largest area is found in the Bangkok Plain which constitutes in fact the Chao Phraya delta. This clay domain occupies almost 4% of the total area of the country. Most of them are poorly drained and are commonly classified as epiaquepts or endoaquepts. Due to their varying mineralogy, the nutrient status is generally relatively high, especially with respect to potassium. Because of their high clay content, ample supply of water and low-lying topography, these soils are ideally suited for the production of paddy wet-land rice in the rainy season. The average yield ranges from 3 to 4 tonnes per hectare, depending on crop management, flood containment and water availability. However, about 11% of the total area in the central plain consists of typical acidic sulphate soils which is not favourable for rice production. With respect to engineering purposes, these soils give problems for the construction of highways because of their high shrink-swell capacity, prolonged water-logging and strong acidification. 5.8. Kaolinite-gibbsite

domain

Soils containing kaolinite and gibbsite are only found in very limited areas in the northern highlands. Most of them are highly weathered and are derived form basic or intermediate igneous rocks under udic moisture conditions. Due to the steep topography, the use of these soils is limited to wood production and watershed protection.

6. Conclusions This paper gives an overview of the clay mineralogy of soils in Thailand. This study is a part of the National Soil Survey Program which has been carried out over the last twenty years. Thailand is a large country with a number of climatic regimes and geological areas submitted to complex biological and physico-chemical processes; the

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latter are reflected in the vegetation, the soils and the associated mineral assemblages. From a mineralogical point of view, it was possible to differentiate between eight clay mineral assemblages and three other land types. This information is actually being used for the evaluation of potential agricultural applications, for the determination of fragile or sensitive areas requiring appropriate conservation and/or preservation, for the justification of large scale development projects having an impact on the ecosystem, for the planning of construction works like highways, pipelines,... and finally for the development of environmental rules related to chemical pollution.

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