Introduction

About 10% of the non-ice covered landmass of the world is occupied by “wet soils”. Of this, 80% is caused by ground water and 20% by surface water stagnation. Hydrological function of the wetlands refers to the vital function of flood control and ground water recharge and discharge (Nishat et al. 1993).

Wetlands constitute a part of natural heritage for human being. For thousands of years wetlands have played a significant part in the development of human society. In humid climate, particularly with a concentration of rainfall during the rainy season, favours seasonal saturation of soil with water and seasonal flooding of low-lying lands. An abundance of low-lying lands therefore contributes to the existence of wetland areas. Many of the wetlands of the tropical Asia are the result of a combination of humid climate with a heavy concentration of rainfall and an abundance of low-lying land like floodplains, deltas and wide coastal low-land (Kyuma, 1985). The proportion of wetland soils to total land area is probably higher in South and Southeast Asia than in any other major tropical region of the world. Most of these soils, which are already producing rice, are very productive (Murthy, 1978).

Bangladesh has the highest wetland to total land ratio in the world. Wet soils are acquiring increasing importance in land development and environmental protection. Knowledge of the duration and periodicity of wetness is important in evaluating possible use of soils and formulating remedial measures for chemical and physical changes caused by floods. In most cases such changes are apt to impose permanent characteristics. Variations in the rainfall regime and seasonal fluctuations strongly influence perched water table. The impact of wetness also depends on the period of the year during which it occurs, either during the growing period or the “dead season”. Wetlands constitute about 70% of the territory of Bangladesh (Saheed, 1984). Fertility level of the wetland soils in the country is low to moderate and is believed to be enriched by siltation during flooding. The land use in wetland soils is basically traditional depending on the timing and duration of flooding.

Bangladesh is a land – hungry country with a high pressure of population leading to increasing number of people that are landless. Its wetland resources have suffered considerably from the impacts of increased population.  Wetlands of our country are shrinking fast but these soils are principally used for rice cultivation. Pressure from increasing population demands to bring more land under paddy cultivation. Another important feature is that wet soils of the major area in Bangladesh are problematic such as peat soil, saline soil, acid sulfate soil, sodic soil etc. It is thus obvious that study of wetland soil in Bangladesh occupies an important position so far as their characteristics and management are concerned because 95% of the total rice area of Bangladesh is on wetland soils.

Brahmanbaria was formerly a sub-division of Comilla district. It lies between 23˚ 39΄ and 24˚16΄ north latitudes and between 90˚ 44΄ and 91˚ 51΄ east longitudes. The total area of this district is 1927 sq. km. About 83 % of the area is subject to seasonal flooding. The Surma-Kusiyara flood plain exists in this district and is seasonally flooded but in most places remains flooded for the greater part of the dry season. Basins are moderate to deeply flooded. Soils are moderately fine and medium textured which has developed on unconsolidated alluvial sediments. Rice is by far the most extensive crop in this district. Broadcast Aman partly mixed with Aus occasionally followed by Rabi crops is grown on about 43% of the land.

Unfortunately, wet land soils of the Brahmanbaria district as a whole have been subject to the least amount of scientific study. Though recently SRDI has done some analysis at upazila level but researches on these soils are inadequate. In fact, low land soil of the Brahmanbaria district is agriculturally productive and contains high quantity of soil nutrients. Our present knowledge on the characteristics of these soils is, therefore, very limited. A more intensive and detailed study is needed at the present time to determine the potentialities of these soils for agricultural production. The present investigation is undertaken, therefore, to make a detailed study of the soil resources in wetlands of Brahmanbaria district of Bangladesh.

The objectives of this study were as follows:

1.         i.            To study some selected wetland soils with reference to their morphological, physical, chemical, physico-chemical and mineralogical properties.
2.       ii.            To furnish information on clay mineralogical composition of the soils.
3.     iii.            To investigate the changes in selected characteristics of the studied soils with reference to their physical, chemical and physico-chemical properties.
4.     iv.            To shed light on genesis and classification of the soils.

Review of Literature

With the target of carring out morphogenic investigation on the soils of the wetlands of Brahmanbaria district, it was felt necessary to review the available literatures on similar soil elsewhere so as to provide information on their nature and properties.

Definition and concept of wetlands

The RAMSAR Convention has defined wetland as “ areas of marsh, fen, peat land or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of which at low tide does not exceed six meters”. Thus term “wetland” groups together a wide range of inland, coastal and marine habitats which share a number of common features
(Dugan, 1990).

As defined by the convention, wetlands include a wide variety of habitats such as marshes, peatlands, floodplains, rivers and lakes and coastal areas such as salt marshes, mangroves and seagrass beds but also coral reefs and other marine areas no deeper than six meters at low tide, as well as human–made wetlands such as waste- water treatment ponds and reservoirs.

The range of wetland habitats which come under the mandate of the RAMSAR Convention is enormous. According to their basic biological and physical characteristics IUCN has identified a total of 39 categories of wetlands of which 30 are natural wetlands and 10 are man-made (Dugan, 1990).

Wetland soils can be defined as “soils whose development and properties are strongly influenced by topography or permanent saturation in the upper part of the land”. In fact, wetland soils or wet soils are known under different names: hydric soils, stagnosols, meadow soils, aquic soils etc. (Dudal, 1992).

Haor, boar, beels and jheels are commonly identified as freshwater wetlands in Bangladesh and they have the characteristics of four landscape units –floodplain, freshwater marshes, lakes and swamp forests. Floodplains are the areas that undergo periodic flooding as a river channel overflows with flood water. Freshwater marshes are more or less permanent shallow water dominated by reeds etc. Lakes, natural or man-made, are deep waterbodies (Dugan, 1990). Beels may be taken to be a combination of freshwater marshes, lakes and swamp forest. Boars are beels which are actually oxbow lakes and jheels are freshwater marshes. Haors are combination of floodplains and beels which in monsoon season go under water and in dry months are isolated as the floodplains dry up.

Human society derives aesthetic enjoyment, inspiration and a feeling of relaxation from their scenic beauty. Archaeological findings confirm that many of the earliest human settlements such as those of the Mesolithic period were dependent on wetlands for food, water, clothing and shelter. The civilization in Asia and the Far East developed in floodplain environment. The major river systems of the world have supported the development of rich and sophisticated civilizations, and many diverse societies have evolved effective systems for deriving benefits from the rich natural productivity of the wetlands. The rich biodiversity, which these lands support, is of particular international significance. One noteworthy example of great important to human society is the deep water rice that has evolved in the Ganges- Brahmaputra river basins, containing genes which enable these paddy plants to rise above flood waters providing even under severe flooded conditions (Nishat et al. 1993).

Classification of wetlands

Most wetland soils are classified as either Aquents or Aquepts or as Histosols where the decomposition of organic matter is retarded because of peraquic conditions. In a sense, they are totally immature or only weakly developed as soils and have poorly expressed profile morphology.

A classification of wetland is given below:

Classification of wetlands (Nishat et al. 1993)

(iii) Coral reefs.

2. Intertidal(i) Rocky marine shores, including cliffs and rocky shores.

(ii) Intertidal mobile unvegetated mud, sand or salt flats, salt marshes and mangroves.

1.2. Estuarine1. Subtidal(i) Estuarine waters; permanent waters of estuaries and estuarine systems of deltas.

2. Intertidal(i) Intertidal mud, sand or salt flats, with limited vegetation, marshes, mangrove swamp, snipa swamp.

1.3. Lagoonal (i) Brackish to saline lagoons with one or more relatively narrow connections with the sea.

1.4. Salt lake (i) Permanent and seasonal, brackish, saline or alkaline lakes, flats and marshes.2. Freshwater  2.1. RiverinePerennial(i)                 Permanent rivers and streams, including waterfalls.

(ii)               Inland deltas

Temporary(i)                 Seasonal and irregular rivers and streams, riverine floodplains, including river flats, flooded river basins, seasonally flooded grassland.

2.2. LacustrinePermanent

(i)                 Permanent freshwater lakes.

(ii)               Permanent freshwater ponds.

2.3. PalustrineSeasonal

Emergent (i)    Seasonal freshwater lakes. Permanent freshwater marshes and swamps.

(i)                 Seasonal freshwater marshes on inorganic soil, seasonally flooded meadows.

(ii)               Peatlands, including acidophilous, ombrogenous, or soligenous mires.

Forested

(i)   Shrub swamps, including shrub-dominated freshwater marsh.

(ii)               Freshwater swamp forest.

(iii)             Forested peatlands.

3. Man-made Wetlands

3.1. Aquaculture/Mari culture

(i) Aquaculture ponds, including fish ponds and shrimp ponds.

3.2. Agriculture

(i)                 Ponds, including farm ponds.

(ii)               Irrigated land and irrigation channels.

(iii)             Seasonally flooded arable land.

3.3. Salt Exploitation

(i) Salt pans and salines.3.4. Urban/Industrial

(i)                 Excavations, including gravel pits, borrow pits and mining pools.

(ii)               Wastewater treatment areas,

3.5. Water storage areas

(i)                 Reservoirs holding water for irrigation and/or human consumption.

(ii)               Hydro-dams.

Wetlands in Bangladesh

The wetland soils in Bangladesh can be grouped into two broad classes: organic and mineral. The organic group (peats) consists of around 74000 ha. These peats (Histosols) are distributed over Bangladesh in different depressions, of which the Gopalganj-Khulna basin areas have the largest areal extent. Hemists, Saprists, and Fibrists occur in Bangladesh. Seven soil series have been established for Histosols (SRDI Staff, 1965-86). Mineral soils forming under the wetland conditions are the most extensive in Bangladesh (SRDI Staff, 1965-86). Mineral soils forming under the wetland conditions are the most extensive in Bangladesh (SRDI Staff, 1965-86). Out of total of 483 soil series, around 416 have developed under aquic moisture regime (Table- 2)

Of the soil orders Inceptisols occupy more than 50% of the total area. Hydraquents are the deeply flooded soils (Saheed and Hussain, 1992). Haplaquepts (Endaquepts) are the most extensive wetland soils in Bangladesh. Aeric and Typic Haplaquepts are the most dominant wet subgroups of Bangladesh, followed by Typic and Aeric Fluvaquents. Around 17 soil series have been classified under the Aeric Albaquept subgroup where ferrolysis is the dominant pedogenic process. Aeric Haplaquents and Typic Psammaquents also occur in Bangladesh. Typic sulfaquepts and Sulfic Haplaquepts occur along the sea coasts near Cox’s Bazar and also in areas adjoining the mangrove forest of the Sundarbans in the southwest. Some Cambic Arenosols and Gleyic alisols in Bangladesh have also been occurring under aquic moisture regime.

Distribution of wetland soils of the world

The wetland soils of the world make up about 10% of the world’s land mass. The principal suborders of wetland soils and their distribution are given is Table- 4. Moorman (Moorman, 1978) reviewed the classification of rice soils in a recent paper including the classification of most wetland soils of the tropics. Because climate is a major factor in agricultural production the wetlands divided into two parts, such as:

a. Non tropical wetland soils.

b. Tropical wetland soils.

a. Non tropical wetland soils

Large areas of Aquepts and Histosols dominant landscapes in Alaska, Canada, Russia, but these soils have little or no potential for food production. Short growing seasons followed by long periods when the ground is frozen make it impractical to drain these soils and impossible to grow crops other than vegetables on them.

Table- 4: Distribution of principal wetland soils of the world

[Source: USDA, 1975]

Wetland soils occupy large areas in the middle latitudes of the northern hemisphere. One of the largest areas is in the delta of the Mississippi River in the United States. Most of the land in this area has been drained and is now producing crops. Haplaquepts, Ochraqualfs and Udifluvents (Bartelli, 1974) are the dominant soils; they are fertile, generally flat and variable in texture. Histosols and Hydraquents are extensive in the southern part, where a large percentage of the land remains in forest. Major cultivated crops are rice cotton & soybean.

The Atlantic and Gulf Coastal plains are is the largest area of wetland soils in the United States and is the area with the most potential cropland (Bartelli, 1974).b. Tropical Wetland Soils

Wetland soils are fairly extensive throughout the humid tropics. South and Southeast Asia have the highest percentage, although large areas exist in South America. Africa has a few large areas, but they constitute only a small percentage of the landmass. Os born (Osborn, 1953) represents one – fifth of the world’s land is about one – third desert, and that all of its irrigated land makes up only 0.1% of the continent. Wetland soils occupy a larger area than irrigated land, but compares to the total the area is still quite small. There are, however, several large areas, principally in central Africa, where wetlands are common. Tropaquepts and Tropaquents dominate both areas, although soils with better natural drainage are intermingled. These soils vary in texture and chemical properties and have limited suitability for conversion to productive use.

Tropical South America probably has more wetland with high potential for development than any other continent. A high percentage of its wetland soils lie in the Amazon basin, which occupies approximately 40% of the continent (Osborn, 1953). Tropaquents and plinthaquults are associated with Oxisols and other Ultisols on the flood plain and terraces of the Amazon.

In northern South America, coastal areas in Venezuela, Guyana & Surinam are dominated by wetland soils. In Guyana and Surinam, most wetland soils are planted to sugarcane and other food crops, including rice. In this area, large expanses of Vertisols are associated with Tropaquepts and Plinthaquulls. The Rupununi Savanna in Brazil and, Guyana is a region typified by a long wet season and dry season. As a result, most soils are either flooded or waterlogged, or both, for many months. This region, which occupies more than 2700 Km² in Guyana alone (Eden, 1970). In India, west Bengal has a large concentration of Ochraqualfs, Haplaquepts, and Haplaquents (Murthy, 1978), ranging in texture from Sandy loam to clay and formed mostly in old alluvium.

Wetland soils in Sri Lanka are located mainly in inland valleys, although some are on coastal plains (Panabokke, 1978). Many of the inland valleys, where most of wetland soils produce rice, are dominated by Aqualfs and Aquults. Aquepts and Hemists are locally extensive on minor flood plains and of coastal plains. Most of these soils have low pH, low CEC, and low base saturation.

The major wetland soils in Malaysia are Aquepts and Aquents (Paramananthan, 1978). Most are formed in marine or alluvial sediments and are the least productive rice soils in the country. Low pH in some areas and sulfate retard rice production.

Aquepts dominate the wetland soils of Indonesia (Soeprapthohardjo et al, 1978), some of the largest areas being on the north coasts of Java and on the east coast of Sumatra. Sulfur deficiency may be a problem in some of these rice soils. Other countries where wetland soils are important are the Philippines, Kampuchea, Laos, Vietnam and Thailand (Dudal, 1964).

2.5 Distribution of wetlands in Bangladesh

                 Bangladesh

[Source: Ali M. Y. 1990]

Table- 6:  Area of different types of wetlands in Bangladesh

[Source: Akonda, 1989]

Soil processes in wetlands

IRRI (1985) published some soil process mechanisms in wetland soils like physical processes (dynamics of water movement; gas exchange and seasonal fluctuation of groundwater; alternate wetting and drying condition; regeneration of structure; formation of restrictive pans, surface crusts and clods; galgai formation; textural differentiation), chemical processes (redox processes; migration, transformation, segregation of Fe, Mn, Al and other micronutrients and macronutrients; salinization, alkanization carbonate accumulation, ferrolysis; pH changes; chelation and complexation) and different biological processes (organic matter transformation and humus formation; decomposition and nutrient cycling; Biological nitrogen fixation and immobilization; nitrogen losses through volatilization and denitrification processes; rhizosphere effect such as exudation, ion exchange etc.)

Genesis and stages of soil formation under seasonally flooded

The soils in the wetland areas remain seasonally water saturated or permanently are in waterlogged condition. Seasonally saturated soils are dry in some of the year if not irrigated. The periods of water saturation vary from soil to soil depending on location in the landscape. The ground water table in these soils fluctuates very widely. The fluctuating ground water tables play important role in the genesis of theses soils. The stages and pedogenic processes are varied and complex in the seasonally flooded soils.

Gleyans is the name given to the shiny surfaces of soil cracks and pores formed by the deposition of material washed from the soil surface or the topsoil under seasonally flooded condition. These coatings generally are continuous, thick and of a uniform grey colour. The colour of the gleyans generally is that of the topsoil: midgrey where the topsoil is midgrey (when wet); dark grey where the topsoil is dark grey.

In English gley is “yellow and grey mottling in the soil produced by partial oxidation and reduction of iron caused by intermittent waterlogging”. The definition in German, Spanish, Portuguese, Italian and Dutch languages is essentially the same as above. Features of partial gleization are expressed by the presence of mottles, which are caused by alternate wetting and drying i.e. oxidation and reduction conditions. The main morphological features of hydromorphic soils are dark coloured organic mineral horizons in the surface soils, Brown or grayish brown B– horizon enriched in sesquioxide mottles, cloddy structure in the topsoil and absence of structure in the subsoil.

Brammer, 1996 noted the stages of soil formation in the wetland soils of Bangladesh are given below:

1. Initial deposition of alluvium
2. Ripening
3. Early development of mottles
4. Homogenization
5. Development of structure in the subsoil
6. Formation of subsoil coatings
7. Acidification and decalcification of topsoils
8. Ferrolysis, if the soil is acidic
9. Formation of ploughpan
10. Oxidation of subsoil
11. Formation of peat ( specially in some soils)

The most important horizon in a hydromorphic soil is the gley horizon. Thorp and Smith (1949) introduced the new name “gleys” as a great soil group in the soil classification system of the United States. It was defined as, “an intrazonal group of poorly drained Hydromorphic soil with dark coloured organic horizon of moderate thickness underlain by mineral gley horizons”.

According to Joffe (1949) the popular Russian idea of gley is “a more or less compact, sticky loam or clay material which is not, however, as sticky as the loam or clay, frequently with more or less clearly pronounced light greenish blue tinge”.

In the Soil Survey Mannual (Soil Survey Staff, 1951), a gley horizon has been defined as “a layer of intense reduction, characterized by the presence of ferrous iron and neutral grey colour that commonly change to brown upon exposure to the air”. The gleization process involves saturation of the soils with water for long periods in the presence of organic matter. In English gley is “yellow and grey mottling in the soil produced by partial oxidation and reduction of iron caused by intermittent water logging”.

Soil Survey Staff (1975) reported that when reducing condition prevails due to water saturation for a long period of time, and the regime is defined as aquic moisture regime. The aquic soil moisture regime implies a reducing regime that is virtually free of dissolved oxygen because the soil is saturated with ground water or by water of the capillary fringe.

Dudal and Moormann (1964) termed the artificial man-induced water regime as “anthraquic” and the superficial soil horizon that formed under this water regime was called “anthraquic epipedon”. Anthraquic epipedon may comprise both the cultivated layer and the underlying ploughpan. Both the layers have grey base colours and are strongly iron stained along root channels. The cultivated layer becomes strongly reduced when water logged or flooded.

Smith (1983) stated that reduction and gleying processes that result from biological activity under anaerobic environments constitute only part of the properties diagnostic for soils with aquic moisture regime. “Surface water gley” or “pseudogley” or “inverted gleys” are the principal morphological features of soils that develop under aquic moisture regime. Karmanov (1966), Wada and Matsumeto (1973) and Mitsuchi (1974) noted that poorly drained low land soils do not change much, when used as paddy land.

In the waterlogged soils of Bangladesh, there is commonly a strongly oxidized layer and in places a coating of iron oxides, but no iron pan, at the base of the ploughpan (Brammer, 1971).

Brinkman (1977) noted that surface water gley soils are extensively developed on seasonally flooded landscapes. They may be acid hydromorphic soils with albic horizons and containing less clay than the deeper horizons, and with a seasonally fluctuating pH in the surface horizon. The oldest of the Holocene floodplain landscapes in Bangladesh have soils in which only the upper 10 or 15 cm have less clay and contain some albic materials. He considered it premature to attempt a classification of paddy soils with an expression of ferrolysis.

The surface water gley soils also occur on many other floodplains in Bangladesh (Brammer and Brinkman, 1977). The Tista, the old Brahmaputra and the northeastern part of the Meghna river floodplains as well as the old Meghna estuarine floodplains contain soils in which the surface horizon has an anomalously low clay content and seasonally fluctuating pH.

Brammer and Brinkman (1977) correlated surface water gley soils in Bangladesh with the hydromorphic soils developed on the floodplain landforms occurring widely in south east Asia (e.g. Vander Kevie, 1972), the degraded rice soils of Burma (Karmanov, 1968); the “aquarizems” of Japan (Kyuma and Kawaguchi, 1966), and the “anthrasols” (Dudal and Moorman, 1964) that form in areas long used for seasonal wetland rice cultivation. Hydromorphism is the common soil forming process in all these soils.

Hassan (1984) studied the soil formation in the floodplain areas of Bangladesh and observed that the soil formation in this region takes place in several stages. Initially, the sediments have low bulk density and high water content. In the ripening stage moisture is lost irreversibly resulting in an increase in bulk density. Subsequently, the sediments become homogenized due to bioturbations. At this stage the formation of soil structure begins due to alternate seasonal shrinking and swelling caused by wetting and drying in the wet and dry seasons, respectively. Soils of this region possess either A-C type profile, or locally an A-(B)-C type profile. Biotic factors, depending on the duration of dry and wet periods, contribute to the soil structure formation. Effects of other soil forming factors become diffused due to high ground water table and jubenility of these soils. It is interesting to note that in most cases, the attributes of the parent materials dominate the soil properties.

Saheed and Hussain (1992) reported that in many floodplain soils influence of an aquic soil moisture regime were imposed in the upper part of the profiles, while the lower horizons reflect that of the free drainage.

Ali (1994) noted that an anthraquic epipedon forms in the floodplain soils of Bangladesh under an imposed aquic moisture regime having rice cultivation for a long time.

Seasonally flooded soils of Bangladesh show special features such as coatings (gleyans), ferrolysed layers and anthraquic horizons which have not been recognized in the USDA Soil Taxonomy (Soil Survey Staff, 1975). So, the classification of these soils is tentative and subject to change as new information becomes available.

In the general sol type system of classification in Bangladesh the Noncalcareous wetland soils are classified into nine general soil types Table- 10. The table shows that noncalcareous grey floodplain soils occupied the highest proportion and occupied around 32 percent. Noncalcareous Dark grey floodplain soils covered 11 percent. A major part of these two soils are seasonally flooded.

The wetland soils are mainly classified as Aquepts because of the occurrence of gleyed coating on ped faces or dominant grey colours in the subsurface layers. Most of these soils fall in the typic or aeric subgroups depending on the degree of oxidation or reduction.

All the floodplain soils have grey matrix colours (chroma <2) and mottles. Many of these soils have dark-grey coloured topsoils and their subsoil coatings are also dark-grey coloured. Organic matter content are not high but dark humus staining was developed under natural vegetation. Soils on the old Meghna estuarine floodplain and most basin soils on the old Brahmaputra floodplain are classified as Noncalcareous Dark Grey Floodplain Soils.

Characteristics of wetland soils:

Morphological characteristics and properties:

Wetland soils cover a relatively important area of the world’s land mass and may be found in practically every climatic zone. They may point to the presence of a high groundwater table or to an impermeable layer close to the surface, or they may result from natural or artificial flooding of the soil during part of the year.

Wetland soils are characterized by hydromorphic features (halomorphic, gypsimorphic, calcimorphic or redoximorphic ones, depending on the volume of the catchment area) whose arrangement corresponds to differences in redox or water potentials in space and to their changes in time. Hydromorphic features often coincide with but are not identical to redoximorphic features (e.g. iron П sulfides, iron Ш oxides. Wetland soils may be submerged soils or floodwater soils or ground-water soils or surface- water soils. The best micromorphological indicator of hydromorphism is the presence of typic sesquioxidic features. Manganese segregation (mainly coatings and hypocoatings) dominate in slightly hydromorphic materials. With increasing hydromorphism, iron hydroxide segregations appear in the groundmass and low chromas in the peds.

Sawy and sadek (1989) studied morphological properties of some poorly drained soils from the Nile delta. They found that the soils had large void structures, such as compound packing, vughs and channels and preserved illuviation argillans and ferriargillans. The translocation of clay was found to be a very active process at the beginning of profile development.

Smith and Beecroft (1983) from the study of the morphology and water regimes at three recent alluvial soils of New Zealand found that with increased duration of saturation, mottles tend to decrease in size and abundance and have more diffused boundaries. The matrix colors of the soil horizon, in conjunction with the nature of the mottles, broadly reflect the different moisture regimes at the three sites although the distinction between imperfectly and poorly drained soils was not clear.

Das (1977) reported that the soils of Brahmaputra valley are formed both old and new alluvium of the river Brahmaputra and its tributaries. The materials were of mixed origin. The soils of the old alluvium were in two different relief environments, namely, upland with good drainage and lowlands with nearly hydromorphic conditions subject to submergence. The soils are acidic which might be due to the nature of the alluvium itself. These soils showed signs of release of iron under gleying conditions.

Physical properties:

Physical properties of soils do not change their pattern easily in comparison to the morphological and chemical properties. The soil physical properties thus impart stable and most useful contribution in soil classification purposes and in the diagnosis, management and improvement of cultivated soils.

Soil colour

Among the conspicuous features of soil, colour is the most important and is highly useful for its identification and characterization. Soil scientists commonly use grey colours
(Chroma < 2) as an indicator of seasonally or permanently saturated and reduced soil conditions.

The colour of the topsoils of Acid basin clays is grey to dark grey, and has numerous brown and yellow mottles along root channels (Hussain, 1992). Because of ploughing the topsoil is sometimes compacted. These soils developed large polygonal blocks when dry.

Soils of the haor areas of Sylhet and Mymensingh district, Arial beel and Comilla basin have heavy textured soil. The subsoil is grey to dark grey heavy clay and has strong yellow to red mottles (Hussain, 1992). The agricultural potential of these soils is limited by the moderately deep to very deep seasonal flooding.

SRDI Staff (1965-86) noted that overall soil colour ranged from olive to olive brown on ridges and grey to very dark grey in inter ridge depressions and basins. Sehgal et al. (1968) studied the soils of the Sutlej floodplain area and found that these soils were light grey to light yellowish grey even when the soils were mostly poorly drained.

Brammer (1971) noted that the textures of soils are known to affect the brightness of colour in some soils of Bangladesh. Usually the light textured soils tend to have lighter colours and the heavy textured soils tend to have darker colours. The reduction and segregation of iron in the soil, however, had been intense enough to produce horizons dominated by grey or olive grey colours with chroma of 2 or less. In the heavy textured soils, organic matter might have a role in producing dark grey colour.

Singh et al. (1989) found variation in soil color in some imperfectly drained soils which were attributed to different characteristics of the parent materials. Poorly drained soils have hue of 2.5Y with a chroma of less than 2. Micromorphologically, medium to coarse structure in argillic horizons reflected varying degree of plasma segregation and accumulation.

Daniels et al.  (1973) have indicated that reducing conditions result in grayish colour in poorly drained soils and oxidizing conditions are responsible for the brighter colours in better drained soils. A study of seasonal fluctuations of water table in soils have shown that soil colours can be used as a general indicator of saturated and reduced conditions, as well as movement of ground water table (Mackintosh and Hust, 1978; Zobeck and Ritche, 1984; Pickering and Veneman, 1984; Evans and Franzmeir, 1986).

Buol et al. (1973) stated that since colours are good indicators of soil behaviour, and the environment in which they form, they may be helpful in arriving at conclusions concerning their best uses and management.

SRDI Staff (1974) surveyed the soils of the Brahmaputra floodplain area and noted that soils formed in the seasonally flooded old Brahmaputra floodplain have surface soil colours ranging from olive to grey and olive grey in ridges and grey to dark grey in lower slope of ridges and basins. Subsoil colour ranges from grey to dark grey in ridges and grey to mixed dark grey in basins.

Usually in young soils colour is an indication of parent materials while in freely drained mature soils, it is an indication of climate. Generally low colour development equivalent in the floodplain soils is an interesting feature (Hussain et. al. 1981). This is due to their poor drainage conditions and annual flooding.

During flooding there is a net loss of iron from these soils which helps in developing a grey to yellowish grey hue. Gleization is thus a general soil forming process in these soils (Hussain and Majumder, 1992).

Coventry and Williams (1984) studied the quantitative relationships between morphology and current soil hydrology in some Inceptisols in Australia and observed a strong relationship between the soil colour and the current ground water table. They also found that the grey colours occurred when there was saturation of at least five for at least 21 weeks.

Sharma and Dev (1985) studied different geomorphic surface in the riverine plain of the Punjab and found that the soils showed wide variation in hues ranging from 7.5 YR to 5 Y depending on their physiographic location. Soils located in the lower terrains showed grey colours (hue 5Y) with low chromas suggesting poor drainage conditions and a fluctuating ground water table.

Singh et al. (1989) found variations in soil colour which were attributed to differential characteristics of the parent materials. On the basis of morphological characteristics they concluded that the poorly drained floodplain soils have hues greyer than 10 YR, value 5-6 and chroma less than 2; whereas moderately to well drained soils have redder hues and higher chromas.

Okusami and Rust (1992) the hydromorphic soils from some inland depressions, alluvial plains and coastal sediments. A 10YR hue was observed in addition to other typical morphological features of hydromorphic soils. Redder hues within a 10YR matrix colour indicated an iron accumulation zone which reflected the aerobic or anaerobic fluctuation zones.

Szogi and Hudnall (1992) emphasized that in studying the seasonally wet soils the terms “episaturation” and “endosaturation” must be differentiated because the different nature and duration of perched and ground water tables. The endosaturation conditions are related to the occurrence of horizons with oxidizing conditions above reduced horizons. In saturated zones of soils that have true water tables, the redox potentials are low, particularly if the water table is not very mobile.

Khan (1995) studied some benchmark soils from the seasonally flooded recent floodplains of Bangladesh and observed that the top soil colours generally ranged from grey to dark grey while the sub soil colours ranged from olive grey to grey.

Mazumder (1996) and Ali (1994) studied some soil series developed on the Brahmaputra alluvium and found that topsoil colour ranged from grey to dark grey to dark grey while the subsoil colour is olive is olive to grey with abundant mottles.

Concretions and mottling

Charkrabarti et al. (1984) examined the morphological features of five profiles of alluvial soils from Brahmaputra and Surma valleys of Assam and found that upland leveled soils were devoid of mottlings and concretions and were moderately developed. Soils affected by high water table resulted in the formation of iron mottlings and were weakly developed. Profiles of Brahmaputra valley indicated the development of a weak argillic horizon, and the Surma valley soils, which remained submerged for most part of the year, did not have iron concretions.

In the floodplain soils of Bangladesh, mottles are the most common redoximorphic feature. They are the reliable indicators of aquic moisture regime (Saheed and Hussain, 1992).they can be formed by both epiaquic and endoaquic mechanisms in these soils.

Sidhu et al. (1978) have reported the movement of reduced Fe and Mn by mass flow and diffusion during the rainy months, and their precipitation after the rainy season, when conditions are favourable for oxidation, to be the possible stages in the alluvium in the genesis of Fe-Mn concretions and mottles in the soils developed on seasonally flooded alluvium in the Punjab.

From the study of the morphology and water regimes of three recent seasonally flooded soils developed on a floodplain in New Zealand, Smith and Beecroft (1983) found that with increased duration of saturation, mottles tend to decrease in size and abundance and have more diffuse boundaries. The matrix colours of the soil horizons, in conjunctions with the nature of the mottles, broadly reflect the different moisture regimes at the three sites although the distinction between imperfectly and poorly drained soils was not clear.

Coatings

A coating is an important property in the seasonally flooded soils in the floodplain areas. Brammer (1971) called these as flood coatings that has shiny surface is formed by deposition of material washed from the soil surface or the top soil under seasonally flooded conditions. These coatings are typically continuous, thick and of a grey colour. Brammer (1971), further reported that the colours of the coatings were same as that of the top soils, grey where the top soil is grey (moist); dark grey when the top soil is dark grey. This indicates that the materials have been derived from the top soil and not from suspended materials in the flood water. It also indicates that the flood coatings are not pressure coatings. Coatings of the lowland soils may develop in soils under cultivation, forest and grass land, and on floodplain as well as terrace land; the common factor in their occurrence being seasonal flooding.

According to Veneman et al, soil materials that are subject only from time to time to short period s of saturation shows little or no mobility of the iron. Therefore the high chroma of the matrix is preserved. Manganese is more readily reduced than iron and can be translocated to the ped surfaces, where it is precipitated as manganese coatings and hypocoatings. Soil materials saturated during several consecutive days show a clear mobility of the iron present. Only the largest pores are filled most of the time with air, and the ped interior remains practically constantly wet and is the seat of reduction of iron and manganese compounds which give rise to low chromas inside the peds. In wetter conditions coatings of iron oxyhydrates are deposits on the walls of voids. Manganese is more readily reduced than iron and leached down the profile or may be present as black diffuse nodules. When the soil material remains completely wet during several months, much of the iron and manganese is removed from the profile. Inside the peds low chromas are therefore observed. Iron and manganese are deposited as coatings around voids.

Particle orientation in flood coatings has not been studied well in the seasonally flooded soils of Bangladesh. They are quite extensive in the soils of the Ganges, Meghna and Brahmaputra floodplains. Brammer (1971) proposed the name gleyans for these coatings since the coating materials are always gleyed and usually have a grey colour (reduced). SRDI Staff (1970 and 1975) studied the nature of flood coatings in the seasonally flooded soils developed on the floodplains of Mymensingh district by the old Brahmaputra River. In these soils cracking is common, flood coatings are enormously present.

Soil Texture

Soil texture is one of the most fundamental and permanent characteristics that has direct bearing on structure, porosity, adhesion, consistency and physico-chemical behavior of soils.

SRDI Staff (1965-86) working on the soils of Comilla districts stated that the soils developed in the Meghna and Brahmaputra floodplains almost everywhere showed a distributional pattern of friable silt loams to silty clay loams on the ridges and clays in the basins. Some clays in older floodplain areas were very heavy and cracked widely when dry and flood coatings were also quite common in them.

Generally, the alluvia of most rivers passing through Bangladesh are predominantly silty. But sediments of rivers show some distributional trends. Sandy materials usually occur near active and abandoned river channels, and clayey materials occur in the basin sites. As one moves from the river banks towards the back-swamps the texture of the soil materials gets gradually finer. Peat may sometimes accumulate in the depressional sites, if it is permanently wet. The major river systems in Bangladesh are reported to have varied in their extent of sandy, silty and clayey deposits. The sediments of the north Bengal rivers draining directly from the Himalayas are relatively coarse textured and those of river Meghna draining the central parts of Sylhet basin are relatively fine textured. In general, the proportion of fine textured sediments increases from north to south across Bangladesh (FAO, 1971).

The texture of the substratum of Acid basin clays is usually silty and sometimes clayey and is permanently reduced (Hussain, 1992).

Brammer (1971) stated that silt loams and silty clay loams predominate on the old Meghna estuarine floodplain, whereas silty clays and clays occur extensively on the old Brahmaputra floodplain. He noted that loam was the dominant textural class in Bangladesh soils followed by clay.

Hussain and Chowdhury (1980) reported the physical properties of a number of cracking clay soils and observed that clay was the dominant fraction followed by sand and silt. The mean clay content of the soils was 55 percent.

SRDI staff (1970) found that most ridge soils in old Brahmaputra floodplain are silt loam to silty clay loam and inter ridge depressions they are mostly silty clay loam to silty clay.

Okusami and Rust (1992) pointed out that the soil texture varied quite widely with land types in their study of hydromorphic soils from inland depressions, alluvial plains and coastal sediments.

Soil Structure

The importance of soil structure in soil classification and soil productivity can hardly be overemphasized (Soil Survey Staff, 1975). Alternate wetting and drying situations play an important role in soil structure formation. Wetting which leads to expansion and gelation and drying which leads to construction and degelation are extremely important phenomena in structure formation, and hence, in profile differentiation with respect to one another and also with respect to structure development. Wetting and drying and expansion and construction affect structure formation because they affect orientation of the clay particles with respect to the plant roots (Bear, 1965).

The structure of the soils of acid basin clays is coarse prismatic to blocky. Some structural units have shinny pressure faces. The pressure faces are occasionally wedge-shaped (Hussain, 1992).

SRDI Staff (1975) found that the moderately well to imperfectly drained subsoils have weak to moderately strong prismatic and blocky structure in the old Brahmaputra floodplain area. The imperfectly to poorly drained soils have moderately strong to very strong prismatic structure; and the poorly to very poorly drained soils have strong prismatic and blocky structures.

Brammer (1996) stated that polygons provide the prismatic structure which is typical of most loamy and clayey floodplain soils. Clay material cracks horizontally and usually develop both blocky and prismatic structure. Prismatic structure may be broken to angular blocky or subangular blocky in less clayey materials.

Mazumder (1996) studied some Brahmaputra floodplain soils land found that prismatic to angular blocky structure developed in the subsurface horizons of the profiles whereas the surface soils were massive.

Soil Moisture

Soil moisture retention is strongly related to the surface area per unit mass of the soil. It is also related to the texture of the soil and the clay mineral types.

Das et al. (1974) studied water retention characteristics of 12 alluvial soil profiles from West Bengal. Soil varying in textures from sandy loam to sandy clay loam showed increasing trend of water holding capacity with depth (42.9-58.7%) and the available  water ranged from 7.1 to 23.3 percent in Gheora and from 7.7 to 16.0 percent in Mehrauli profile.

Chemical properties:

Submergence causes changes in the nutrient status of the surface soils. So the nutrient dynamics in wetland soils are in a complicated situation. There is not only a vertical and lateral redistribution of various nutrient species including organic matter but a change in the forms of the individual nutrients as well.

Soil Reaction (pH)

Soil reaction is the most important single chemical characteristic which has a great influence on different physical, chemical and mineralogical property of the soils. Suitability of soil as a medium for the plant growth and desirable micro-organisms depends upon whether the soil is acid, neutral or alkaline and therefore this property play an important role in pedogenesis and classification.

In most of the wetland soils of Bangladesh, pH value, in general, is around 7.0 which become alkaline on drying when the soils are calcareous and the noncalcareous soils become acidic. Gangetic alluvium, being calcareous and the noncalcareous, has higher pH when dry than other river alluvia. With depth, in almost all soils, there is an increase in pH (Saheed and Hussain, 1992). Like organic matter, the total nitrogen contents in the mineral wetland soils range from 0.05 to 0.1 percent.

Ponnamperuma (1965 and 1972) observed that when an aerobic soil was submerged, its pH decreased during the first few days, reached a minimum, and then increased asymptotically to fairly stable value of 6.7 to 7.2 a few weeks later in a 1:1 soil water suspension. He reported that the overall effect of flooding is an increase in pH of most acid mineral soils. The increase in pH of most acid mineral soils was due to the reduction of Fe³⁺ to  Fe²⁺ and a decrease in pH of alkaline soils which was due to CO₂ accumulation.

Sehgal et al. (1968) reported pH in some soils of the Sutlej floodplain of Punjab ranges from 7.9 to 9.1. It is possible that these soils were calcareous. Mujib (1969) and Brammer and Brinkman (1977) concluded that the increase of pH with depth is a common feature in the seasonally flooded soils of Bangladesh.

Hussain and Swindale (1974) noted that the increase of pH with depth is a characteristic property of the hydromorphic soils in Hawaii.

Hussain et al. (1980) studied the properties and genesis of some pedons from the Tippera surface and observed that the soils were strongly acidic at the surface and the pH gradually increased with depth. The gradual increase of pH with depth was an important characteristic feature of all the floodplain soils in Bangladesh (Brammer, 1971).

Chakraborty et al. (1984) studied the morphology and physico-chemical properties of some alluvial soils of Assam and reported that the pH of the soils increased with profile depth.

Hussain and Chowdhury (1980) studied some soils from Ganges delta in Bangladesh and found that the reaction (pH) of the soils neutral to slightly alkaline with a pH in the range of 6.7-8.0. The pH values increased with increasing depth. Rahman et al. (1992) also reported similar feature in some calcareous Gangetic alluvial soils. In the noncalcareous soils this feature is even more regular (Brammer, 1971).

Organic Matter and C/N Ratio

In wetland soils organic matter is the substrate for anaerobic microbial activity that results in oxygen depletion and characteristic change in redox potential, pH, and nutrient availability. Soil organic C accounts for 0.1-40% of wetland soils. Most mineral wetland soils of tropical Asia have only up to 3% organic matter content is very low. More than half of the soils have organic matter in the range of 1 to 2% (Huq et al. 1993).

SRDI (1965-86) analyzed a large number of samples from many representative soil profiles covering almost all the areas of Bangladesh and found that the organic matter contents of soils were generally low; ranging from 0.3 to 1.5 percent in upland soils, 1.5 to 2.0 in medium low land areas and 2.0 to 3.5 percent in the low land areas. In beel areas, the organic matter content was about 4.0 percent (Rahman, 1990).

Hussain and Chowdhury (1981) studied the pedochemistry of some cracking clay soils in the basin areas of Dhaka and Comilla districts and observed that organic matter in the surface horizon was quite high and decreased with profile depth.

According to the classification proposed by BARC (1989), the percentage of organic matter within the limit of 1.0-1.7 fell in the low range. Huq (1990) commented that most agricultural soils of Bangladesh have low organic matter content.

Mujib (1968) found 1.37 percent organic matter in some soils of Brahmaputra floodplain of Bangladesh. Sidhu et al. (1994) noted that the organic carbon content in some floodplain soils decreased irregularly with depth due to their fluvial nature. These soils were Entisols. Saheed and Hussain (1992) reported that both organic carbon and total nitrogen contents in the mineral wetland soils in Bangladesh were low.

Physico-chemical properties:

Cation Exchange Capacity (CEC)

Cation exchange phenomena are the most important properties in soils. Cation exchange phenomena are considered as an index of soil fertility as well as soil quality. It plays an important role in the genetic processes of soils.

SRDI Staff (1975) reported that CEC of the top soil of most of the Brahmaputra floodplain area having less than 2% organic matter ranged from 9-23me/100g soil. The CEC of subsoils was usually slightly higher ranging from 10-28me/100g of soil. The values of CEC slightly increased with depth due to their increased clay content. The CEC in the wetland surface soils of Brahmanbaria district is 21.22 cmol (p⁺) Kg^-1.

Chatterjee and Dalal (1976) reported that the CEC value of some soils from Bihar and West Bengal decreased with depth from 4.1 to 10.2me/100g.

Karim and Islam (1956) reported that the average CEC of clay and silt fractions of some soils and sediments of Bangladesh were 27 and 12 cmol (p⁺) Kg^-1 respectively.

SRDI staff (1969) reported an increase in CEC values with the increase of clay contents in the Sara and Ghior series.

Exchangeable Cations

Wright et al. (1955) stated that the cations such as Ca⁺⁺, Mg⁺⁺, Na⁺ and K⁺ are known to be sensitive to leaching during weathering and soil formation. In well drained normal soils Ca⁺⁺ is the dominant cation. Kanehiro and Chang (1956) noted that as the gleization process progresses, the exchangeable Ca⁺⁺/Mg⁺⁺ ratio tends to approach unity. It has been reported that under the gleization process of soil formation, exchangeable Mg⁺⁺ becomes the dominant cations in the exchange complex (Sunders, 1959).

Ponnamperuma (1964) reported that submergence of soil causes an increase in the concentration of ions in the soil solution, as indicated by the rise in specific conductance. In a reduce soil, these ions are chiefly Ca⁺⁺, Mg⁺⁺, K⁺ and Na⁺ which are not involved in the reduction process, the increase in their concentration is a secondary effect of submergence and reduction.

Ferrolysis is a process of soil degradation of silicate minerals under alternate cycles of oxidation and reduction with removal of basic cations from the soil and eventually destroying the clay particles (Brinkman, 1970). This process is dominant in the wetland soils of old floodplain areas of Bangladesh (Karim et. al. 2001).

Hussain and Swindale (1974) studied the grey hydromorphic soils of Hawaii and reported that the average exchangeable Ca⁺⁺/Mg⁺⁺ ratio was 1.5. Islam and Islam (1973) studied some submerged soils of Bangladesh and found that the exchangeable Ca⁺⁺ concentration first increased after submergence and then decreased. He noted that the velocity and magnitude of increase of exchangeable Ca⁺⁺ varied in different soils. In all soils, the peak values were obtained in the ninth week of submergence.

Walia and Chamuah (1992) found that the exchange complex of the flood affected soils of Brahmaputra valley are dominantly saturated with Ca⁺⁺ and Mg⁺⁺ cations followed by Na⁺ and K⁺.

Base Saturation Percentage

Bychenko and Komarovkaya (1971) reported 90% base saturation in the floodplain soils of Afanasiev area of Kirvo region. Increase of base saturation with depth was reported by Mazumder (1976) in some deep water rice soils of Bangladesh. Ali (1994) observed high degree of saturation in some alluvial soils of Brahmaputra and Gangetic floodplains.

Macro-Nutrients

Habibullah et al. (1980) reported that the total K in the soils of the Brahmaputra floodplain and in the Grey Terrace soils ranged from 1.39 and 2.29 percent, respectively.

Brar and Sekhon (1985) studied a large number of soils from the Indus basin belt in the Punjab and Uttar Pradesh. They studied reserve K levels in soils in relation to organic carbon, clay and texture. According to Islam et al. (1988) the total soil K in 29 rice growing soils from Bangladesh ranged from 1.04 to 3.12 percent.

Hussain et al. (1989) indicated that available N showed a significant positive correlation with organic matter and total N in some submerged soils of Bangladesh. SRDI Staff (1991) studied soil series of Meghna floodplain and observed that the mean amount of nitrogen is below the critical level (75 ppm), the mean phosphorus content is medium (13-25 ppm) and the mean potassium content is high.

Hussain et al. (1992) studied the properties of four pedons from Bhola district and observed that the available nitrogen ranged from 42 to 95 ppm, available phosphorus from 6 to 12 ppm and available potassium ranged from 200 to 452 ppm. The concentration of available nitrogen, phosphorus and potassium were higher near the surface and decreased downward in the profile.

Islam et al. (1988) indicated that soils of Bangladesh are rich in total K but quite poor in available K. They further pointed out that the readily exchangeable potassium varied between 8 and 189 ppm with an average of 53 ppm. Exchangeable potassium accounted for 0.33 to 0.65 percent of total K and had significant correlations with the clay contents and CEC values of the soils.

Elahi et al. (1993) studied K status of 12 extensively occurring agricultural soils from northern region of Bangladesh and showed that total K₂O content in soils ranged from 1.53 to 3.04 percent.

Micro nutrients

The amounts of free oxides in soils are very important in evaluating mineral weathering and pedogenic changes in soils (Mckeague and Day, 1966; Blume and Schwertmann, 1969). Orlov (1992) stated that the forms of iron and manganese in soils and their vertical distribution in the profiles reflect the trend and peculiarity of the soil formation process. Quantity of free Fe₂O₃ in soils is important in determining their genesis.

Arduino et al. (1984, 1986) indicated that the free Fe₂O₃ / total Fe₂O₃ ratio is a good criterion for indicating the relative age of soils and its development. They noted that the percentage of total iron (Fe_t) extracted by dithionite (Fe_d) iron increased with increasing age of the soils.

Joffee (1960) stated that the surface enrichment of free iron oxide was probably due to the fact that ferrous iron is oxidized easily to form ferric oxide at the surface, while in the sub soil ferrous oxides does not change.

Mckeague (1965) stated that the low free iron oxide is neither due to the leaching of iron nor to the accumulation of iron in mottles but presumably resulting from reduction.

Brinkman (1970) reported that during the anaerobic phase of soils free iron is reduced with continuous oxidation of organic matter and formation of hydroxyl ions. The ferrous iron displaces other cations and the displaced cations are leached from the soils.

Hussain et al. (1992) stated that the free iron oxide content in the floodplain soils of Bangladesh was low due to loss of iron when reduced along with draining water thus making the colour of the surface soil grey or dark grey. He reported that at a depth of around 40-80 cm in all the studied pedons an enrichment of free iron oxide occurred, which was probably due to their fixation in the profiles in the form of mottles. This is very common feature in the floodplain soils growing rice.

Joffee (1960) noted that the movement and deposition of manganese dioxide and its distribution in the soil profiles were significant criteria in pedogenic study as it behaves more or less like iron oxide. In general manganese oxide is more easily soluble than iron oxide.

Daniels et al. (1962) stated that, poor drainage condition was probably the important factor that caused the decrease in free iron and manganese oxides contents of the top soil. Karim (1984) identified aluminium substituted goethite in some seasonally flooded rice soils in Bangladesh.

Mineralogical properties:

Brammer (1996) reported that floodplain sediments of the Surma and other eastern rivers contains low amount of feldspars and micas and their total content of easily-weatherable minerals usually is about 10 percent. However some rivers such as Someswari and Matamuhuri bring the deposits richer in amphiboles. Deposits in the Sylhet basin are also richer in amphiboles (25-55 percent of the heavy fraction) than most alluvium derived from the Tertiary hill sediments. At the same time, they are richer in heavy minerals like epidote (20-40 percent) and zircon than Brahmaputra deposits. The clay mineralogy of floodplain sediments is also strikingly different from that of hill sediments and the Madhupur Clay. Teesta and Brahmaputra river sediments and old Meghna deposits have a mixture of kaolinite, illite and chlorite. Young Meghna estuarine deposits have these same clay minerals but they also contain significant amounts of smectite.

Brinkman (1977) worked with the surface water gley soils in Bangladesh. He noted that in the soils of the old Brahmaputra sediments easily weatherable minerals comprise about 30%, a third of which was mica. From X-ray diffractographic data he found that vermiculite account for about 54% of the clay fraction in the lowest horizon and this was progressively replaced by soil chlorite, which increases to 30% in the surface horizon. He roughly estimated 25 % illite, 20% kaolinite and 10-20 % inter-stratified minerals in those soils.

Hassan and Razzaq (1981) and Hussain et al. (1989) reports high content of mica, kaolinite and smectite in the clay fraction of soils formed on the Gangetic alluvium. The variation in the content of smectite with depth was reported to be insignificant which led them to conclude that this mineral was allogenic in origin and the post-depositional changes are very small.

Floodplain deposits of the Surma-Kusiyara and other small eastern rivers have characteristically low content of feldspars and micas. Their weatherable mineral content is reported to be around 10 percent (Hussain, 1992).

In the floodplain soils of Bangladesh there is little evidence of any change in clay mineralogical composition due to pedogenesis, except in the topsoils of some older, noncalcareous soils where ferrolysis reactions appear to have destroyed some of the original clay minerals forming a small amount of amorphous silica-alumina-iron mineral (Brammer, 1996).

Clay synthesis possibly takes place in some poorly drained soils. Small quantity of montmorillonite is reported to have been synthesized in some samples taken from a basin clay on the Surma-Kusiyara as well as the Gangetic floodplains together with kaolinite and illite (Brammer, 1996).

Table- 8: Minerals in the sand and silt fractions and the weatherable mineral contents in some noncalcareous

floodplain soils of Bangladesh

Source: Huzing, 1971; Egashira, K and Yasmin, M. (1990) and Hussain, M. S. (1998)

Nt* = Non-transparent; I** = Interstratified minerals

Kapoor et al. (1982) studied the clay mineralogical composition of soils from four profiles developed on alluvium by suing X-ray diffraction technique and found dominance of illite followed by mixed-layer minerals, chlorite, smectite, chloritized smectite and vermiculite. The illite present in the soils was found to consist of both the dioctahedral and trioctahedral varieties and the latter appeared to have undergone transformation to smectite-like minerals through intermediate stages of (10-14 A) interstratifications.

Puri et al. (1983) studied the silt and clay mineral composition in soil samples from nine different locations representing five soil groups of Gangetic alluvium. They found that these soils were coarse to moderately fine textured and moderately to poorly drained with a predominance of mica followed by chlorite, smectite and vermiculite. They concluded that these minerals were relatively high.

Vinayak (1984) investigated the clay mineralogy of three typical saline-sodic soils if the Indo-Gangetic alluvial plains by X-ray diffraction and chemical techniques. He found that illite was the dominant clay mineral in all the soils. He characterized the saline sodic soils by the presence of smectite as a second dominant mineral. It was, however, not detected in the associated cultivated soil. Minor amounts of chlorite and kaolinite were present in all cases. He reported that imperfect drainage might have resulted in the transformation of illite to smectite. Illite, kaolinite and chlorite in these soils were probably inherited.

White (1985) studied the clay mineralogy of some representative soils from the Brahmaputra and the Ganges floodplains of Bangladesh. From the X-ray diffractographic data he reported that among the Brahmaputra floodplain soils an exclusive combination of mica-kaolinite vermiculite has been found in the younger member of the soils. He stated that apparently most of these kaolinite and vermiculite minerals are part of the original alluvial deposition, while chlorite and smectite are the products of in-situ weathering. The relatively older soils of the old Brahmaputra floodplain, with a mild acidic environment in the pedons, have traces of chlorites, but not smectite.

Ali (1994) studied the effects of alternate wetting and drying cycles on pedogenic processes of some representative Bangladesh soils. He stated that the Gangetic alluvial soils were found to contain clay minerals like smectite, illite chlorite and kaolinite. Their contents varied with respect to soils and their management. In Brahmaputra alluvium soils, he observed that the dominant clay minerals were illite, chlorite and kaolinite.

In a study of some Brahmaputra floodplain soils Majumder (1996), observed that the dominant clay minerals were mica, kaolinite, chlorite and a small amount of vermiculite. A small amount of smectite was also detected in the subsoils of Sonatala soil series.

Wetland conditions and characteristics of Bangladesh

Agro-Ecological Regions (AER)

The Land Resources Appraisal of Bangladesh (FAO 1988) classified the whole of the Bangladesh into 30 different agro-ecological regions in which depth and duration of seasonal flooding has been one of the component information layers. Among them 24 AEZs have floodplain soils. Distribution of various land types in different agro-ecological regions has been incorporated as an Annexure. Wetlands occupy about 50% of the areas of these agro-ecological regions and about 17% of the total areas of Bangladesh. However, these estimates exclude the water bodies like rivers. Figure- 3 indicates the various agro-ecological regions where ‘wetland’ and ‘wetland soils’ are present. The regions 3, 10, 11, 17, 22 and 28 also contain some medium to lowlands which at times of the year become wetlands. It is interesting to note that wetlands occur at contour lines as high as 15 meters. This phenomenon is principally due to local hydrology and geomorphologic formations.

Geomorphology, geology and physiography of wetland soils of Bangladesh

Depending on the land levels in relation to seasonal flooding (depth of flooding and duration of flooding) six broad land types (Table- 7) are recognized in Bangladesh (FAO, 1988).

Of these land classes medium lowland through bottomland are to be considered “wetlands” and “wetland soils”. These lands have constraints of various degrees so far as their uses are concerned. A generalized map of the inundation land types is shown in figure 4. The area distribution of various land classes are shown in Table- 8.

Fig. 3: Agro ecological region in Bangladesh (BARC, 1999)

Table- 9: Land Types in Bangladesh on the basis of Inundation Depth

[Source: FAO-UNDP, 1988]

Table- 10: Area distribution of various Land Classes

[Source: SRDI Staff (1965-86)]

Fig. 4: Inundation land type map (BARC, 1999)

Geology:

There are three major geological formations in Bangladesh which are important to the context of soils in general and ‘wetlands’ and ‘wetland soils’ in particular. These are: Tertiary hill sediments in the northern and eastern hills, the Modhupur clay of the Modhupur and Barind tract in the center and west and recent alluvium in the floodplains and estuarine areas. Geologically, the Bengal basin is an active tectonic region where some areas are believed to be undergoing subsistence, thus causing the formation of a few synclines (Morgan and McIntire, 1959). Flood water stands in synclines have given rise to many ‘wetlands’ in areas which are topographically at higher elevations. Thus, one can see the occurrence of wetlands in the inland valleys inside older land formations of terraces and hills (Saheed and Hussain, 1992).

Unconsolidated floodplain sediments occupy the greater part of the country. The floodplains of the Ganges, the Brahmaputra and the Meghna cover approximately 40% of Bangladesh (Khan, 1991). The floodplain sediments are far from homogenous in age, texture and mineralogy. They have been deposited under piedmont, meander floodplain, estuaries and tidal conditions in different areas, new alluvium in still being deposited near active river channels, but most floodplain land has apparently received little new alluvium for hundreds years of more. Rivers have changed their courses from time to time in the past, abandoning and re-occupying various parts of their floodplains and thus providing sediments of different ages in different areas. Some floodplains areas have also been uplifted in Sylhet and Mymensingh areas, and there are numerous sand filled earthquake fissures in part of these areas.

Most floodplain sediments have high silt content. This is particularly true in the case of Brahmaputra/Jamuna and Meghna sediments. Tista floodplains and west of Ganges floodplains have sandy sediments in the sub-surface horizons while most of the Ganges floodplains have clay deposits on the surface. Peat has accumulated in some permanent wet basins throughout the country. In most areas they are at shallow depth; it can be up to 5m thick as in the Gopalganj-Khulna peat basins.

Geomorphology:

Each year 2.4 billion tons of sediments are transported by the major rivers to Bangladesh, having a profound effect on the geomorphology of the floodplains and the coastal plains.  Tremendous amount of sediments is deposited on the floodplain areas which plays an important role in case of changing its geomorphological features.

An understanding of geomorphology is especially important in Bangladesh where differences between soils are partially related to their positions in the landscape. The country comprises hill, terrace and floodplain areas. It may be classified into four distinct regions:

1.   The eastern and northern frontier hilly regions comprising of the eastern hilly regions, hills of Lalmai and northeastern Sylhet district and a narrow strip of a series of low hill ranges and isolated circular and elongated hillocks represented by recent alluvium along the northern frontiers of the districts of Sylhet and Mymensingh. (Hills)
2. The Great Table Land (Terraces)
3. Floodplains of the Ganges, the Brahmaputra and the Meghna river systems
4. The Delta (Reef Floodplains)

The floodplains of the Ganges, the Brahmaputra and the Meghna cover approximately 80% of Bangladesh. Numerous swamps have developed in the floodplains of the Brahmaputra and the Meghna. The Chalan Beel is the largest wetland of the area.

The floodplains and the delta are studded all over with clusters of wetlands or swamps, both big and small, commonly called haors or beels. These freshwater swamps appear to be tectonic in origin. Those of the districts of Rajshahi and Pabna and the Delta seem to have formed due to changing stream courses at short intervals and rapid building up of high levees by the streams. The swamps have also developed in many of the valleys of the hilly regions due mainly to poor drainage.

Physiography and Sedimentation:

The term physiography includes combination of the geological material in which particular kinds of soil have formed and the landscape on which they occur. Thirty four physiographic units and subunits are recognized in Bangladesh (FAO, 1988).

The main wetlands and their physiographic units are as follows:

A. Lower Atrai Basin

B. Lower Punarbhaba Floodplain

C. Gopalganj-Khulna Beels

D. Arial Beel

E. Surma-Kusiyara Floodplain:

The meander floodplains of the rivers flowing into the upper Meghna catchment area are included in this unit. The rivers include the Surma, Kusiyara, Manu, Dhalat and Khowai as well as smaller rivers flowing from Shillong plateau on the northern side. All these rivers originate in hill areas in India. These rivers pour into a vast low-lying area, the Sylhet basin. This basin seems constantly of have been sinking due to earth movements. The lowest parts, even in the north adjoining the Indian border and 300 km from the coast, are less than 5 meters above MSL (Nishat et al. 1993). Seasonal flooding is very deep in this sub-unit. The alluvial sediments deposited in this sub-unit are predominantly silts and clays. Derived from tertiary hill sediments, the alluvium has a much lower content of weatherable minerals. Despite the sudden floods that occur, bringing with them vast quantities of sediments, the main river channels appear to be much more stable in their courses than Brahmaputra and Ganges channels probably because the eastern rivers carry little sand by the time they reach this area and because silt and clay floodplain sediments are themselves denser than the less consolidated, mixed-textured sediments of the Ganges and the Brahmaputra floodplains. Sylhet Basin mainly comprises extensive, low-lying basins (the haors) bordered by relatively narrow, high ridges. The relief is locally irregular due to erratic deposition of new sediments. Clay soils predominate, and peat occurs locally. Seasonal flooding is mainly deep, more than 5 meters in the basin centres. Water quickly drains from the floodplain ridges after the rainy season, but large areas in the basins remain wet or submerged for most or all of the dry season.

F. Floodplains:

The floodplains contain noncalcareous recent alluvial sediments rich in weatherable minerals with illite as the dominant clay mineral. The exception is the Ganges alluvium which is calcareous at younger stage and montmorillonite forms an important part of its clay fraction. On lower sites, most of the soils are seasonally flooded, poorly to imperfectly drain with loams and clays as sediments. Flooded topsoils are near-neutral upon drying. They become acidic in non-calcareous soils and alkaline in calcareous soils.

The floodplain soils have formed in river and piedmont alluvium ranging from very recent to several thousand years old. Textually, these soils are silt loams or sandy loams on intermediate sites to silty clays or clays in basins. Flooding with silty water occurs in lands close to river channels, hill foot areas and on unembanked parts of tidal and young estuarine floodplains. Flooding depth varies with physiography, being deepest in basins.

Importance of wetlands for Bangladesh

Wetlands in Bangladesh have great ecological, ecological, economic, commercial and socio-economic importance and values. They contain very rich components of bio-diversity like flora and fauna of important local, national and regional significance.

The principal functions of wetlands are:

Ground water recharge, ground water discharge, storage of flood water, shoreline stabilization and reduction of erosion, sediment trapping, nutrient retention/removal, support for food chains, fisheries production, habitat for wildlife, recreation, natural heritage values, biomass production, water transport, bio-diversity preservation and micro-climate stabilization (IWRB) 1992, Dugan 1990).

Wet lands in Bangladesh are a very rich depository of vegetations, aquatic plants, reeds and algae. The floral composition is relatively uniform throughout the haors, jheels, beels, and boars but the dominance varies seasonally. Wetland soils have distinctive advantages and disadvantages for food production. In addition to their ample water supply, they are usually level and often occur in large land units, making large – scale farming feasible. Other advantages include low erosion disadvantages include cost of development & difficulty of management. Some wetland soils have special problems such as salinity, high Na-content, low pH, or poor physical properties following drainage. Wetlands are essential breading, rearing, and feeding grounds for many species of fish & wildlife. They are also important for producing food for humans and domestic animals. International recognition of these some times conflicting values have led to an ever increasing need to classify and characterize wetland soils in relation to food production.

Wetlands are nationally important for a variety of reasons:

Biodiversity:

The large and varied wetland environment is rich in species diversity. Of more than 5,000 species of flowering plants and 1,500 species of vertebrates, of which approximately 750 are birds and over 500 are coastal, estuarine and freshwater fish, some 400 vertebrate species and between 200 and 300 plant species are judged to be dependent on wetlands for all or part of their life spans. Wetlands provide habitat for a rich variety of resident and migratory waterfowls, a number of endangered species of international interest, and a large number of species of commercial importance.

Fish:

The inland capture fishery is the important subsector in terms of total catch, source of employment and supply of animal protein. It is based on the country’s vast freshwater resources and some 270 species of fin and shellfish which inhabit them. Essential habitats for the inland fisheries comprise open and closed water habitats, including rivers, canals, floodplains, haors, beels, baors and small roadside depressions. Although discrete in the dry season, these water bodies become interconnected during the monsoon and provide critical habitats for completion of the life cycles of a large number of fish species.

Agricultural diversity:

There are many local varieties of rice, conservatively estimated to number in the thousands, as well as other existing or potential commercially-important plants, which provide a valuable gene pool to ensure continued development of improved varieties for the future. All wetlands are subject to sedimentation composed of clay soils rich in organic matter, and the vast flooded areas of wetland are covered by crops which can tolerate waterlogging and inundation. Before the introduction of mechanized dry- season irrigation in the sixties, deepwater rice and the broadcast aman rice (floating rice) used to be the major crop in the wetlands during the rains. This crop was sometimes mixed with short duration Aus rice to be harvested in June allowing broadcast Aman to grow till November.
Tourism:

It remains an infant if not non-existent economic activity, but there is substantial foundation for the view that the country’s natural resources, especially the Sundarbans, could support the development of this sector.

Other economic activities:

The country’s wetland resources support a significant range of economic activities other than fishing, such as extraction of reed and other plant products, harvesting of aquatic vegetation, herbs, etc.

Degradation of wetland soils

Since independence in December, 1971 there has been an accelerated expansion of physical infrastructure in the floodplains and hoar areas. In recent years, decentralization of administration at the upazila level also led to a rapid expansion of roads and feeder roads evening the rural areas of the hoar basins. These infrastructures were often done without proper planning or due regard to natural water flows. These poorly planted roads and drainage structure created water logging and also had serious impact on the water regimes in the flood plains.

The degradation of wetlands in Bangladesh was mainly due to:-

Increased of population and expansion of human habitats; expansion of agriculture and subsequent conversion of wetlands through drainage into rice fields; flood control and irrigation projects for enhancement of agricultural productivity; national, local and rural infrastructures like ill-planned roads, narrow culverts etc.; over-felling of wetland trees; over-grazing by livestock; over-fishing and associated disturbances; siltation due to degradation of the watershed areas which are often transboundary in nature; indiscriminate control/ regulation/ use of waterflows of main river systems in the upper riparian; and pollution of water due to industrial, urban, agrochemical and other types of pollutants including pollution from transboundary sources.

Degradation of the wetlands in Bangladesh has created the following impact:

(a)    Serious reduction in fish habitat, fish population and diversity;

(b)   Extinction and reduction of wildlife including birds and reptiles;

(c)    Extinction of many indigenous varieties of rice with the propagation of high yielding varieties.

(d)   Loss of many indigenous aquatic plants, weeds and shrubs;

(e)    Loss of natural soil nutrients and loss of natural reservoir;

(f)    Increase in the recurrence of flashfloods;

(g)   Deterioration of living condition.

Strategies and plan for better use of wetlands:

The following strategies and research plans are envisaged for a better understanding and consequently better use of the wetlands (Nishat et al.1993):

(a)    A thorough inventory of the various types of freshwater wetlands under different agro-ecological regions.

(b)   Thorough physical, physico-chemical and chemical analyses of water, soil and vegetation of the wetland ecosystem; this has to be done initially every month for three consecutive years and then to be continued periodically as a monitoring work; this will help loads brought in by the major rivers and their tributaries to the various catchment areas and the quanta leaving the system; this will help assess the dynamics in a macro level;

(c)    Quantify the total loads brought in by the major rivers and their tributaries to the various catchment areas and the quanta leaving the system; this will help assess the dynamic in macro level;

(d)   Evolve water management programmes efficient for both rainy season and dry periods; creating dams around the basin areas might help protect lands from flooding but not allowing water to be stored will create adverse effect, like soils becoming acids, in non-calcareous floodplains and organic wetlands; however, in calcareous floodplains where the waterbodies become dry during the dry season, the wetlands could be reclaimed by creating dams etc. so that the cropping intensity could be increased;

(e)    Diversification of the use of water bodies needs to be attempted through an efficient water management programme;

(f)    Survey and monitoring for various pollutants in the wetlands, particularly in areas where intensive agriculture is practised or near the industrial and urban periphery;

(g)   Assess the possibility of using the mucky clay or clayey muck for various soil amendments other than its use as fuel; and

(h)   Develop personnel and manpower trained in the wetland systems of the country.

Environmental conditions of the area under study

Geology and landforms of Brahmanbaria district:

Except for a minor area of the low hills and terrace in the extreme east, the whole of the district is occupied by a relatively smooth, nearly level to gently undulating landscape of floodplain ridges and basins.

The subunits are given below:

1. Middle  Meghna floodplain
2. Old Meghna estuarine
3. Salda floodplain
4. Sylhet basin
   1. Surma-Kusiyara floodplain
   2. Titas floodplain
5. Piedmont alluvial plain

Sylhet basin is a vast depressed area lying between the Surma-Kusiyara floodplain and the Old Brahmaputra floodplain. The sediments appear to have been derived mainly from the hills to the north and from the Surma and Kusiyara rivers. The relief comprises high river levees surrounding extensive basins (haors), the centres of which stay wet through the dry season. The whole area is subject to flash floods, and most of the land is deeply or very deeply flooded in the monsoon season. The difference in the elevation between river banks and haor centres can be five meter or more. Clays predominate in the basins, with peat in some basin centres (Brammer, 1996).

Surma – Kusiyara floodplain is formed by the sediments brought in by the rivers draining into the Meghna catchment area from the Northern and Eastern hills (Rahman, 2005). Some small hill and piedmont areas near Sylhet, too small to map separately, are

Brahmanbaria District

Inundation land types

Fig. 7: Distribution of land types on the basis of flooding depth

(Source, SRDI, 1973)

included within the boundaries. Elsewhere, the relief generally is smooth, comprising broad ridges and basins, but it is locally irregular alongside river channels. Surma-Kusiyara unit occupies minor areas in the extreme north. It comprises alternate narrow long ridges and deep narrow inter- ridge depressions with some broad basins. The relief is generally is smooth, comprising broad ridges and basins, but it is locally irregular alongside river channels. The soils are mainly heavy silt on the ridges and clays in the basins (Brammer, 1996).

Middle Meghna floodplain consists of a complex, rather irregular landscape of floodplain ridges and inter-ridge depressions, cut offs, ox-bow lakes with fresh spill deposits along active channels (SRDI Staff, 1973). The whole landscape is seasonally flooded by the Meghna and exposed to river erosion by fresh deposits in each monsoon season. Seasonal flooding from the Meghna River is mainly deep. Basin sites are submerged early and drain late (Brammer, 1996).

Old Meghna estuarine comprises smoothed out rather low ridges with broad basin areas.  The relief is almost level, with little difference in elevation between ridges and basins (Brammer, 1996).

Climate:

a. The area has a pronounced tropical monsoon climate. Three main seasons found here: the monsoon (or rainy) season from May to October during which about 84 percent of the total annual rainfall is received; the dry season (or winter) from November to February which has very little rainfall and has the lowest temperatures and humidities of the year; and the pre-monsoon (or hot) season from March to April which has the highest temperatures and evaporation rates of the year and during which occasional thunder showers fall. Total annual rainfall is 2420 mm (in 2005) and 2430 mm (in 2004) (BBS, 2006). The rainy season excess of rainfall over evaporation is about 44 inches and in dry season excess of evaporation over rainfall is about 11 inches. There is plenty of rainfall during monsoon and most of this rainfall occurs during the months of May, June and July.  Normal duration of the rainy season (monsoon) as well as distribution of the rainfall vary from year to year and no kind of periodicity could be established between a drought year and a per humid or flood year. In some years short spells of drought and flood alternate (which have significant influence on agriculture) was found.

b. The climatic condition of the district is similar to that of other districts in the eastern region of the country. Hot summer, long rainy season and pleasant spring-cum-winter are the main noticeable seasons prevailing in the locality. The summer begins at the end of March and merged with the rainy season which continues up to September. Winter lasts from early November to late February. The highest and lowest mean temperatures are 33.5˚C (in 2005) and 12.3˚C (in 2005) (BBS, 2006) in the months of May to January respectively.

c. The average relative humidity is around 78%.

Hydrology:

The main rivers in the Brahmanbaria district are the Meghna, the Dhaleswar, the Titas, the Bansi, the Baliaguri, the Pagla, the Saldha and the Buri.The Meghna river flows along of the margin area. The Meghna is navigable throughout the year. All other rivers are navigable during the rainy season. The flowing length of the rivers is about 184 Kms. The river area is 95.64 Kms (BBS, 2006). The Titas taking off from the Meghna several miles north of Bhairab bazar has meandered through the study area and finally has joined the Meghna near Nabinagar several miles downstream of Bhairab bazar. Several creeks draining Tripura hills join the Titas.

Normally the flood- level of the Meghna River starts rising from May due to increased discharge of its tributaries viz. Old Brahmaputra, Dhaleswar rivers. River-level and discharge are usually at their highest ion July to September, from November it starts receding and finally touches lowest discharges in January-March. Creeks draining the Tripura Hills swell rapidly and flow in spate for a few days at a time after heavy pre-monsoon rainfall in its catchment areas i.e. Tripura Hills. The dry season discharge of most rivers is very small.

Table- 12: Climatic Data of Comilla Meteorological Station

[Source: Bangladesh Meteorological Department, 2006]

On the Old Meghna estuarine floodplain, flooding is mainly due to stagnation of rain water. However flood levels are controlled by changes in rainy-season water-levels of the Meghna, its tributaries and distributaries. These changes are usually gradual and even in deep basins broadcast aman can be grown without much danger of crop damage or loss. Basin depressions or abandoned channels that remain wet for the greater part or all of dry season are normally used for Boro rice. The piedmont alluvial plain comprises mainly basins, basin depressions and valleys. These are generally shallow to moderate or deeply flooded.

The Sylhet basin subdivided into the Surma-Kusiyara floodplain and Titas floodplain because of its lower elevation receives run-off from the Tripura Hills, neighbouring piedmont apron and adjoining Old Meghna estuarine floodplain as well as spillings from the Meghna river. In the rainy season it becomes a enormous lake more than 12 feet deep. Deep flooding, sudden rise of flood level and current water prevent the cultivation of Kharif crops in most of the area. However Boro rice is extensively grown because large areas remain wet for most or all of dry season (SRDI Staff, 1973).

Drainage:

About 95% of the area is affected by seasonal flooding. Depth and duration of the flooding vary with topographical position and are mainly controlled by the rainy- season water-levels of the main rivers and their tributaries. Ridges are mainly seasonally flooded from few weeks up to 4-5 months. Basins and basin depressions are flooded more deeply and for a longer time. They receive run-off water from adjacent ridges and usually are subject to rapid rise of the flood level. The Surma-Kusiyara floodplain is seasonally very deeply flooded and in most places remains flooded for the greater part of the dry season. This area is subject to flash floods in the pre-monsoon, monsoon and post monsoon seasons, so the extent and depth of flooding can vary greatly within a few days. Normal flooding is mainly shallow on the ridges and deep in the basins, with flood depths tending to increase towards the margin with the Sylhet basin. The centres (haor) stay wet in the dry season (Brammer, 1996). Sylhet basin is also subject to early flash flood and basin centres stay wet for most or all of the dry season (Brammer, 1996).

Vegetation:

Irrigated boro paddy is the main crop: local Boro in basin centres subject to early flooding; HYV boro on the higher margins of basins. Aus followed by transplanted Aman are the main practice on shallowly flooded ridge, with some mixed Aus plus Aman alone on basin margins. The cultivation of dryland rabi crops is mainly confined to loamy river-bank soils. Some basin land that cannot be irrigated is under red swamp or grassland used for dry season grazing (Brammer, 1996). The main constraints on improved agricultural production in both areas are: the heavy rainfall experienced susceptibility to flash flood, deep and prolonged flooding of basin sites and the prevalence of heavy soils that are difficult to cultivate when wet or when dry (Brammer, 1996).

The land use in this area is mainly depends on the duration and depth of seasonal flooding and by the suitability of soil moisture in the dry season. On shallowly flooded land farmers grow aus followed by transplanted aman. Where sufficient dry season moisture is available, a dry-land crop is often grown after the main harvest. Dryland rabi crops cannot be grown in soils which remain wet for some weeks after the end of rainy season. Where flooding become too deep transplanted aman is cultivated and where rapid rise of the flood level occur, broadcast aman or sometimes mixed with aus is grown. Basin depressions with sufficient water for dry season irrigation are used for boro cultivation. On comparatively high land where water cannot be kept on the land, only aus is grown, followed by a dryland crop at the end of the rainy season. Rabi crops mainly include khesari, mustard, till, gram, sweet potatoes and potatoes.

Table- 13: Crop suitability ratings by soil series, phase or land type in

different wetlands of Brahmanbaria district

[Source: SRDI staff, 1973]

Note: mll = medium lowland, vll = very lowland, B. Aus = Broadcast Aus, T. Aus =Transplanted Aus, B. Aman = Broadcast Aman, T. Aman =Transplanted Aman; Suitability class: Well suited = 1, moderately suited= 2, poorly suited = 3, not suited = 4

Soil:

Most of the soils of Brahmanbaria district are seasonally flooded, have silt to clayey textures and moderate or low contents of organic matter. Except in a minor area of hills and terrace in the east; all soils are developed in recent and sub recent alluvium. The Northern part of the district has grey silty loam of the non-saline phase of the old lower Meghna estuarine floodplain. The central part of the district is covered with dark grey clay of the Titas floodplain.

The soil in the eastern region contains silty clay loam. The piedmont alluvium admixture of sandy and silty alluvium of middle Meghna floodplain is prevailing in the southern region. The overall basin is poorly drained and deeply flooded in the monsoon season. In the different parent materials, the soils have developed different properties, mainly due to difference in texture, drainage, kind of sediment, age of the deposits and vegetation. Most soils are dark grey or very dark grey but mottled to varying degrees with brown or yellowish red. Top soils are generally puddled for rice cultivation and usually have a compact ploughpan at a depth of 3-4 inches. Most top soils are near neutral in seasonally reduced condition and become medium to strongly acid when dry. The soils have moderate content of organic matter and soil reaction is mainly acidic (Brammer, 1996). Fertility level is medium to high.

The soils of the Surma-Kusiyara alluvium are mainly heavy silts on the ridges and clays in the basins. This area is subject and depth of flooding can vary greatly within a few days. Normal flooding is mainly shallow on the ridges and deep in the basins, with flood depths tending to increase towards the margin with the Sylhet Basin. The basin centres (haors) stay wet in the dry season (Brammer, 1996). Mainly silty clay loam ridge soils covering about 25 percent of the total soil area on the Surma-Kusiyara floodplain and only 10 percent in the Sylhet basin. Most soils of the Sylhet basin have subsoil with mixed grey and dark brown colors, strong prismatic and blocky structure and the reaction of soils ranges from strongly acidic to near neutral. Organic matter content of the soils is moderate (Brammer, 1996). Soils in basin depression are grey or dark grey in color and remain wet for most or all of the dry season.  Soils of the Titas floodplain are poorly drained and black to grey clay soil which remains wet in most of the dry season. Soils formed in the Old Meghna estuarine floodplain have dark grey or very dark grey subsurface soil. Nearly all soils have subsoil with mixed dark grey and yellowish brown colours, strong prismatic structure, conspicuous dark grey coatings and a slightly acid to mildly alkaline reaction. Soils in basin depressions are dark grey to very dark grey in colour and remain wet for most or all of the dry season.

The soils of the Middle Meghna floodplain are poorly drained and have grey or olive-grey as dominant matrix colour. About one half of the area is occupied by finely stratified sandy and silty alluvium. Moderately shallow loamy soils over a stratified substratum cover most of the ridge and inter-ridge depression sites. In relatively sheltered parts, where no river erosion or thick deposition has taken place for the last decades, loamy soils have developed moderate to strong prismatic structure with grey cutans along ped faces. The subsoil reaction varies from slightly acid to mildly alkaline.  Basin soils of the Old Meghna estuarine floodplain are poorly drained and have a humous dark grey to very dark grey A horizon between the ploughed layer and the subsoil. Silty clay loam texture predominates in the basin soils. Part of the soils has very dark grey or black coatings along ped faces in the subsoil.

About three-fourths area of the Salda floodplain is deeply flooded and sudden rise in flood level takes place. Sediments carried by the Meghna water have produced fine textured very dark grey basin soils. Soils occupying ridges are shallowly to deeply flooded, mixed olive and grey or dark grey colours and loamy texture.

Materials and methods

Selection of soil sampling sites and sample preparation:

On having a preliminary idea about the different location of Brahmanbaria district, the next job for its soil study was to select the individual soil sampling sites as accurately as possible to have a homogenous and representative sampling of soil pedons. Soil sampling sites was based mainly on the land type i.e, it must be low to medium low land which are seasonally flooded, the age of the soil materials and vegetation. The soil sampling sites are recorded in Table- 14 with some of their respective environmental characteristics. Four soil series were collected from Sarail upazila which is shown in Figure- 8. After studying soil profile soils are collected for laboratory study.

Methods of the field study of soils:

A pit of 1m × 2m up to a depth of 130 cm was excavated for each soil profile. For understanding the soil properties, the soil profiles were studied and described morphologically in the field according to the system of Soil Survey Staff (1951). The environmental conditions of the study area is given in Table – 14. The horizons of each pedon were described morphologically.

Fig. 8: Map of Sarail upazila showing the location of soil sampling sites

Table- 14: Soil sampling sites and their environmental characteristics

Field description of the studied soils:

The soils were subjected to detailed study in the field. The environmental characteristics

and general morphological properties of the soil profiles are presented below:

Balaganj Series (P-1)

Balaganj series are seasonally shallowly flooded, poorly drained soils developed in mixed textured alluvium occupying upper part of ridges. They have a mixed grey and dark brown, friable, silt loam to loam subsoil with strong to moderate subangular blocky structure and patchy to continuous grey cutans on ped faces.

Typical profile     : Balaganj Series

Phase                    : Medium highland phase

Location                : 24° 06′ 494N – 91° 07′ 622E

Village                   : Dhorantighat; P. S.- Sarail, District: Brahmanbaria

Topography          : Gently sloping ridge.

Landuse                : Transplanted Aman, Boro.

Drainage               : Poor. Flooded up to 3-4 feet about 4-5 months

Sampling date      : 23^rd March, 2006

Profile Description:

Range in Characteristics

a. Profile characteristics:

Texture of the both topsoil and subsoil are silt loam. Colour of the topsoil is light yellowish brown and subsoil varies from grey to dark grey. Structure of the soil is fine subangular blocky.

b. Environmental characteristics:

These soils occupy upper to lower part of ridges and narrow levees on the Surma-Kusiyara floodplain. Medium highland phase is flooded up to 1-3 feet in the monsoon season for about 2-3 months; medium lowland flood hazard phase is flooded up to 4-6 feet for about 4-5 months in the monsoon season and become drought in the dry season flood hazard takes place. Lowland flood hazard phase is flooded deeply in the monsoon season and become drought in the dry season and flood hazard takes place.

c. Vegetation and Landuse:

At present these soils are used for transplanted aman- rabi crops.

PHOTO-A

Phagu Series (P-2)

Phagu series consists of poorly drained, seasonally deeply flooded soils developed in the basins and basin margins in the Surma-Kusiyara floodplain. They have a grey to dark grey, clay subsoil usually with strong prismatic and angular blocky structure and continuous grey or dark grey cutans along ped faces.

Typical profile      : Phagu Series

Phase                     : Low land

Location                : 24° 05′ 572N – 91° 08′ 384E

Village                   : Tiorkona; P. S.- Sarail, District: Brahmanbaria

Topography          : Nearly level basin.

Landuse                : Boro – fallow.

Drainage               :  Flooded up to 7-6 feet deep for about 5-6 months.

Sampling date      : 24^th March, 2006

Profile Description:

Range in Characteristics

a. Profile characteristics:

These soils have a grey to dark grey in colour, finely mottled generally firm to very hard clay topsoil with medium angular blocky structure, overlying grey mottled yellowish brown, firm clay subsoil with strong angular blocky to prismatic structure and slightly acid in reaction.

b. Environmental characteristics:

These soils occur on the Surma-Kusiyara floodplain and in Titas basin. They are seasonally flooded up to 9-12 feet for about 6 months.

c. Vegetation and Landuse:

The present landuse of these soils is for boro cultivation.

PHOTO-B

Nasirnagar Series (P-3)

Nasirnagar Series comprises seasonally very deeply flooded, very poorly drained, moderately fine textured soils developed in the Titas valley of the Sylhet basin. They are deeply flooded in the monsoon season and remain wet for most of the dry season. They have a dark grey to very dark grey, mottled brown, silty clay loam subsoil.

Typical profile      : Nasirnagar Series

Phase                     : Low land phase

Location                : 24° 05′ 914N – 91° 06′ 643E

Village                   : Kalikascho; P. S.- Sarail, District: Brahmanbaria

Topography          : Slope of basin.

Landuse                : Boro – fallow.

Drainage               : Poor. Flooded up to 6- 7 feet for about 5-6 months.

Sampling date      : 24^th March, 2006

Profile Description:

Range in Characteristics

a. Profile characteristics:

The colour of the topsoil ranges from very dark grey to dark grey and that of subsoil grey to light yellowish grey. Texture of both subsoil and topsoil is silt loam.

b. Environmental characteristics:

These soils occur on the Surma-Kusiyara floodplain and in Titas basin. They are seasonally flooded up to 8-12 feet in the monsoon season and remain wet for most of the dry season.

c. Vegetation and Landuse:

These soils are mainly used for boro-fallow. Part of the area is covered by grasses.
PHOTO-C

Nabinagar Series (P-4)

Nabinagar Series comprises seasonally very deeply flooded, very poorly drained soils developed in fine textured alluvium on the Titas valley of the Sylhet basin. They have a very dark grey, silty clay to clay A horizon with common brown mottles overlying a silt loam to silty clay loam oxidized substratum.

Typical profile           : Nabinagar Series

Nabinagar Series       : Lowland phase

Location                      : 24° 09′ 966N – 91° 07′ 161E

Village                         : Dhorantighat; P. S.- Sarail, District: Brahmanbaria

Topography                : Basin depression.

Landuse                      : Boro – fallow.

Drainage                     : Very poor. Flooded up to 8-10 feet about 7-8 months.

Sampling date            : 23^rd March, 2006

Profile Description:

Range in Characteristics

a. Profile characteristics:

The colour of the topsoil ranges from very dark grey to black and that of subsoil olive brown to grey. Texture ranges from silty clay loam to silty clay. Colour of the subsurface A horizon is dark grey; the mottle colour ranges from dark brown to grey, it usually occupies less than 20% of the soil mass. The contrasting layer is mostly brightly oxidized olive to light yellowish brown and is usually medium to moderately fine textured.

b. Environmental characteristics:

These soils occupy basin depressions, infilled cut-offs on the Titas valley of the Sylhet basin. They are seasonally flooded up to 10-12 feet for about 7-8 months in the monsoon season and remain soggy for most of the dry season.

c. Vegetation and Landuse:

These soils are mainly used for a single cropping of boro-fallow.
PHOTO-D

Laboratory methods:

Soil samples from each horizon of the profile were collected in polythene bags. The bags were sealed properly precluding moisture loss from the samples and transferred as quickly as possible to the laboratory for relevant analyses.

Prior to analysis, the representative soil samples were air dried under shade. The soil samples were then gently ground with rolling wooden rod and passed through 2mm
(10 mesh) sieve and mixed thoroughly. The samples were then preserved in plastic pot for laboratory analyses.

Table- 15: Parent material of the selected soil series and their expected

areas

[Source: SRDI staff, 1973]

Methods for Physical Analysis

і. Hygroscopic moisture

Hygroscopic moisture content of soils was determined by drying the air-dried soils in an oven at 105˚C-110˚C for 24 hours (Black, 1965).

іі. Particle size distribution

The particle size analysis of soils was carried out by combination of sieving and hydrometer methods as described by Day (1965). Textural classes were determined by Marshall’s triangular coordinate curve as devised by USDA (1951).

Methods for Chemical Analysis

і. Soil reaction (soil pH)

The pH (H₂O) of soils was determined at a soil-water ratio 1:2.5 using a pye pH meter. A same ratio of soil-1N KCl solution was used for determination of pH (KCl).

іі. Organic carbon

The organic carbon content of soils was determined volumetrically by wet oxidation method as described by Jackson (1967).

ііі. Total N

The total nitrogen in soils was determined by Kjeldahl’s method as described by Jackson (1967).

Methods for physico-chemical Analysis

і. Cation exchange capacity (CEC)

Cation exchange capacity of soils was determined with 1N NH₄OAc method buffered at pH 7 (Jackson, 1967).

іі. Exchangeable cations (Ca⁺⁺, Mg⁺⁺, Na⁺, K⁺)

The exchangeable cations were determined from 1N NH₄OAc (pH 7.0) extract as described by Jackson (1967). Ca⁺⁺ and Mg⁺⁺ were determined by atomic absorption spectrophotometer, while Na⁺ and K⁺ were analyzed by Gallenkamp flame photometer.

ііі. Base saturation percentage (BSP)

The percent base saturation was calculated by using the following formula.

% Base saturation =                                                                 × 100

Methods for Mineralogical Analysis

Surface soil samples representing the A (top) horizon from each of the five soil profiles were selected for the determination of mineralogical composition in the clay fraction. To achieve successful dispersion the following pretreatments were used for the removal of flocculating and cementing agents.

The soil samples were treated with in NaOAc-acetic acid buffer (pH 5.0) to destroy the free carbonates and organic matter was destroyed by 30% H₂O₂ treatment (Jackson, 1967). Free oxides of Fe & Mn were removed from soils by citrate-bicarbonate-dithionite extraction method as described by Aguilera and Jackson (1953) and Mehra and Jackson (1960).

After removing soluble salts, carbonates, organic matter, MnO₂ and Fe₂O₃ the soils were dispersed with dilute sodium carbonate solution at pH 9.5 (Jackson,1975) and the clay fraction (< 2μ) was separated by sedimentation & decantation processes as described by Jackson (1975).

Identification of mineralogical composition was carried out by X–ray diffraction (XRD). The separated clay fraction was treated by 1M KCl and 0.5N MgCl₂ respectively to make K⁺ and Mg⁺ saturated clay and wash them with ethanol to remove excess salts. Suitable amount (0.4 ml) of sol was dropped on a glass slide covering two third of its area, air dried, and X–rayed (parallel powder mount). In addition to the air dried specimen, the Mg saturated clay was X – rayed after saturation with glycerol; and K –saturated clay after heating at 300°C and 550°C for 2h. The XRD pattern were obtained using Shimadzu XRD 6000 diffractometer with Ni – filtered Cu K radiation at 40k V and 30 mA and at a scanning speed of 2.0°/min over a range of 3-35°2θ.

Approximate mineral contents of clay fraction estimated on the basis of the relative peak intensities in the XRD patterns (Islam and Lotse, 1986). The intensities ratio of two components P and Q in a multi – component mixture can be related to their weight ratio as follows:

Ip/Iq = Kp.q.Wp/Wq

Where Ip and Iq are the intensities of the P and Q components in XRD, Wp and Wq are the weight proportion of P and Q components and Kp.q, a constant value is the intensity – weight coefficient P and Q components.

Results and discussion

Morphological properties

The coded morphological properties of the wetland soils of Brahmanbaria district have been presented in Table- 16. Among the morphological characteristics color, mottling, consistence, structure, texture, and root distribution have been recorded and discussed.

Colour

Colour is the most obvious characteristic of any soil that is easily seen by even a lay-man. An enthusiastic observer in a given area can often relate soil colour to specific physiographic location of the soil in that area. It is also possible to make some generalized statements. For example, in temperate climate dark-coloured soils are related with high presence of organic matter than light coloured soil. The red colour soil is related with the presence of free iron oxide (unhydrated). Yellow colour of the soil indicates the presence of iron oxide. Grey or whitish colours of soil caused by the presence of quartz, kaolinite and other clay minerals, carbonates of lime and manganese and ferrous iron. Under reducing conditions, the soils usually appear grey or bluish grey in color. In some instances relict colour that is those inherited from the parent materials, may be present in the soil. The variation of soil colours may be related to variation in submergence time, reduction and movement of iron from the surface downward. Thus the variation on drainage condition is responsible for variation in soil colours.

The matrix colour of the soils was found to be a mixture of grey, dark grey, black and yellowish brown. The dominant hue, when moist, in two profiles was 10YR while in the other two profiles it was 5Y. These hues in these soils have been developed due to prolonged submergence during rainy season and drying condition during the dry months. These hues are related to the loss of free iron in these soils. Under moist condition, the colour of the surface horizon in the studied soils were found to be grey (5Y 5/1), black (5Y 2/1), very dark grey (10YR 3/2), and light yellowish brown (10YR 5/8).

Soils of the Meghna floodplains become increasingly oxidized in the subsoil, so that yellow or brown colour dominates over grey colour in many soils which are not permanently wet. Some of these soils show dark coloured topsoil and subsoil coatings, probably developed under natural vegetation (swamp) before the soils were cleared for cultivation. Floodplain and poorly drained soils of Surma-Kusiyara floodplain soils have grey matrix colours and their mottles and ped coatings are indicative of seasonal wetness (SRDI staff, 1973).

It was noted earlier that the studied soils are basically poorly drained. Most of the soils have low chroma. On the basis of chroma these studied soils can be classed as “hydromorphic soils” (Soil Survey Staff, 1994). The causes of hydromorphism are that these soils are inundated for a considerable part of the year by rain water as well as river waters which cause the reducing condition of these soils. The reducing condition is the cause of formation of grey colour in the studied soils. This grey colour of the soils is caused by removal of free iron oxides from these soils by draining water. Amounts of free iron oxide in the seasonally flooded soils have been reported to be low (Khan et. al., 1997; Ali, 1994; Muzib et al., 1969)

Mottles

The formation of various coloured mottles is generally associated with seasonal fluctuation of ground water table (Mckeague, 1965). Due to alternate wetting and drying conditions abundant quantities of mottles have formed in all the four soil profiles of the studied area. The colours of mottles were a combination of dark reddish brown to dark yellowish brown (Table- 16). Mottled horizons occur in the subsoil zone in these soils. Mottles were present most of the profiles but Balaganj and Phagu contains mottles throughout the profile.  The size and contrast of the mottles show variation from soil to soil. The occurrence of gley horizons in the features of most hydromorphic soils (Hussain et al., 1989).

The hydromorphic soils are known by different names in various parts of the world but they have mottles depending on the degree of hydromorphism (Hussain and Swindale, 1974; Vander, 1972). These soils are known as degraded rice soils in Burma (Karmanov, 1968) and aquarizems in Japan and in some other Southeast Asian countries (Kyuma and Kawaguchi, 1966).

Consistence

Soil consistency designates the manifestation of the physical forces of cohesion and adhesion acting within the soil at various moisture contents. These manifestations include- a) the behaviour towards gravity, pressure, pull or thrust. b) the tendency of the soil mass to adhere to foreign bodies or substances, and c) the sensations which are evidenced as feel by the fingers of the observer. Finally, it may be concluded that consistency represents the composite expression of the forces of cohesion and adhesion that determine the ease with which a soil can be reshaped or ruptured.

Results of soil consistence are important for soil management especially during tillage. This assumes importance when the soils are ploughed in wet conditions. The consistence of the soils under study has been determined under two moisture conditions – moist and wet.

Except for the soils of the Nabinagar series consistence of surface soils of all the studied profiles was found to be slightly sticky and slightly plastic when wet. In subsurface soils of Phagu series very sticky consistence also found. In moist condition, the consistence of the soils of Balaganj series was friable, while in Nabinagar, Nasirnagar and Phagu series it was firm (Table- 16).

Texture

Soil texture means the size distribution of soil particles. Natural soils are comprised of soil particles of varying sizes. The soil particles called soil separates (i.e, sand, silt and clay). Soil texture refers to the relative proportion of soil separates in a given soil. It also refers to the fineness or coarseness of the soil.

The field texture of the soils was determined by “feeling method”. The field texture ranged from silt loam to silty clay loam except in the Nabinagar series in which the texture ranged from silty clay to clay (Table- 16). Loamy and clayey texture of the soils of Brahmanbaria district is an indication that they are highly suitable for agricultural use especially for rice cultivation under waterlogged condition. The variation in clay, silt and sand content in most profiles suggests that texture is mainly due to sedimentary variations rather than a result of soil forming processes.

Silty clays and clays are predominate, with mainly silty clay loam soils covering about 25% of the total soil area on the Surma-Kusiyara floodplain and only about 10% of the Sylhet basin(Brammer, 1996). Texture of the soils of Surma-Kusiyara and Old Meghna estuarine floodplain soils are silt loam and silty clay loam to silty clay and topsoils are heavier in texture (SRDI Staff, 1973). Wetlands are not, however, confined to low-lying lands. Under heavy concentration of rainfall, even terraces and fans are flooded or saturated with water, as long as the land surface is level or depressional and soils are heavy textured and impervious (Huq and Kamal, 1993).

Table- 16: Coded* morphological properties of the studied soils

*According to the method of Soil Survey Staff (1988); B**=Boundary

Structure

Soil structure is the arrangement of soil particles into aggregates and the forces between them and the consequences of the arrangement on soil pores. It also refers to the aggregation of individual soil particles into larger units or peds with planes of weakness between them.

In the surface layer of all the studied soil profiles, the structure was found to be massive. Structure could not develop in the surface horizon as they have been ploughed for many years with puddling as an usual feature. The subsoil of Balaganj and Nabinagar series showed strong medium and medium subangular blocky and fine subangular blocky structure. The subsoil of Phagu series showed strong coarse prismatic and medium angular blocky structure. In Nasirnagar series, medium angular blocky structure was found. From the studied soils, it may be concluded that medium subangular blocky and medium angular blocky structure dominate in the wetland soils of Brahmanbaria district. SRDI staff (1973) during the reconnaissance survey with large number of soils of the studied area (Brahmanbaria) reported almost similar structures in the studied soils. Wet basins of the Sylhet basin soils have blocky or prismatic structure which is developed below the surface layer and brown mottle colour showing some oxidation (SRDI Staff, 1973).

Coatings

Brammer (1996) defined coatings as the shiny surfaces of soil cracks and pores formed by deposition of materials washed from the surface soils under seasonally flooded condition. The sides of cracks and pores in most of the wetland soils of Bangladesh are coated with such materials. These coatings are called gleyans are uniformly grey; mid-grey if the surface soil layer is grey (when wet); dark grey if the surface layer is dark grey.

Continuous thick dark grey to continuous dark grey cutans along ped faces are found in Balaganj and Phagu series. The coatings on ped faces may possibly be due to mechanical downward washing of material from the soil surface through cracks when the soils are in waterlogged condition and ploughed.

Most of the soils of Sylhet basin are poorly or imperfectly drained having conspicuous cutans along ped faces and pores in the subsoil (SRDI Staff, 1973).

Horizon boundaries

It is the important morphological feature in soil profiles. Clear smooth boundary was found in Balaganj, Phagu and Nabinagar series. Abrupt smooth boundary was found in Balaganj and Phagu series. With respect to horizon boundaries, the soils under the present investigation are more or less similar.

Physical properties

The physical parameters of soils are important criteria in the land use and also land management. Some of these properties were studied in the field as well as in the laboratory. Results of physical properties of the studied soils are presented in Table- 17.

Particle size distribution

The studied soils were generally fine textured and showed quite a narrow range in textural class from silt loam to clay among the horizons (Table – 17). Texture varies both within the profile and on different parts of the relief. The topsoil was much lighter in texture. Subsoil texture was closely related to position on the relief, with silt loam on the slope and silty clays or clays in basins. Most of the studied soils were located near the basin margins and bottom lands of seasonally flooded areas, the soils were mostly silt loam and clayey in texture.

Sand fraction

Sand fraction of the studied soils varies from 3 to 29 percent with a mean value of 10 percent (Table – 17). The surface horizons of the four studied profiles have 16, 6, 7 and 5 percent sand, respectively. This indicates that sand is not the dominant size fraction in these soils and its quantity is just not sufficient to provide a congenial physical condition of the soil for plant growth. The highest value of sand was found in the soils of the Balaganj series while the lowest value wass found in the soils of the Nabinagar series.

Table- 17: Particle size distribution and textural class of some wetland

soils under study

HM* = Hygroscopic Moisture

Based on the sand content, the studied pedons show the following gradation.

Balaganj > Nasirnagar > Phagu > Nabinagar

The trend of vertical distribution of sand fraction in the various soil profiles is presented in Figure- 13. The vertical distribution of sand indicates that its content is almost similar in Phagu and Nasirnagar series. In Balaganj series, the sand percent in the C-horizon was 29 percent and very lower content of sand fraction was found in the C-horizon of Nabinagar series. Except Nabinagar series sand percentage were gradually higher in lower horizon than those of the upper horizon. This irregularity in sand distribution pattern is thought to be related to the depositional process of the parent material. In other words the irregularity is geogenic rather than pedogenic in nature.  It may be concluded that the parent materials of the present soils were homogeneous. The very low percentage of sand fraction in the studied soils indicates the general characteristics as observed by Brammer (1971).

Silt fraction

Silt was the dominant fraction of the soils of Brahmanbaria districts. The silt content of the studied soils ranged from 37 to 77 percent with a mean value of 62 percent. The result presented in Table – 18 shows that except Nabinagar series, silt was the dominant size fraction. However, the vertical distribution of silt fraction is more or less similar in the soil profiles (Figure- 14). Such a high level of silt content is possibly the consequence of nature of the parent material. Based on the percent silt content, the studied pedons show the following gradation.

Nasirnagar > Phagu > Balaganj > Nabinagar

The sand/silt ratio of the studied soil profiles ranges from 0.07 to 0.46 with a mean of 0.17 (Table – 17). The highest mean of sand/silt ratio is found in the Balaganj series whereas the lowest ratio is found in the Nabinagar series.

Clay (colloidal) fraction

The colloidal fraction represents the clay fraction. The clay content of the studied soils varied from 8 to 60 percent with a mean value of 26 percent (Table – 17). This indicates that clay was one of the dominant size fractions in most of the investigated soils. The highest average value for clay was observed in the Nabinagar soil (49%) and the lowest value was in

the Balaganj profile (14%). Based on the percent clay content, the studied pedons show the following gradation.

Nabinagar > Phagu > Nasirnagar > Balaganj

The high clay content in the Nabinagar soil may be attributed to the higher clay content in their parent materials. It has been stated that these soils have formed in the lower part of the depressions where more clay is accumulated compared to that in the levees. The vertical distribution pattern of clay in the soil profiles is shown in Figure – 15. It is evident that the distribution of clay in the profiles is very much irregular. This irregularity in clay distribution may be attributed to geological processes rather than the pedological ones. Increase in clay content in lower horizons is found in Nabinagar series. High accumulation of clay in the lower horizons is a common feature of the seasonally flooded wetland soils of Bangladesh. Impoverishment of clay in the upper horizons of the soil profiles is an important feature in some seasonally flooded soils where ferrolysis is a common soil forming process (Brammer, 1964; Brinkman, 1971). The silt/clay ratio of the studied soils varies from 0.61 to 8.00 with a mean value of 3.36.

The highest decrease in clay content was observed in top 15 cm of Brahmaputra Floodplain, Meghna River Floodplain and Old Himalayan Piedmont Plains(about 50%) followed by Tista Floodplain, Barind Tract, Madhupur Tract and Chittagong Coastal Plain(30-40%). But in lowland situation, decrease in clay content in Surma Kushyara Floodplain is comparatively low (about 20%) (BARC, 1999).

To sum up the results of particle size distribution in the investigated wetland soils, it may be stated that –

1. Silt is by far the dominant size fraction in the studied wetland soils and therefore, played a significant role in moulding the physical properties of these soils.
2.  Clay is the dominant size fraction in the Nabinagar series.
3. The vertical distribution of sand and clay fractions in the profiles is irregular.
4. Except Nabinagar series the vertical distribution of silt content in the studied profiles are more or less regular.
5. The mean sand, silt and clay contents in the studied soils are 12, 62 and 26 percent respectively.
6. The sand content is much lower in these soils (average 12%). This is the common feature in many of the wetland soils in the seasonally flooded basin areas of Bangladesh.
7. Finally it may be concluded that the parent materials of the soils under the investigation can be considered as more or less homogeneous.

Hygroscopic Moisture

The amount of moisture retained by air-dry soils (Hygroscopic Moisture) has been presented in Table – 17. The hygroscopic moisture in the soils ranged from 1.37 to 6.10 percent. The mean value of hygroscopic moisture in the soils is 3.78 percent. The highest hygroscopic moisture percent was recorded in the Phagu series. The soils of the Balaganj series contain the lowest hygroscopic moisture. Such a variation in moisture content retained by air- dry soils is possibly due to the difference in their clay and organic matter content. A highly significant positive correlation was found between percent clay and percent hygroscopic moisture content in the studied soils, the ‘r’ value being + 0.65 (Fig. – 19).

Chemical properties

Soil reaction (pH)

Sol reaction is the most important characteristics of soils in the wetland area. There can be three types of soil reaction: acid, alkaline and neutral. Soil acidity is common in all regions where precipitation is high enough to leach appreciable amount of exchangeable bases from the surface layers of the soils so that the exchange complex is dominated by hydrogen ions. The soil pH plays a very important role in grouping the soils into different classes. It has direct influence on the availability of nutrients to plants and hence special emphasis is given on it. It has a profound influence on many factors connected with the suitability of a soil for agricultural use and hence special importance has been given to it (Truog, 1961).

The pH value of the studied wetland soils ranged from 6.1 to 7.4 with a mean of 6.9 (Table- 18). The soils, therefore, have slightly acidic to neutral in reaction. In each soil profile there is a trend of increasing pH with depth (Fig- 20). Nevertheless, the surface soil of each profile shows a slightly acidic in reaction. The increase of pH with depth is a common feature in many of the seasonally flooded soils of Bangladesh. This has been

attributed to the alternate oxidation and reduction conditions in the seasonally flooded, poorly drained soils (Ponnamperuma, 1985). Most of the wetland soils of Bangladesh contain low organic matter ranges from 1 to 2%. The decomposition and mineralization of organic matter are interrelated with successive microbial changes and are accompanied by a stepwise bio-chemical and chemical reduction of the soils resulting in the lowering of redox potential and changing of pH to near neutral (Huq et al.1993).

In most wetland soils of Bangladesh the pH value, in general, is around 7.0 (Saheed and Hussain, 1992). Dry topsoil pH of Surma-Kusiyara floodplain and estuarine ranged from 4.5 to 6.3. It is lower in tertiary sediments which ranged from 4.2 to 4.9. The subsoil pH of Surma-Kusiyara floodplain ranges from 4.5 to 6.2, in the estuarine floodplain it is 5.3 to 7.1. The substratum of Surma-Kusiyara floodplain soils ranges from 5.5 to 6.8, in estuarine floodplain it is 5.8 to 7.2 (SRDI staff, 1973).

When an 1N KCl solution was used in place of water for determination of pH in the soils there was a decrease in pH value (Fig. – 18). This change in pH value has been denoted as ∆pH. The ∆pH values of all the soils under study were negative. This indicates that the clay minerals having permanent negative charge are more than those having pH dependant negative charge. The ∆pH value of the soils ranges from -0.6 to -0.9, indicating that the soils contained very small quantity of reserve acidity (Table- 18). This indicates that the studied wetland soils of the basin area contain clay minerals having more permanent negative charge than the pH dependent negative charge. This is expected as the soils remain inundated with water both from rain or river during the monsoon season and some part of the dry season, when the H⁺ in the exchange phase is washed out. The low ∆pH value may also be indicative of the low cation exchange capacity which is a notable feature of these soils. The distribution pattern of ∆pH value within the soil profiles was more or less regular. The ∆pH value was lower in the surface soils than in the subsoils.

Table- 18: Soil reaction of the studied soil

ΔpH = pH (KCl) – pH (H₂O)

One of the main objectives of ∆pH determination was to get a measure of exchangeable H⁺ on the surface of the soil colloids. A portion of the exchange sites on the surface of soil colloids is occupied by exchangeable H⁺. Under this circumstance, when 1N KCl is added to the soil, the dissociated K⁺ ion replaces the absorbed H⁺ and the replaced H⁺ ion on coming to the solution phase depresses the pH value of the soil suspension.

The vertical distribution of pH (H₂O) in all of the studied profiles showed an increasing trend with the increase in depth (Fig. 17). The pH of the studied soils was slightly acidic near the surface horizon and gradually became neutral in the deeper layer. This increase has been attributed to the alternate oxidation and reduction conditions in the seasonally flooded, poorly drained soils (Ponnamperuma, 1985). The increase of pH with depth is a common feature in many of the seasonally flooded soils of Bangladesh (Mujib, 1969; Brammer, 1971 and Matin, 1972). But the pH values of the Nasirnagar series did not show the increasing trend with depth. With the receding of the ground water table, the ferrous iron is oxidized and the pH naturally drops first in the surface horizon or up to oxidized zone. In the subsoil zone the soluble bases have only restricted movement where the internal drainage is poor. For this reason, the pH in the subsoils tends to remain at a higher level as compared to that in the surface soils. A highly significant positive correlation was found between pH (H₂O) and pH (KCl) in the studied soils, the ‘r’ value being + 0.94 (Fig. – 19).

Fig. 14: A comparison of pH (H₂O) with pH (KCl) in the studied wetland soils

Organic matter

Organic matter is the store house of all plant nutrients. It influences physical, chemical and physico-chemical properties of soils far out of proportion to the small quantities present. Organic matter supplies energy and body-building constituents for most of the microorganisms which is important for soil. Role of organic matter in improvement of soil structure, water and nutrient holding capacities in light soils, release of available nutrients from native sources, control of soil erosion and supply of food and energy for beneficial soil microbes are well established facts (Islam, 1993). In most of the mineral wetland soils of Bangladesh, organic matter content is very low. More than half of the soils have organic matter in the range of 1 to 2% (Huq et al. 1993).

SRDI has analyzing about 11,000 samples from 2500 representative soil profiles covering agriculturally important areas of Bangladesh found that the organic matter contents of soils were generally low; it ranges from 0.3 to 1.5% in upland soils, 1.5 to 2 in medium lowland and 2 to 3.5% in the lowland areas. In beel areas, this fraction is about 4% (Rahman, 1990).

Low-lying areas of most floodplains have a good reserve of organic matter, higher than that in high land or medium-high land. These soils remain under water for a considerable period of time of a year. So, little decomposition of organic matter can occur. Moreover a large number of aquatic weeds grow which add organic matter to these soils (FAO-UNDP, 1988). The rapid decomposition and loss of organic matter from the soil are the usual process in tropical climatic condition. Many reports show rapid depletion of the soil organic matter resulting in decline in soil fertility and subsequent decrease in crop yield (Portch and Islam, 1984; Bhuiya, 1987 and BARC, 1997).

Organic carbon percentage in the Surma-Kusiyara floodplain ranges from 0.3 to 1.5 percent in the topsoil of the floodplain and ridge soils and from 1.5 to 5.6 percent in basin soils (SRDI Staff, 1973). The organic carbon was found in the studied soils ranges from 0.34 to 0.90 with a mean value of 0.63 percent.

Results of the organic matter content of the studied soils from the wetland areas of Brahmanbaria district indicate that the percentage of organic matter ranges from 0.58 to 1.55 with a mean of 1.10 percent (Table- 19). The Nabinagar series contains high amount

Table- 19: Organic matter and nitrogen contents of the studied soil

OC* = Organic carbon; OM** =Organic matter; C/N*** = Carbon / Nitrogen

of organic matter throughout the profile. All the soil profiles contained higher amount of organic matter in the surface than the subsoil (Table- 19). From the results it may be concluded that the studied soils contain low amount of organic matter (BARC, 1989). This finding was in agreement with the findings of Huq (1990), who reported that most agricultural soils of Bangladesh have low organic matter content.

Medium content of organic matter was found in the surface soils of Balaganj, Nabinagar, Nasirnagar and Phagu series which contains 1.44, 1.55, 1.51 and 1.48 percent organic matter. Since these soils remain flooded for 6-7 months every year, the organic matter decomposition is being retarded seasonally. Moreover the medium amount of organic matter at the surface may be attributed to maximum root activities of growing crops as well as natural and artificial addition of fresh and partly decomposed organic materials in the form of farm yard manures and crop residues. The highest mean value of organic matter was found in the soils of the Nabinagar series. Low content of organic matter may be caused by rapid decomposition of organic residues because of high temperature and rainfall, higher cropping intensity under tropical conditions. Based on the percent organic matter content, the studied pedons show the following gradation.

Nabinagar > Phagu> Nasirnagar > Balaganj

The organic matter content in the studied soils show a general tendency of decrease with depth in all the pedons and the decrease is more or less gradual which is a sign of their maturity (Table- 19).

Total Nitrogen and C/N ratio

Total nitrogen contents in the studied wetland soils ranged from 0.05 to 0.11 percent with a mean of 0.08 percent (Table- 19). The highest mean value of nitrogen content among the profiles was found in the Balaganj and Nasirnagar series whereas the lowest amount was found in the Phagu series. The total nitrogen contents in the mineral wetland soils ranges from 0.05 to 0.1 percent (Huq et al. 1993). In Surma-Kusiyara floodplain and old Meghna estuarine floodplain soils nitrogen ranges from 0.02 to 0.09 percent in topsoils. Both carbon and nitrogen contents generally decreases regularly with depth (SRDI Staff, 1973). These features were also similar in the studied soils.

The vertical distribution pattern of total nitrogen follows closely the sequence of organic matter (Fig. 20). Higher amount of nitrogen content are found in the surface horizons than the underlying horizons. A positive correlation was found between organic matter and total nitrogen contents of the soils with a correlation (r) of +0.76 (Fig. 21). The quantities of organic matter and total nitrogen are low in the seasonally flooded wetland soils of Bangladesh (SRDI Staff, 1973).

The C/N ratio of the studied soils ranged from 5 to 11 with a mean value of 8 (Table- 19) This indicates that the organic matter fraction is highly oxidized even if these soils remain flooded for more than 5 months or more in a year. The microbial activity in these soils appears to be quite vigorous. The C/N ratio within the soil profiles showed irregular patterns.  This sort of irregular pattern is quite common in the floodplain soils of Bangladesh (Majumder, 1996, Hussain et al., 1989). Jenny (1960) also noted that in some great soil groups the C/N ratio was variable within the single soil profile. The C/N ratio of the soils of Surma-Kusiyara and old Meghna estuarine floodplain soils was mainly between 6 and 15 (SRDI Staff, 1973).

Physic-chemical properties of the soils

Cation exchange capacity

The cation exchange capacity (CEC) is an important soil physico-chemical property that is used for classifying soils in Soil Taxonomy and for assessing their fertility (Brady, 2002). CEC is defined as the amount of a cation species bound at pH 7 or another suitable pH depending on the method used for its measurement. Sometimes it is called Total Exchange Capacity or Base Exchange Capacity. CEC is a very important property of soils for predicting their quantity and types of clay minerals and their nutrient holding capacity (Landon, 1991). It also plays a vital role in determining the trend and type of pedogenic processes of soils (Buol et. al., 1980). The weathering stage of soils can also be conveniently determined or predicted by their cation exchange capacity.

The cation exchange capacity of the studied wetland soils ranges from 13.14 to 24.56 cmol (p⁺) Kg^-1 with a mean value of 19.51 cmol (p⁺) Kg^-1 (Table- 20). These results indicate that the clay rich soils have higher CEC values. Because fine textured soils tend to have higher CEC than sandy or coarse textured soils. Sandy soils have lower CEC than clayey soil because the coarse textured soils are commonly lower in both clay and humus content. Increase in CEC values with increase in clay contents in soils has been reported by many authors (Gupta and Misra, 1970 and Pathak et. al., 1980). Again the cation exchange capacity of most soils increases with increasing pH. The cation exchange capacity of the soils is pH dependent (Jackson, 1958; Chapman, 1965). Cation exchange capacity of soils are influenced by a number of factors, which may be enumerated as organic matter content, clay content, free iron oxides etc. (Campbell and Claridge, 1975; Dawson et. al., 1991).    At low pH values, the cation exchange capacity is also generally low (Brady, 2002).

The highest mean value of the CEC among the profiles was found in the Nabinagar series (21.94 cmol (p⁺) Kg^-1)and the lowest in the Balaganj series (16.60 cmol (p⁺) Kg^-1). The variation of CEC values of the pedons studied reflects the influence of both the clay and organic matter content of these soils. The CEC of mineral wetland soils ranges between 10 and 20 cmol (p⁺) Kg^–1. More than 60% of the mineral wetland soils have CEC values ranging from 10 to 20 cmol (p⁺) Kg^-1 and these values rarely exceed the value 45 cmol (p⁺) Kg^-1. The CEC of organic soils may exceed 200 cmol (p⁺) Kg^-1 (Wiklander, 1965).

The CEC is relatively higher in organic wetland soils than mineral wetland soils because of substantial presence of clay (around 60 percent) in addition to the organic matter (Huq, 1993). Brinkman found CEC of clay around 27 cmol (p⁺) Kg^-1; and of silt + sand around 5 cmol (p⁺) Kg^-1 in the soils of Surma-Kusiyara floodplain (SRDI staff, 1973). The distribution patterns of cation exchange capacity in the studied profiles showed more or less an irregular trend with depth (Fig.-22).

Exchangeable cations

Results of exchangeable cations in the studied soils are presented in Table- 20. The amount of exchangeable Ca⁺⁺ ranges from 5.23 to 11.52cmol (p⁺) Kg^-1 with a mean of 9.31  cmol (p⁺) Kg^-1. Exchangeable Ca⁺⁺ is by far the most dominant metal ion in the soils. Similar result was also reported by SRDI Staff in most of the soils of Brahmanbaria district (SRDI Staff, 1973). The highest exchangeable Ca⁺⁺ was found in the soils of Nasirnagar series while the lowest value was found in Balaganj series. The predominance of Ca⁺⁺ was reported by George et. al. (1958) in some humic gley soils of Ohio. The vertical distribution of exchangeable Ca⁺⁺ in the profiles shows an irregular trend. It was observed that all the profiles contain higher amount of Ca⁺⁺ in their surface soils due to their seasonal submergence. The percentage composition of the exchangeable Ca⁺⁺ varies from 48 to 60 percent with a mean of 54 percent (Table- 21).

The exchangeable Mg⁺⁺ content in the soils ranged from 3.90 to 8.08 cmol (p⁺) Kg^-1 with a mean of 6.33 cmol (p⁺) Kg^-1. On the quantitative basis, exchangeable Mg⁺⁺ comes after exchangeable Ca⁺⁺. The highest mean value of exchangeable Mg⁺⁺ among the profiles was found in the Nabinagar series [7.40 cmol (p⁺) Kg^-1] and the lowest in the Balaganj series [5.02 cmol (p⁺) Kg^-1]. The percentage composition of the exchangeable Mg⁺⁺ varies from 32 to 40 percent with a mean of 36 percent (Table – 21). The Ca⁺⁺/Mg⁺⁺ ratio of the studied soils varies from 1.2 to 1.9 and the mean ratio was 1.5 (Table- 22). In almost all the soils of the Surma-Kusiyara and old Meghna estuarine floodplain, Ca⁺⁺ exceeds Mg⁺⁺, but the ratios are variables; most of them lie between 2 and 4 and the trend of Ca⁺⁺/Mg⁺⁺ ratio down the profile often irregular (SRDI Staff, 1973).

The highest mean Ca⁺⁺/Mg⁺⁺ among the profiles was found in Nasirnagar series and the lowest in the Nabinagar series. Hussain and Chowdhury (1980) reported an exchangeable Ca⁺⁺/Mg⁺⁺ ratio of around 2 in some poorly drained wetland soils of Bangladesh. The Ca⁺⁺/Mg⁺⁺ ratio in soils decrease with increasing maturity of the soils (Buol et. al., 1980). The exchangeable Ca⁺⁺/Mg⁺⁺ ratio of agricultural soils of California was 2.49 and of Lanna soil in Sweden was 3.06 (Bohn et. al., 1979). It appears that the seasonally flooded soils of Bangladesh are similar to those of the other countries in respect of Ca⁺⁺/Mg⁺⁺ ratio. The above results of exchangeable Ca⁺⁺/Mg⁺⁺ ratio indicate that the seasonally flooded soils in Bangladesh are immature.

The amount of exchangeable Na⁺ ion varied from 0.87 to 1.25cmol (p⁺) Kg^-1 with a mean of 1.04cmol (p⁺) Kg^-1. The highest mean exchangeable Na⁺ within the profile was found in Nabinagar series [1.13 cmol (p⁺) Kg^-1] whereas the lowest was found in the Phagu series [1.00 cmol (p⁺) Kg^-1]. The seasonally flooded soils are so intensely washed that Exchangeable Na⁺ can never be a problem. The vertical distribution pattern of exchangeable Na⁺ in all the profiles was more or less irregular.

The amount of exchangeable K⁺ in the studied soils ranged from 0.28 to 0.94 cmol (p⁺) Kg^-1 with a mean value of 0.48 cmol (p⁺) Kg^-1. The highest mean exchangeable K⁺ within the profile was found in Nabinagar series [0.85 cmol (p⁺) Kg^-1] whereas the lowest was observed in the Nasirnagar series [0.31 cmol (p⁺) Kg^-1]. The exchangeable K⁺ content in the studied wetland soils indicates that these soils are not deficient in K⁺ and little amount of potassic fertilizer will be needed. The percent composition of exchangeable K⁺ ranges from 1 to 6 with a mean of 4 (Table- 21). The highest percentage was found in the soils of the Nabinagar series and the lowest in those of the Nasirnagar series. In an “ideal soil” the cationic composition in the exchange complex was as follows: Ca⁺⁺ 65 percent; Mg⁺⁺ 10 percent; K⁺ 5 percent; H⁺ 20 percent (Toth, 1965). On this basis the wetland soils under the present investigation may be considered as very close to the “ideal soil”. Generally in the studied soils, the value of Ca⁺⁺ and Mg⁺⁺ increased gradually from basin to basin depressions. Exchangeable Na⁺ and K⁺ content were high in basin depressions.

Base saturation percentage

The percentage of the CEC of any soil that is satisfied by the base forming cations is termed as the base saturation percentage (BSP). The BSP of a soil is very important for predicting the running genetic processes in soil as well as its stage of development. The BSP values have also been used in soils for their classification (Soil Survey staff, 1975).

The percent base saturation of the studied soils ranged from 78 to 95 with a mean value of 88 (Table- 20). The high base saturation may be due to rapid replenishment of bases from the weathering minerals. The highest mean value among the profiles was found in the Phagu series (93) and the lowest in the Balaganj series (83). The BSP was found to be lower in the surface horizon of the soils and higher in the underlying horizon. In Bangladesh, Ali (1994) noted that the base saturation was high in some alluvial soils of the Brahmaputra floodplains. A negative correlation was found between pH (H₂O) and percent base saturation in the studied soil, the ‘r’ value being – 0.13 (Fig. – 21).

Clay mineralogical composition

Clay mineral contents can be used as important criteria for soil classification (Soil Survey Staff, 1975). The use of clay mineral data in the comprehensive soil classification system as well as soil family differentiation characteristics was noted by (Buol et. al., 1980). These minerals are best determined by X-ray diffraction analysis and some other techniques. They also provides physical support for plants and the presence or absence of particular mineral which gives clues to how soils have been formed.

X-ray diffraction technique was used to identify the minerals in the clay fraction. X-ray diffractograms of clay fraction of some selected soil samples under the present investigation have been presented in Figs. 26-29. For the identification of minerals in the clay fraction it was assumed that the 17Ǻ or 18Ǻ peak in the glycerol solvated samples were an indication of the presence of smectite (Montmorillonite) mineral. The peak at 14Ǻ which collapsed when heated to 550˚C indicates the presence of vermiculite. The peak at 14Ǻ which does not collapse on heating at 550˚C is due to chlorite. The peak at 10Ǻ in the glycerol solvated clay samples was caused by the presence of mica. The peak at 7.1Ǻ disappearing on heating the sample at 550˚C confirmed the presence of kaolinite mineral. A number of small peaks in the region of 10Ǻ to 14Ǻ, some of which persist on glycerol solvation of the clay, suggested the presence of interstratified minerals most likely of the kaolinite, illite and vermiculite. Presence of quartz is indicated by the presence of 3.35 Ǻ.

Amounts of minerals determined by the above method are presented in Table- 23. In the present soils Kaolinite is the dominant mineral closely followed by mica. A small quantity of quartz is present in all the soils. Small quantities of montmorillonite and interstratified minerals are present in some specific soils. These results appear to be in conformity with the findings of White (1985) and Khan et. al. (1997) for some soils of Bangladesh.

The studied soils contain around 26 percent mica with around 25 percent vermiculite. Fanning and Keramidas (1977) pointed out that mica in most soils originate mainly from soil parent materials and tend to weather to other minerals with time. They are more prevalent in clay fraction of younger and less weathered soils (Entisols, Inceptisols, Alfisols).The studied soils contain 25 percent vermiculite which indicates that the transformation of mica is considerable. Douglas (1977) stated that soil vermiculites are  always reported to occur as an alteration product of muscovite, biotite and chlorite. According to Douglas et. al. (1977) the transformation sequence is:

Mica → Vermiculite → Hydroxyl – Aluminium – interlayered Vermiculite

————————–> K-content decreases ———————————–>

The soils under study contain around 12 percent kaolinite and 3 percent smectite (Table-23). Brady (1994) and Jackson (1964) stressed that kaolinite represents a more advanced stage of weathering than those of any other major types of silicate clays and formed from the decomposition of silicates under conditions of moderate to strong acid weathering environment which results in the removal of the alkalies and alkaline earth metals. Gupta et. al. (1984) stated that kaolinite usually forms through the weathering of feldspars. In the floodplain soils of Bangladesh the kaolinite mineral is thought to be allogenic in nature.

The semi-quantitative estimation of clay minerals shows that the dominant mineral in the clay fraction of the studied soils was mica occupying more than 26 percent of the clay fraction (Table- 23). The second dominant mineral in the clay fraction was vermiculite. The clay fraction is found to contain interstratified minerals of mica-vermiculite and mica-vermiculite-smectite respectively. The mean chlorite content in the clay fraction of the present soils was 13 percent.

Saheed (1985) reported that there were three groups of mineralogical association in Bangladesh soils: mica, vermiculite and kaolinite in most floodplain soils; smectite along with mica in Ganges Floodplain soils; and mica and halloysite in terrace soils.

White (1985) conducted an experiment on the qualitative estimation of clay mineralogical composition of a large number of soils of Bangladesh and reported that mica (muscovite) and kaolinite were the predominant minerals in the clay fraction of most floodplain soils. He speculated that mica was transformed to vermiculite under acidic condition in the Brahmaputra, Meghna and Tista Floodplain soils; while under

Table- 23: Semi-quantitative estimation of minerals in the clay fraction of studied soils

¹Abbreviations: M = Mica; S = Smectite; Vt = Vermiculite; Ch = Chlorite; K = Kaolinite; Vt–Ch = Vermiculite – Chlorite intergrade, M/Vt and M/Vt/S = Interstratified minerals of Mica – Vermiculite and Mica – Vermiculite – Smectite respectively; Q = Quartz; G = Geothite.