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A Preliminary Evaluation of the Clay Mineralogy of the Sundarban Soils

M. M. Hasan
Director (Retd.)
Bangladesh Forest Research Institute Chittgong

M. A. Razzaq
Divisional Officer (Retd.)
Forest Chemistry Division Bangladesh Forest Research Institute Chittagong

Abstract/ Executive Summary:

Preliminary investigations were carried out for the determination of clay mineralogy of the Sundarban soils. A total of 50 pedons were examined over a north-west to south-east transect running through the Sundarbans. Finally, four pedons were selected for laboratory investigation. Based on the observations made from the field investigations, as well as from the results of the physicochemical analyses, it appeared that pedologically the soils were very young and possessed no diagnostic horizon. The dominant minerals in the clay fraction are weathered mica and smectite with traces of kaolinite, chlorite and inter stratified minerals. The clay minerals were identified mainly by the X-ray diffraction technique supported by other techniques. It was observed that, so far as the clay mineralogical composition was concerned, the Sundarban soils had homogeneity both in vertical and in horizontal directions. Slight variation in the clay mineralogical composition and content occurring among the pedons are the inherited properties of the soil materials.

Keywords: Cay mineralogy, Pedons, Sundarban, X-ray diffraction technique

Location: The Sundarban, Bangladesh

Start Date: 01-07-1975

End Date: 30-06-1979

Program Area: Crop-Soil-Water Management

Commodity: Land management

Objectives:

To investigate the clay mineralogy of the Sundarban soils in Bangladesh

Methodology:

A total of 50 pedons were examined over a transect running across the Sundarbans from the north-west toward the south-east. Because of the homogeneity in morphological properties that exists among the pedons only 10 of them have been collected for the preliminary laboratory analysis. Finally four pedons (MS₁, MS₂, MS₃ and MS₄) have been selected considering the vastness of the area. Pedon MS₁, being artificially drained, has developed a weak prismatic structure in the subsoil. Samples have been collected with the help of a dutch auger. Arbitrary depths of 0-25, 25-50, 50-75 and of 75-100 cm have been used for soil sampling because of the absence of recognizable horizon differentiation. The cation exchange capacity (CEC) was determined by using neutral 1N NH₄OAc solution. For the identification of clay minerals the combination of characters as observed from the X-ray diffraction, differential thermal analysis (DTA), thermogravimetric analysis (TGA), cation exchange capacity (CEC) and the specific surface area per gm of clay have been used. The samples were heated from the room temperature (20^oC) to 1000^oC at a heating rate of 20^oC/min. The DTA thermographs were interpreted according to standard methods. The TGA was done by a Dupont 950 thermogravimetric analyzer. The samples were heated form the room temperature (20^oC) to 1000^oC at a heating rate of 15^oC/min. The surface area of the Na-saturated oven-dried clay samples was determined by EGME technique.

Source of Information: Bano Biggyan Patrika (a journal of forest science), Vol. 10, No. 1 & 2, January & July, 1981

Article Link (Online Journal):

Funding Source:

1. Government

Budget: 1,00,000.00

Total Budget (Tk.) : 1,00,000.00

Achievement/Findings:

It appears from this study that all the pedons possess similar clay mineralogical composition. There is no vertical differentiation of clay minerals within the pedons, except in cases of some minor local variations. The prevailing pedo-environmental conditions (very poor drainage, high base status of the alluvium and of the inundating water) of the Sundarbans are favourable for the neosynthesis of smectite. But in consideration of the active alluvial nature of the sediments and the homogeneity in the vertical distribution of clay minerals within the pedons the smectite present in these soils is considered as the inherited one rather than the product of pedogenesis in situ. The presence of kaolinite in the clay fraction also conforms to this assumption. The interstratified minerals which are observed from the X-ray diffractograms constitute both the swelling and the non-swelling ones. This is apparent from the fact that some of the 2/1 and 2/1/1 phyllosilicates (mica, vermiculite, smectite and chlorite) present in these soils possess transition structures. These transition forms are due to the processes of transformation. Part of the K⁺ions of mica or of the brucite sheet of chlorite may have been replaced by hydrated ions. Inter grade minerals having swelling layers form the smectitic intergrades and those with non-swelling layers form the vermiculitic inter grades. The presence, as well as the variation in the composition of clay minerals within the pedons, cannot be correlated with the pedogenesis in the studied area. This is because pedogenetic influences significant for the alteration of clay minerals are, in fact, lacking in the Sundarban soils. Hence, the clay minerals present in these soils are assumed to have been inherited from the alluvial soil materials on which they had formed.

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