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Morphology and Clay Mineralogy of Some Hydromorphic Soils from the Ganges Delta in Bangladesh

M. S. Hussain
ASP, KMP, Khulna, Bangladesh.

M. J. Uddin
Department of Soil, Water and Environment, University of Dhaka, Bangladesh

S. Ferdous
ASP, KMP, Khulna, Bangladesh.

Abstract/ Executive Summary:

Four representative pedons from the Lower Ganges delta area were studied for their morphological and clay mineralogical characteristics. The parent materials of the soils were calcareous Gangetic alluvium which have been decalcified over time. Being located in the lower part of the delta the soils are heavy textured with the weighted average clay content ranging from 52 to 70 per cent up to a depth of one meter. The soils have developed enormous redoximorphic features including redox concentration in the middle zone and a reduced matrix (gleys) in the lower zone of the profiles. Smectite was the dominant mineral with weathered mica, and chlorite, along with small quantities of kaolinite and mixed layer minerals. The smectite mineral was found to be an iron-rich beidellite type with an estimated chemical formulae of (Si_6.80 A1_1.20) (AI_2.41 Fe_1.02 Mg_0.57) 0₂₀ (OH)_4.0 X_1.20). There is a considerable quantity of weathered mica in the soils from which potassium could be released. Layer silicate clay is the dominant fraction in the bulk mineralogy of soils.

Keywords: Mineralogical analysis, Ganges delta, X-ray diffraction, Soils

Location: Khulna and Satkhira districts, Bangladesh.

Start Date:

End Date:

Program Area: Crop-Soil-Water Management

Commodity: Land management

Objectives:

Objective of the study was to examine the redoximorphic features and their genesis in some soils of the lower Ganges delta area in Bangladesh and also to determine their clay and bulk mineralogical composition.

Methodology:

Twenty four soil samples from four representative soil profiles were collected on horizon basis. The names of soil series are Amjhupi, Bajoa, Karnalkati and Harta. Parent materials, physiography and location of these soil series are presented. Amjhupi series cornpries intermittently to seasonally shallowly flooded, imperfectly to poorly drained soils developed in older Gangetic alluvium and occur as noncalcareous. Bajoa series comprises seasonally flooded, poorly to very poorly drained soils developed on Ganges tidal floodplain. Karnalkati series includes poorly drained dark grey to grey, noncalcareous silty clay to clay subsoils. This soil is very strongly acid in the topsoil and have yellowish patches in the subsoils indicating the properties of acid sulphate soils. Harta series comprises seasonally flooded poorly to very poorly drained soils developed in Ganges tidal floodplain and have peaty materials in the lower zone of the profile (SRDI Staff 1973). Cation exchange capacity of soils was determined by 1N NH₄0Ac method as described. Soil pH was measured at a soil water ratio of 1 : 2.5 using a glass electrode pH meter. The electrical conductivity was determined on an extract of 1 : 5 soil water ratio. Base saturation percentage (BSP) was calculated using the ratio of total exchangeable bases and the cation exchange capacity multiplied by 100. The average particle size distribution up to the depth of 120 cm was determined from the particle size distribution data of Ferdous (1998). For clay mineralogical analysis, the soil samples were treated with 1N NaOAc-acetic acid buffer (pH 5) to dissolve the free carbonates. Organic matter was oxidized by treating the soils with 30% H₂0₂ . Free oxides of Fe and Mn were removed from soils by the citrate-bicarbonate-dithionite extraction method as described by Mehra and Jackson (1960). The soils were dispersed with dilute sodium carbonate solution at pH 9.5 and the clay fraction (< 2μ fraction) was separated by sedimentation and decantation method. Small aliquots of clay samples were Mg- and K saturated by repeated washings with normal solutions of MgCl₂ and KCl, respectively. The clay samples were then washed with 95% alcohol and acetone until free of cr and the clay samples were stored in water. Slides were prepared from Mg- and K-saturated clay samples. Mg-saturated air dried clay slides were glycerol solvated in vacuum in an atmosphere of glycerol. The K-saturated air dried clay slides were heated at 350° and 550°C for two hours. All the air dried, 350 and 550⁰C heated, and glycerol solvated clay samples were subjected to X-ray diffraction analysis using a GEOL X-ray diffractometer with Ni-filtered CuKα radiation (at 30 KV, 20 mA) with a scanning speed of 2° 2θ per minute. Identification of clay minerals was made mainly on the basis of their characteristic basal reflections (c-axis length) following the procedure of Jackson (1975). A semiquantitative estimation of clay minerals was made on the basis of characteristic peak height and width. The X-ray diffraction peak at 7.2Å, disappearing on heating the clay to 550°C, was taken as an indication of the presence of kaolinite in the clay fraction.

Source of Information: J. Asiat. Soc. Bangladesh, Sci., 32(2): 257-268, December 2006

Article Link (Online Journal):

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

In the clay fraction of the Ganges alluvium 2:1 expanding lattice type mineral is dominant with an admixture of vermiculite, kaolinite and mica. The soils with such a mixed clay mineralogy are unlikely to show extreme physical and physico-chemical properties. With such a combination of clay mineralogical composition, the soils are expected to present the physical properties quite ideal for agricultural management. The smectite mineral was found to be an iron-rich beidellite type with an estimated chemical formulae of (Si_6.80 A1_1.20) (AI_2.41 Fe_1.02 Mg_0.57) 0₂₀ (OH)_4.0 X_1.20). There is a considerable quantity of weathered mica in the soils from which potassium could be released. Layer silicate clay is the dominant fraction in the bulk mineralogy of soils.

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