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Chemical and Mineralogical Properties of Some Benchmark Soils in the Flood Plains of Bangladesh

Z. H. Khan
Department of Soil Science, University of Dhaka, Dhaka-1000, Bangladesh

A. R. Mazumder
Department of Soil Science, University of Dhaka, Dhaka-1000, Bangladesh

M. S. Hussain
Department of Soil Science, University of Dhaka, Dhaka-1000, Bangladesh

S. M. Saheed
Soil Resource Development Institute, Farmgate, Dhaka-1215, Bangladesh

Abstract/ Executive Summary:

The chemical and mineralogical properties of five benchmark soils from Bangladesh have been studied. Free Fe₂0₃ content in these soils is low due to loss of iron along with the drainage water during the reduction phase. An accumulation of free Fe₂0₃ has been found to occur in the middle zone of the soil profiles which is possibly due to their fixation in the form of mottles. Free Fe₂0₃ is significantly correlated with the free MnO₂ content of the soils. The SiO₂/R₂O₃ molar ratios of these soils indicate the occurrence of non-uniformity in the soil parent materials. The SiO₂/R₂O₃ molar ratio of the clay fraction implies that clay mineralogy is mixed with an abundance of 2:1 layer lattice type minerals. Mica is the dominant mineral in the clay fraction of most of these soils. The content of smectite is high only in the Gangetic alluvium soils. The other minerals in the clay fraction of these soils are kaolinite, vermiculite, chlorite and interstratified minerals. The clay minerals appear to be inherited from parent materials with very little in situ mineral transformation.

Keywords: Benchmark soils, Chemical and Mineralogical Properties

Location: Department of Soil Science, University of Dhaka, Dhaka-1000, Bangladesh

Start Date:

End Date:

Program Area: Crop-Soil-Water Management

Commodity: Soil fertility

Objectives:

To determine the chemical and mineralogical properties of 34 soil samples of Benchmark soils in the the Barisal, Ghior, Gangachara, Sonatala and Ghatail soil series of Bangladesh

Methodology:

A total of 34 soil samples from five extensive soil profiles representing the Barisal, Ghior, Gangachara, Sonatala and Ghatail soil series were collected on horizon basis. The soil samples were processed and analysed in the laboratory for chemical and clay mineralogical properties. Free oxides of iron and manganese were extracted by sodium dithionite-citrate-bicarbonate method of Mehra and Jackson (1960). Iron and Mn in the extract were determined by atomic absorption spectrophotometer. Total analysis of the soil samples and the clay fractions thereof was carried out by Na₂C0₃ fusion method. Aluminium, iron, calcium and magnesium in the fusion extract of soils and clays were determined by using an atomic absorption spectrophotometer. Potassium was determined by a flame photometer. For clay mineralogical analysis the soil samples were treated with hydrogen peroxide and sodium acetate to destroy organic matter and carbonates and with sodium dithionite-citrate-bicarbonate to remove free oxides of iron and manganese; thereafter clay fraction was separated from deferrated and dispersed soil suspension by gravity sedimentation method (Jackson 1975). The clay samples were Mg- and K- saturated by repeated washings with normal solutions of MgCI₂.6H₂0 and KCI, respectively. The excess of the saturating solution was washed out with alcohol and acetone until free of CI and the saturated samples were stored in water. Slides were prepared from samples of Mg-saturated and air dried; Mg-saturated and glycerol solvated; K-saturated and air dried; K-saturated and heated at 550°C for one hour, These slides were scanned with GEOL X-ray diffractometer with Ni filtered Cu Kα radiation (at 30 kV, 40 mA) at a scanning speed of 2°2 θ per minute. The minerals in the clay fractions were identified following method of Jackson (1975). A semi-quantitative estimation of minerals in the clay fraction was carried out following the method of Johnson et al. (1963).

Source of Information: Journal of the Indian Society of Soil Science. Vol. 45. No. 2. pp 362-369 (1997)

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Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

The seasonal submergence and drying set the conditions of alternate oxidation and reduction, which are the most striking features of the pedochemical environment in these soils. These alternate wetting and drying situations hasten the process of soils profile development. Formation of structure in these soils is relatively quicker (Brammer 1971). The enrichment of Fe₂O₃ in the middle zone of soil profile is possibly due to the effect of these alternate oxidation and reduction conditions. This kind of mobilization and fixation of iron in the soils indicates that gleization is possibly the major pedogenic process. As noted earlier, the post-depositional mineral alteration in these soils appears to' be minimal.

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