Clay mineralogy of some soils of Bangladesh: A consultancy report

Abstract

This consultancy was initiated through IADS/BARC (Dr. Sam portch and Dr. M.A. Mannan) who arranged for the analysis of 115 samples of soil from Bangladesh. The agreement called for qualitative analysis of the clay mineral fraction of the soil samples. The clay fractions (less than 2 microns ) were obtained by sedimentation after a brief ultrasonic treatment to disperse the soils. Oriented specimens were prepared as films on glass slides and diffractograms recorded on a General Electric XRD-5 X-Ray diffractometer in the Department of Agronomy, Purdue University, West Lafayette, IN 47907. In addition to diffractograms of untreated specimens of all 115 samples, an additional 56 diffractograms were made of selected samples which had been heated to 500 degrees C. or solvated wigh ethylene glycol in order to properly identify and distinguish between possible vermiculite, and smectite (monomorillonite) phases. Qualitative estimates of the composition were made on the basis of parts per ten for all 115 samples and the analyses prepared in table form. In addition, In addition, three copies of the tabulated results and sets of the 171 diffractograms were delivered to---------------- Most of the parent material is of alluvial origin; it appears that the dominant parent material in the clay fraction is muscovite mica, along with moderate amounts of kaolinite. the mica is weathered to vermiculite under acidic conditions and under basic conditions it weathers to sectite of montmorillonite. Under acidic conditions aluminum polymers resulting from dissolution and decomposition of the mineral phases may be exchanged onto the expanded mica of vermiculite; this may reduce the capacity of the weathered micaceous clay to "fix'' potassium. X-Ray diffractograms show the prosence of amorphou material in some of the highly weathered soils; this was confirmed by chemical data on oxalate extractable aluminum. The 001/002 intensity ratio for mica suggests that only the resistant core of the muscovite mica remains where " ferrolysis" has occurred. the amorphous material could be involved in phosphorous fixation. Some mica materials have been weathered to a "high charge" vermiculite; this would have the capacity to fix large amounts of potassium. This appears to be true for the BAU Mymensingh samplesl from the potash experiments. Management practices which localize potassium fertilizer application would minizize possible fixation problems. Soils having an 001/002 intensity ratio for mica around 2.0 have been intensely weathered and only the resistant core of the mica is left. Such soils are likely to show response to potassium fertilization.