Mineralogy of Some Benchmark soils of the Coastal Plain, West Bengal

Abstract

Mineralogical studies of sand, silt and clay fractions of four benchmark soils (Ballartop, Chandipur, Narayanpur and Patibunia) occurring on coastal plain, West Bengal were conducted to identify the minerals present in these fractions and also to understand their transformation in this ecosystem. Ballartop, Chandipur and Narayanpur soils were developed on very gently sloping to nearly level plain with poor drainage whereas Patibunia soil was developed on degraded mangrove with imperfectly drainage condition. The soils were dark grayish brown to dark gray in colour except Narayanpur soils which was gray to dark gray throughout the profile with olive brown to dark brown mottles and the texture of the soils varied from silt loam to silty clay. The soils were neutral to moderately saline (pH 6.7 to 8.1) except the soil of Ballartop (pH 4.7- 8.2). The EC value ranged from 0.72 to 8.3 dSm−1 and the clay content varied from 19.8 to 51.9%. The CEC of the soils was low to high and ESP of these soils ranges from 6.1 to 33.5.

The mineralogical study revealed that in the heavy sand fraction, the minerals were present in the order: opaque > limonite > biotite > phlogopite > hornblende > pyroxene > goethite > kyanite and the other minerals are in trace. In the light sand fraction, quartz was the dominant mineral (43–89%), followed by muscovite (6–48%), potash feldspar (1–14%) and plagioclase feldspar (1–11%). The minerals in silt fraction followed the order: mica (42–67%) > kaolinite (9–24%) > chlorite (6–16%) > vermiculite (5–15%) smectite (1–10%) > K- plagioclase > feldspar feldspar with minor to trace amount of 10–14Å mixed layer mineral and 8Å smectite/kaolinite (sm/k). In clay fraction, mica was the dominant mineral followed by kaolinite and varying amount of smectite, chlorite and vermiculite. Significant amount of smectite (26–33%) was present in Narayanpur soils and in some horizons of Chandipur soils. The mineralogy of Ballartop, Chandipur and Patibunia soils was “illitic” and that of Narayanpur soils was “mixed”. The higher peak intensity ratio of 001/002 mica reflection of silt and clay fractions indicated the dominance of tri-octahedral mica which have significant amount of reserve potash.