Siltation pattern and pedogenesis in the coastal zone of Bangladesh

Abstract

Siltation pattern and pedogenesis in the coastal zone of Bangladesh were studied. Suspended sediment load carried by the two rivers, the Bishkhali and the Arial Khan rivers of the coastal zone in Bangladesh was determined on seasonal basis for three consecutive years in order to observe siltation pattern. Suspended sediment load in the Bishkhali river was very high (9.4 kg per sq. meter per month) in the dry season as compared to that in the wet season (4.6 kg per sq. meter per month) whereas in the Arial Khan River the suspended sediment load was low in dry season (4.1 kg per sq. meter per month) as compared to that in the wet season (13.06 kg per sq. meter per month) due to severe water turbulence of the rivers. The mechanical, chemical, physico-chemical and elemental compositions of the sediments were determined and the importance of siltation in soil genesis was also studied. Eight soil profiles from eight extensive soil series viz. Bhola, Nilkamal, Pirojpur, Hogla ,Muladi, Barisal, Jhalakati, and Ramgati occurring in the coastal floodplains of Bangladesh were studied morphologically in the field, and soil samples were collected on genetic horizon basis for their laboratory analyses. All these soils remain seasonally flooded for varying periods during the monsoon months and develop some unique morphological features. Development of a cambic horizon is the most notable morphogenetic property in these soils. Particle size distribution showed wide variations in their clay fractions in the soil profiles. The quantity of clay was higher in the basin soils than in the ridge soils. Silt was the dominant size fraction in the sediment of the Bishkhali and the Arial Khan rivers. The chemical properties of the soils such as pH, organic carbon, total nitrogen, cation exchange capacity, exchangeable bases, free oxides of Fe and Mn, free lime and the fusion analysis of the sediments, whole soils and their clays were determined. Results of fusion analyses of the sediments, soils and their clay fractions were presented. Both in soils and sediments silica was the most abundant element comprising a large part (58 to 65%) of the elemental composition of them. The other dominant elements were in decrasing order of abundance: SiO 2 > Al 2 O 3 > Fe 2 O 3 > MgO > CaO > K 2 O > TiO 2 >P 2 O 5 > MnO 2 . The vertical distribution pattern of silica, alumina, and ferric oxide were not uniform. The silica: sesquioxide molar ratios of the soils indicated that the soil parent materials were heterogeneous. Illite was the dominant mineral in the clay fraction of Muladi, Barisal, Jhalakati and Ramgati soils, whereas the other soils had mixed mineralogical composition consisting of mica, smectite, chlorite, kaolinite and trace amount of vermiculite. Gleization seemed to be the major pedogenic process of soil formation. Composition of parent material (sediments) and the associated aquic moisture regime dominantly influenced the genesis of these soils. All the soils meet the criteria of the Inceptisols order of the US Soil Taxonomy. The soils were characterized at the family category level according to the US Soil Taxonomy and correlated with World Reference Base (WRB) system. The studied soils were classed as six families which will be useful for their management implication as well as agrotechnology transfer program.