Morphogenetic and Taxonomic Characterization of the Lower Atrai Basin Soils of Bangladesh

Abstract

The lower Atrai basin or the Chalan beel, as it is popularly known, is one of the large inland depressions in the northern part of Bangladesh, where a large quantity of Boro and Aman rice is grown. This basin was formed when the old Brahmaputra river changed its course into the Jamuna channel. The Jamuna river impeded the flow of the Padma, thereby causing the latter to deposit sediments at the mouths of the Karatoa and Atrai rivers. The diverted flow of these two rivers created the Chalan beel. The major portion of the basin extends over three adjacent districts of Sirajgonj, Natore and Naogaon and very small portion of Rajshahi and Bogra districts. It is located between 24021′ to 24051′ N latitudes and between 88049′ to 89023′E longitudes. The original area of the basin was about 108,800 ha which has been reduced to about 36,800 ha at present caused by siltation. Eight extensive soil series, viz. Binsara, Taras 1, Jaonia, Taras 2, Hasnabad, Laskara, Manda and Mainam from the lower Atrai basin of Bangladesh were studied in the field as well as in the laboratory. The salient morphological features such as colour, colour patterns, texture, structure, mottles, consistency and flood coatings of the soils were studied in the field. In all the pedons prismatic to angular blocky structure developed in the subsurface horizons. Silt was the dominant mechanical fraction of the soils followed by clay and sand fraction. The vertical distribution patterns of sand/silt ratio as well as the clay contents in the profiles were irregular which indicated the heterogeneous nature of the parent materials. Most of the soils were found to have higher bulk density in the Ap2 horizons (plough pan) due to compaction resulting from ploughing for rice cultivation. The important chemical parameters of the soils such as pH, organic carbon, total nitrogen, cation exchange capacity, exchangeable bases, free oxides of Fe and Mn were determined. Moreover, fusion analysis of whole soils and the clay fraction thereof were done. The reaction of the soils was moderately acid to neutral. The ΔpH values of all the soils were negative and which ranged from -1.05 to -1.83 pH unit. The mean organic matter contents of the individual pedons varied from 0.39 to 1.26 percent with a grand mean of 0.96 percent. The organic matter and total nitrogen content in the present soils showed a general tendency of decrease with depth in all the pedons. The percent base saturation was high (71-99) and showed a tendency to increase with depth in all the profiles. Cation exchange capacity of the soils was medium to high with a mean value of 15.25cmol/kg. Free oxides of iron and manganese in the soils were low due to their loss from the profile along with the draining water. Results of fusion analysis of the soils indicate that SiO2 contents in the whole soil were higher than that in the clay fraction in all the pedons. On the other hand the contents of Al2O3 and Fe2O3 in the whole soil were lower than that in the clay fraction. Both in soil and clay fraction SiO2 was the dominant element followed by Al2O3 and Fe2O3. The distribution pattern of SiO2, Al2O3 and Fe2O3 varied with depth. The silica sesquioxide molar ratios of the soils indicated that the soil parent materials were heterogeneous. The soils had illitic mineralogical class except Hasnabad and Laskara pedon. The latter two soils had mixed mineralogy class. All the soils contained high content of mica followed by kaolinite, chlorite, vermiculite and interstratified minerals. The Hasnabad and Laskara soils had mixed mineralogical class consisting of mica, kaolinite, chlorite, vermiculite and moderate amount of interstratified mineral. The higher content of mica in the clay fraction suggested that the soils were well supplied with potassium and there is little need for application of potassic fertilizer. Electron micrographs were used to see the shapes and sizes of minerals in the clay fraction. The soils of the lower Atrai basin have been characterized as “hydromorphic soils” where gleization seemed to be the major pedogenic process. For classification the Manda and Mainam soils meet the requirements of the Entisols and Binsara, Taras 1, Jaonia, Taras 2, Hasnabad and Laskara soils meet the requirements of the Inceptisols order of the US Soil Taxonomy. According to WRB system the Manda and Mainam soils were grouped in to Fluvisols and Binsara, Taras 1, Jaonia, Taras 2, Hasnabad and Laskara soils were grouped into Gleysols. The soils were characterized in the family level of US Soil Taxonomy. This family level classification may facilitate the process of agro-technology transfer with other soils in Bangladesh.