Soil Order a group of soils in the broadest category. For example, in the 1938 classification system, the three soil orders were zonal soil, intrazonal soil and azonal soil. In the present USDA classification there are ten orders differentiated by the presence or absence of diagnostic horizons: Alfisols, Aridisols, Entisols, Histosols, Inceptisols, Mollisols, Oxisols, Spodosols, Ultisols, and Vertisols.
Histosols occur extensively in the gopalganj-khulna beels, and more locally in some old floodplain basins elsewhere. They include seven soil series all classified as peat soils. Mollisols have not been identified in Bangladesh. However, some minor soils classified as noncalcareous dark grey floodplain soils and black Terai soils have been placed in the Mollic Gleysols. Entisols include non-calcareous alluvium, calcareous alluvium soils, strongly acid soils but lacking an umbric A horizon and not sulphidic, with an umbric A horizon but not sulphidic (tista floodplain) and containing a sulphidic layer (occurring on tidal floodplains on the Chittagong coastal plain). Inceptisols include non-calcareous and calcareous grey and dark grey floodplain soils, deep grey terrace soil and grey valley soil as well as some acid sulphate soil, grey piedmont soil and acid basin clay. Vertic properties are present in the subsoil of some calcareous dark grey floodplain soils and acid basin clay and in the Madhupur clay substratum of many shallow grey terrace soils. [Md Sultan Hussain]