Parent Material, Soil

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Parent Material, Soil unconsolidated and more or less chemically weathered mineral and organic matter from which the solum of soils is developed by pedogenic processes. Two groups of inorganic parent materials are recognised: (a) sedentary (still at original sites), also called residual, which develops in place from the underlying rock; and (b) transported. Gravity, water, ices or wind may transport the materials. The gravity-transported materials are called colluvium, which is made up of the fragments of rock detached from the heights above and carried down the slopes. The water-transported materials are divided into three types depending on the environment of deposition. Water transported materials deposited in lakes are called lacustrine; those deposited by streams are called alluvium (fluvium); and materials deposited in oceans are called marine. The materials, which are transported by ice and wind, are called glacial and eolian, respectively. Except in the case of organic soils, the original mineral materials in soils occur due to geologic reasons.

Most of the soils of Bangladesh are relatively young and their properties are heavily influenced by the nature of their parent materials and drainage condition. Almost the entire parent materials are water-transported, carried by the rivers flowing through Bangladesh, and are deposited in different environments. A total of twenty different types of parent materials are identified on which the soils of Bangladesh are developed. These are Tertiary rocks, madhupur clay, Madhupur Clay alluvium, Old Himalayan Piedmont Plain alluvium, Tista alluvium, Atrai alluvium, Karatoya-Bangali alluvium, Jamuna alluvium, Old Brahmaputra alluvium, Surma-Kushiyara alluvium, Middle Meghna alluvium, Lower Meghna river alluvium, Old Meghna estuarine alluvium, Young Meghna estuarine alluvium, Minor eastern rivers alluvium, Ganges river alluvium, Ganges tidal alluvium, Northern and Eastern Piedmont Plain alluvium, Peat and Chittagong Coastal Plain alluvium.

sediments belonging to three distinct ages, viz Tertiary, Pleistocene and Holocene, occur in Bangladesh. The consolidated sediments occurring in Sylhet and chittagong hill tracts are the oldest rock formations. The Madhupur Clay occurring in the Madhupur and barind tracts is regarded to be of Pleistocene age. Unconsolidated floodplain sediments occupy about 80% of Bangladesh and are termed recent alluvium (Holocene sediment). Streams commonly overflow their banks and flood the surrounding area. Sediments carried by the swollen stream are deposited during the flood, with the coarser materials being laid down near the riverbank and finer materials farther away. When rivers change their courses, as they have done many times in the past in Bangladesh, sandy materials may sometimes be found a long way from present riverbanks.

Different rivers bring sediments from areas of different geology. Accordingly, the alluvial sediments of different rivers contain different amounts of such rock materials as micas, feldspars, hornblende, etc. For instance, Tista and Brahmaputra alluvial deposits contain large amounts of biotite (black mica), an easily alterable mineral, but little or no lime. On the other hand, Ganges alluvium contains lesser amounts of biotite but significant amounts of lime. The deposits of Surma, Karnafuli and other rivers draining the eastern hills contain little biotite and no lime. Thus there is great diversity in the parent material occurring in Bangladesh, from which soils with different properties were developed. [Sirajul Hoque] [Hoque, Sirajul Professor of Soil, Water and Environment, Dhaka University]