Sand loose material formed of quartz grains (sand grains) such as that of the beaches or dunes. Sand grains are mostly siliceous (sometimes calcareous, gypseous or of volcanic origin), because it is mainly quartz, derived from the disintegration or fragmentation of crystalline rocks, gives fragments of size between 0.05 mm and 2 mm. After the disintegration of rocks, the sand is mostly transported by water, glaciers or wind. The proportion of sand can vary from soil to soil. Coarse sands are highest in silica while finer ones contain more potassium, calcium, and phosphorus. The size of the very coarse, coarse, medium, fine and very fine sand ranges from 1.00 to 2.00, 0.50 to 1.00, 0.25 to 0.50, 0.10 to 0.25 and 0.05 to 0.10 mm respectively according to the USDA (United States Department of Agriculture) system.
However, in the International System, size varies between 2.00 and 0.20 and 0.20 and 0.02 mm for coarse and fine sand respectively. As regards its importance in the soil, the coarse sand has significance structurally but affects permeability. Since it is composed of relatively undecomposed primary minerals, its contribution to the supply of nutrients is very low and insignificant. Moreover, a high proportion of sand usually leads to the loss of stickiness and plasticity in the soil, making it friable and soft. From an agricultural point of view, the sandy type of soil can be quite easily handled for tillage operations.
Bangladesh is quite rich in terms of sand and silt deposits. Though sand is present in all soils of Bangladesh, it occurs widely in the substratum of soils on the Old Himalayan Piedmont Plain, in some parts of the Old Brahmaputra Floodplain and in the west Ganges River Floodplain. Sands also occur on the chars of Brahmaputra-Jamuna river. These sands are of good quality, and can be used for plaster and fill material. They are replenished every year during the monsoon. Extraction of sand keeps the river channels clear for the free flow of water. Moreover, this will also create job opportunities for local people. However, some portions of the meghna and the karnafuli rivers are affected by tides and hence the sands and silts are slightly saline, and hence of low quality. [Rameswar Mondal]