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Soil the natural medium for growth of land plants. Alternatively, the accumulation of unconsolidated rocks and minerals fragments and organic matter formed in place at the earth';s surface; capable of supporting life.

Practically, soil is a three-phase system covering solid, liquid and gaseous phases. The solid phase comprises both organic and inorganic matter while the liquid and gaseous phases contain water and air respectively. soil organic matter is the residue of living and non-living origins of plant, animal and microorganisms. Dead residues exist in various degrees of decomposition ranging from undecomposed to completely decomposed. The mineral matter mostly supplies the mineral elements derived from rocks. Particles of various sizes are also generated from rocks by weathering. Of these, sand, silt and clay provide mechanical and nutritional support to the growing plants. The chemical contribution of sand to soil is very little. However, its physical contribution to improve pore space is important as this allows the free movement of air and water through the soil. Silt supplies nutrients to plants to some extent. In contrast, clay is the most reactive and is called the plasma of the soil. Both physical and chemical properties of the soil are dominantly influenced by clay fraction. Clay is colloidal in nature and generally possesses a negative charge. Thus, it holds positive ions like calcium, magnesium, potassium and ammonium ions around the exchange sites.

soil water and air occupies the pore spaces of the soil. Under normal conditions, the macro pore space is occupied by air and micro pore space by water. A soil is said to be agriculturally good if the pore spaces are filled with air and water in equal proportions.

The component of soil air is always changing and contains approximately the same composition as atmospheric air. However, the percentage of carbon dioxide and water vapour is higher and that of oxygen is lower in soil air in comparison with atmospheric air. Bangladesh has many different kinds of soils because it has a wide range of environmental conditions, such as parent material, climate, relief, drainage, vegetation, and age.

Based on these conditions the general soil types of Bangladesh can be grouped as follows: Black Terai soils (north of the himalayan piedmont plains); Non-calcareous Brown Floodplain soils (Himalayan Piedmont Plains); Calcareous Brown Floodplain soils (developed in alluvium deposited by the ganges river plus western Jessore and southern Kushtia districts); Non-calcareous alluvium (recent deposits of the brahmaputra, tista, meghna and smaller rivers of the north and east); Calcareous alluvium (recent deposits of the Ganges and the Lower Meghna rivers); Grey Floodplain soils (developed in non-calcareous alluvium and among the most highly productive agriculturally); Calcareous Dark Grey Floodplain soils (occupying most of the Ganges river floodplain); acid basin clays (occurring mainly in the Haor Basin areas); peat soil; acid sulphate soils (mainly found in the Khulna, sundarbans tidal forest and adjoining areas and Chakaria); Grey Piedmont soils (formed in hill outwash alluvium along the hill ranges of Sylhet and the plains of the Chittagong sub-region); Grey Terrace and valley soils (occupying most of the barind tract and the susang hills); Deep Red-Brown Terrace soils (developed on the better drained parts of the madhupur tract, the northeastern Barind, parts of the lalmai hills and locally within the tila ranges of Sylhet and the hills of the Chittagong sub-region); Shallow Red-Brown Terrace soils (widely developed on the Madhupur Tract and very locally in the northwest of the Barind Tract); Brown Mottled Terrace soils; brown hill soils (found in the hills and mountain ranges of Sylhet and the Chittagong sub-region). [Rameswar Mondal] [Mondal, Rameswar Professor of Soil, Water and Environment, Dhaka University]