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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 heart 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]
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