Alluvial Soil a young soil on floodplains and deltas that is actively in the process of construction and has primary characteristics of the alluvium itself. The old alluvial soils often show the development of 'cambic' horizon. These soils are classified in the Inceptisols order of the US soil taxonomy. On the other hand the younger alluvial soils are categorised in the Entisols order.
The alluvial soils have played an important role in the history of mankind. They support a larger proportion of the world's population than do any other great soil groups. All the ancient civilisations originated on alluvial soils. The famous Mesopotamian and the Nile valley civilisations developed on the alluvial soils of the Euphrates-Tigris and the Nile river floodplains respectively. The alluvial soils of the vast Indo-Gangetic plain have been sustaining a huge population over the centuries.
If sufficient water is available most of the alluvial soils may become highly productive even if they have developed from diversified parent material. The alluvial materials of different river basins have different properties. For example, the Gangetic alluvium has an alkaline reaction and is calcareous. In places it contains up to I5% calcium carbonate. The alluvium of the brahmaputra river is acidic in reaction and is noncalcareous. The alluvial soils occupy around 80% of the territory of Bangladesh and contain high quantities of mica minerals. The alluvial soils are heterogenous in age, texture or mineralogy. They have been deposited under different geomorphological conditions in different areas: piedmont plains near the foothills including the himalayas, river meander floodplains, estaurine floodplains and tidal floodplains.
In the alluvial formations the coarser materials are not carried long distances from their sources. The texture of the alluvial materials become more uniform and finer as the distance it is transported increases. Across the country the texture of alluvial materials become finer as one moves from north to south. Most of the alluvial soils are very fertile and are productive here.
Alluvial soils of Bangladesh may be divided as non-calcareous alluvium, calcareous alluvium, acid sulphate soil, peat soil, non-calcareous grey floodplain soil, calcareous grey floodplain soil, non-calcareous dark grey floodplain soil, calcareous dark grey floodplain soil, grey piedmont soil, acid basin clay, non-calcareous brown floodplain soil, calcareous brown floodplain soil, and black Terai soil.
When the alluvial soils in the low-lying areas crack, the surface soil materials enter into the cracks with the first flood water and form coatings which are common features of many alluvial soils. In many alluvial soil profiles groundwater fluctuates throughout the year. Oxidation and reduction conditions alternate in these soils. Most often gleization is the common pedogenic process in the alluvial soils. Alluvial soils may have good drainage as well.
Most of the alluvial soils in Bangladesh are very fertile and are highly productive. Many of them are good producers of rice. The alluvial soils that occur along the coastal areas may be saline as they are affected by tidal action. [Md Sultan Hussain]