Alkaline Soil defined as the soils with pH greater than 7.3. Usually applied to surface layer or root zone but may be used to characterise any horizon or a sample thereof. Alkalinity occurs when there is a comparatively high degree of saturation with base forming cations such as calcium, magnesium, potassium and sodium. The presence of calcium, magnesium and sodium carbonates also can result in a preponderance of hydroxy ions over hydrogen ions in soil solution. Under such conditions, the soil is alkaline, sometimes very strongly. A pH of 9 or 10 may be reached in some soils if sodium carbonate is present.
Noncalcareous grey floodplain soils of Bangladesh contain moderately alkaline (pH 7.6) soils. Generally, the pH is highest where the soils have received new alluvium on the surface and lowest in basin sites. In some regions of manikganj, the topsoil is moderately alkaline. Alkaline soils are found in the regions of the Ganges Tidal Floodplain and under the region of non-saline and non-calcareous soils. They are located in the general soil types of Non-calcareous Alluvium, Non-calcareous Grey and Dark Grey floodplain soils. The active Brahmaputra-Jamuna floodplain is rich in weatherable minerals, especially micas, which are moderately alkaline in reaction but not calcareous. From the Ganges-Jamuna junction downstream, the active floodplain includes mixed alluvium of both rivers, which reduces the average lime content by about 2-5 percent.
Because of the lime, the young alluvium is moderately alkaline in reaction, but it is neither saline nor sodic. Gangetic alluvium is calcareous, while dhaleshwari alluvium is neutral to alkaline but not calcareous. The subsoils are more alkaline in the calcareous layer of Ganges floodplain soils. Non-calcareous alluvium comprises stratified sands and silts occupying recent alluvial deposits occurring adjacent to the Jamuna floodplain and locally along the karatoya river. Cultivated topsoil may be slightly or moderately acid, but other layers generally are neutral to moderately alkaline (pH 6.6-8.2) but not calcareous. Layers of new Jamuna alluvium deposited during high floods in that river are non-calcareous but are slightly to moderately alkaline in reaction. In the Non-Calcareous Dark Grey Floodplain soils, there are small patches in parts of jhenaidah and jessore districts, which are slightly saline at the surface during the hot weather and some patches have strongly alkaline topsoil (pH 8.5-10). In the Old Brahmaputra floodplain and the Old Meghna estuarine floodplain, the cultivated layer generally is slightly acidic to moderately alkaline (pH 6-7.8).
It was found that the greater the concentration of alkalinity of soil solution after each irrigation, the more immediate the alkalinity increase after irrigation. So the pH of the soil solution increased from 7 to 8 up to 9 to 10, which is very harmful for cotton and many other irrigated plants. [Md Khurshid Alam]