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Concept
Alkali soil
Résumé
Alkali, or Alkaline, soils are clay soils with high pH (greater than 8.5), a poor soil structure and a low infiltration capacity. Often they have a hard calcareous layer at 0.5 to 1 metre depth. Alkali soils owe their unfavorable physico-chemical properties mainly to the dominating presence of sodium carbonate, which causes the soil to swell and difficult to clarify/settle. They derive their name from the alkali metal group of elements, to which sodium belongs, and which can induce basicity. Sometimes these soils are also referred to as alkaline sodic soils.
Alkaline soils are basic, but not all basic soils are alkaline.
The causes of soil alkalinity can be natural or man-made:
The natural cause is the presence of soil minerals producing sodium carbonate (Na2CO3) and sodium bicarbonate (NaHCO3) upon weathering.
Coal-fired boilers / power plants, when using coal or lignite rich in limestone, produce ash containing calcium oxide. CaO readily dissolves in water to form slaked lime, Ca(OH)2, and carried by rain water to rivers / irrigation water. Lime softening process precipitates Ca2+ and Mg2+ ions / removes hardness in the water and also converts sodium bicarbonates in river water into sodium carbonate. Sodium carbonates (washing soda) further reacts with the remaining Ca2+ and Mg2+ in the water to remove / precipitate the total hardness. Also water-soluble sodium salts present in the ash enhance the sodium content in water. The global coal consumption in the world was 7.7 billion tons in the year 2011. Thus river water is made devoid of Ca2+ and Mg2+ ions and enhanced Na+ by coal-fired boilers.
Many sodium salts are used in industrial and domestic applications such as sodium carbonate, sodium bicarbonate (baking soda), sodium sulphate, sodium hydroxide (caustic soda), sodium hypochlorite (bleaching powder), etc. in huge quantities. These salts are mainly produced from sodium chloride (common salt). All the sodium in these salts enter into the river / ground water during their production process or consumption enhancing water sodicity.
Source officielle
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