Alkali Soils - Causes

Causes

The causes of soil alkalinity are natural or they can be man-made.

1. The natural cause is the presence of soil minerals producing sodium carbonate (Na₂CO₃) and sodium bicarbonate (NaHCO₃) upon weathering.
2. Coal fired boilers / power plants when using coal or lignite rich in limestone produces ash containing calcium oxide (CaO). 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 Ca and Mg ions / removes hardness in the water and also converts sodium bicarbonates in river water in to sodium carbonate. Sodium carbonates (washing soda) further reacts with the remaining Ca and Mg 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 is 7700 million tons in the year 2011. Thus river water is made devoid of Ca and Mg ions and enhanced Na by coal fired boilers.
3. 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 in to the river / ground water during their production process or consumption enhancing water sodicity. The total global consumption of sodium chloride is 270 million tons in the year 2010. This is nearly equal to the salt load in the mighty Amazon River. Manmade sodium salts contribution is nearly 7% of total salt load of all the rivers. Sodium salt load problem aggravates in the downstream of intensively cultivated river basins located in China, India, Egypt, Pakistan, west Asia, Australia, western USA, etc due to accumulation of salts in the remaining water after meeting various transpiration and evaporation losses.
4. Another source of man made sodium salts addition to the agriculture fields / land mass is in the vicinity of the wet cooling towers using sea water to dissipate waste heat generated in various industries located near the sea coast. Huge capacity cooling towers are installed in oil refineries, petrochemical complexes, fertilizer plants, chemical plants, nuclear & thermal power stations, centralized HVAC systems, etc. The drift / fine droplets emitted from the cooling towers contain nearly 6% sodium chloride which would deposit on the vicinity areas. This problem aggravates where the national pollution control norms are not imposed or not implemented to minimize the drift emissions to the best industrial norm for the sea water based wet cooling towers.
5. The man-made cause is the application of soft water in irrigation (surface or ground water) containing relatively high proportion of sodium bicarbonates and less calcium and magnesium.