Sodicity vs Salinity
We have often heard of ‘saline’ solutions. The word ‘saline’ is associated with salt. Salinity is derived from ‘saline’ and it expresses the level of saltiness of a solution. The term ‘sodicity’ is closely connected to salinity but has the feature of having high concentrations of sodium (Na+) ions in the solution. Ideally both these terms are forms of measurements that give us more information on the properties of the solutions. In general, the term ‘salinity’ is used along with water bodies and soil, but the term ‘sodicity’ is more often connected to soil conditions. Therefore for comparison purposes it is convenient to consider the effect of both these measurements in soil.
As mentioned above, salinity refers to the saltiness of a solution or more correctly it refers to the dissolved salt content present in the solution. When measuring the salt concentrations on a ppt (parts per thousand) scale, if fresh water is labelled as ‘0 ppt’, saline water has a salt content of ’50 ppt’. The level of salinity is also commonly measured in ppm (parts per million), and it can also be measured as a conductivity ratio compared to a potassium chloride (KCl) solution known as the Practical Salinity Scale (PSS) which is a dimensionless unit.
The most common salts that cause salinity are sodium chloride (NaCl), Magnesium chloride (MgCl), Calcium carbonate (CaCO3), bicarbonates (HCO3-) etc. High salinity level in soil is not so favourable for plant growth. When soil water has more salt dissolved in it, it becomes a more saturated/concentrated solution over fresh water. Therefore, instead of plant uptake of water from roots, the water that had entered root cells will leak out as the soil water is more concentrated than the water in cells. This happens to reach a level of equilibrium through a process is called ‘osmosis’, and the plant is said to be under ‘chemical drought’ even though the soil remains moist. Therefore, excess salt in soil is not a positive condition for plants. However, a correct amount of salt is also needed to maintain the proper integrity of the soil. Salt ions (positive ions such as Na+, Ca 2+, and Mg2+) play an important role in keeping the soil aggregates bound together as clay and silt material are often negatively charged.
Sodic soils have an unusually high concentration of sodium (Na+) ions, with a percentage greater than 15% in most cases. The term ‘sodicity’ is derived from the name of the alkali metal sodium itself. Sodic soils have a poor structure and is not very suitable for plant growth. When excess amounts of Na+ is present, it is said that the soils ‘swell’ and it causes dispersion (separation of soil aggregates into small portions). A dispersed soil loses its integrity, becomes prone to waterlogging and is usually harder, making it difficult to the roots to penetrate.
Clay particles are negatively charged, and Na+ helps bind the clay particles together. But often water molecules easily displace the clay particles and solvates the sodium ion. This happens due to the singular positive charge around sodium which attracts only few clay particles to it at a time, making them easily displaceable. Therefore, dispersion occurs when the clay particles are released instead of being bound together. Ca2+, on the other hand, is a better agent in binding clay particles together as it attracts many clay particles around it making them difficult to be displaced by water molecules, thereby protecting the soil’s integrity. Therefore, the addition of gypsum or lime (both contain Ca2+) can improve the condition of sodic soils.
What is the difference between Salinity and Sodicity?
• Saline soils have high salt concentrations than usual, whereas sodic soils have high concentrations of Na+ than usual.
• Saline soils causes a ‘chemical drought’ in soils but sodic soils does not.
• Sodic soils cause waterlogging but saline soils does not.
• Salinity protects the integrity of soil in contrast to sodicity which destroys the structure of soil by causing dispersion.
• Sodicity in soil is easier to correct than high salinity levels in the soil.