Saline and Alkali Soils: Formation, Effects and Control

After reading this article you will learn about:- 1. Characteristics of Saline and Alkali Soils 2. Formation of Saline and Alkali Soils 3. Effect on the Growth of Plants 4. Control.

Characteristics of Saline and Alkali Soils:

Saline soils contain excessive amounts of neutral salts like chloride and sulphate of sodium, calcium and magnesium, and less than 15 per cent exchangeable sodium. Their pH is less than 8.5, due to the presence of excessive amounts of neutral salts. Saline soils are in excellent physical condition and are permeable to water.

Saline-alkali soils also contain enough neutral salts like chloride and sulphate of sodium and calcium and magnesium to maintain their pH value below 8.5, even when they contain more than 15 per cent exchangeable sodium.

Non-saline alkali soils contain more than 15 per cent exchangeable sodium. Their pH is more than 8.5 because they do not contain any neutral soluble salts. They are sticky and plastic when wet and very hard when dry. Water does not percolate through them.

Formation of Saline and Alkali Soils:

Soluble salts originate in the soil from the decomposition of primary minerals. Whenever drainage is restricted and excess amounts of water evaporate from the surface of the land in the arid regions, neutral soluble salts move upward, along with the upward movement of water, and accumulate on the surface as a white crust.

In non-saline alkali soils, soluble salts are washed down by the limited amounts of rainfall in the arid regions when calcium and other ions are replaced from the clay micelle by sodium ions. The clay micelle is then saturated with sodium ions.

They react with the carbonic acid to form sodium carbonate as shown in the following equation:

This sodium carbonate is hydrolyzed to form sodium hydroxide and carbonic acid as shown in the following equation:

Since carbonic acid is a weak acid and sodium hydroxide is a strong base, the latter completely dissociates to increase the hydroxyl ion concentration of the soil solution and its pH. Therefore sodium carbonate is responsible for the increase in soil pH to more than 9.0 or even more than 10. The surface becomes black colour due to the dissolution of humus in the alkaline medium.

Effects of Soil Salinity and Alkalinity on the Growth of Crops:

Saline and Saline-Alkali soils contain excessive quantities of salts, which increase the osmotic pressure of the soil solution. So root hairs absorb less water from saline soils, which contain excessive amounts of chlorides, harmful for crop growth.

Non-saline alkali soils are in a poor physical condition, so roots suffer from poor aeration. They contain borate, bicarbonate and sodium ions even small amounts of borates are toxic to crops. Excessive amounts 01 bicarbonate and sodium ions are also harmful for crops; they make phosphorus and all micronutrients except molybdenum unavailable to crops.

Control of Soil Salinity and Alkalinity:

Underground drainage system should be established.

As saline soils are permeable to water heavy irrigation can be used to wash down the soluble salts by adding gypsum to alkali soils, the sodium ions from the clay micelles are gradually replaced by calcium ions, as shown in the following equation:

Soluble sodium sulphate is washed down by heavily irrigating the alkali soils, when the above reversible equation proceeds in the forward direction, which means that the sodium ions are gradually replaced from the clay micelle by the calcium ions, which ultimately saturate the clay micelle when soil alkalinity is controlled.

If the alkali soil contains free calcium carbonate, then powdered sulphur of ferrous sulphate may be added to the alkali soil, first producing sulphuric acid and then, calcium sulphate, as shown in the following equations:

The calcium from this calcium sulphate gradually replace sodium ions from the clay micelle as shown in the following equation:

Sodium sulphate is washed down by heavily irrigating the field. Ultimately soil alkalinity is controlled.