Salt Affected Soils: Characteristics, Formation and Problems

After reading this article you will learn about:- 1. Characteristics of Salt Affected Soils 2. Formation of Salt Affected Soils 3. Problems.

Characteristics of Salt Affected Soils:

The most important characteristics of salt affected soils like saline, non-saline-alkali, saline-alkali and degraded alkali soils are given in table 15.2.

Formation of Salt Affected Soils:

Saline Soil:

The process by which the saline soil formed is called “Salinization”. Saline soils occur mostly in arid or semi-arid regions. Under humid conditions soluble salts originally present in soil materials and those formed by weathering of minerals generally are carried downward into the ground water and are transported ultimately by streams or oceans.

Saline soils are, therefore, practically non-existent in humid regions, except when the soil has been subjected to sea water in river deltas and low lying lands near the sea. In arid regions saline soils occur not only because there is less rainfall available to leach and transport the salt but also because of high evaporation rates, which tend further to concentrate the salts in soils and in surface waters.

Restricted drainage is another factor that usually contributes to the salinization of soils and may involve the presence of a high ground water table or low permeability of the soil.

Low permeability of the soil causes poor drainage by impeding the downward movement of water. Low permeability may be the result of an unfavorable soil texture or structure or the presence of indurated layers. These indurated layers may consist of a clay pan or a caliche layer of a silica hardpan.

Alkali or Sodic Soil:

It is evident that soil colloids adsorb and retain cations on their surfaces. Cation adsorption occurs as a consequence of the electrical charges at the surface of the soil colloids. While adsorbed cations are combined chemically with the soil colloids, they may be replaced by other cations that occur in the soil solution.

The reaction whereby a cation in solution replaces an adsorbed cation is called cation exchange and is expressed as milliequivalent per 100 g soil. Calcium and magnesium are the principal cations found in the soil solution and on the exchange complex of normal soils in arid regions. When excess soluble salts accumulate in these soils, sodium frequently becomes the dominant cation in the soil solution.

In arid regions as the soil solution becomes concentrated through evaporation or water absorption by plants, the solubility limits of calcium sulphate (CaSO₄), calcium carbonate (CaCO₃) and magnesium carbonate (MgCO₃) are often exceeded, in which case they are precipitated with a corresponding increase in sodium concentration.

Under such conditions, a part of the original exchangeable calcium (Ca²⁺) and magnesium (Mg²⁺) is replaced by sodium (Na⁺) resulting alkali or sodic soils.

Saline-Alkali or Saline-Sodic Soil:

These soils form as a result of the combined processes of salinization and alkalization. If the excess soluble salts of these soils are leached downward, the properties of these soils may change markedly and become similar to those of sodic soil.

As the concentration of these salts in the soil solution is lowered, some of the exchangeable sodium hydrolyzes and form sodium hydroxide (NaOH). This may change to sodium carbonate upon reaction with carbon dioxide (CO₂) absorbed from the atmosphere.

On extensive leaching, the soil may become strongly alkaline, the particles disperse and the soil becomes unfavorable for the entry and movement of water and for tillage operation. At the same time sodium toxicity to plants is increased.

These soils sometimes contain gypsum (CaSO₄.2H₂O) and when it is subjected to intense leaching, calcium dissolves and the replacement of exchangeable sodium by calcium takes place concurrently with the removal of excess salts.

Problems of Salt Affected Soils:

Saline Soils:

There are various problems of saline soils that interfere with the plant growth:

(i) Soils are usually barren but potentially productive

(ii) Wilting coefficient of saline soil is very high

(iii) Amount of available soil moisture is low

(iv) Absorption of water and nutrients.

Excessive salts in the soil solution increase the osmotic pressure of soil solution in comparison to cell sap. This osmotic effect increases the potential forces that hold water in the soil and makes it more difficult for plant roots to extract moisture.

During a drying period, salt in soil solutions may be so concentrated as to kill plants by pulling water from them (exosmosis). Due to high salt concentration plants have to spent more energy to absorb water and to exclude salt from metabolically active sites. At the same time various nutrient elements become unavailable to plants.

(v) Salt toxicity. When the concentration of soluble salts increases to a high level then it produces toxic effect directly to plants such as root injury, inhibition of seed germination etc.

Alkali or Sodic Soils:

Excess exchangeable sodium in alkali soils affects both the physical and chemical properties of soils.

(i) Dispersion of Soil Colloids:

Under alkali conditions, the damage is not due to salt concentration. The influence of exchangeable sodium on the over-all physical properties of soils is associated mainly with the behaviour of the clay and organic matter, in which most of the cation exchange capacity (CEC) is concentrated. The sodium (Na⁺) ion adsorbed by clay colloids causes de-flocculation or dispersion of clay which result in a loss of desirable soil structure and helps for the development of compact soil.

(ii) Other Physical Properties:

Due to dispersion and compactness of soil, aeration, hydraulic conductivity, drainage and microbial activity are reduced.

(iii) Caustic Influence:

It results high sodicity caused by the sodium carbonate (Na₂CO₃) and bicarbonate (NaHCO₃).

(iv) Concentration of Hydroxyl (OH^–) Ion:

High hydroxyl (OH^–) ion concentration no doubt has direct detrimental effect on plants. Damage from hydroxyl ions occurs at pH 10.5 or higher.

(v) Specific Ion Effect:

The presence of excess sodium in sodic soils may induce deficiencies of other cation like calcium (Ca²⁺) and magnesium (Mg²⁺).

The action of sodium in inducing deficiencies of Ca²⁺ and Mg²⁺ appears to be three fold:

(i) Because sodium is comparatively loosely held in exchangeable form, the ions released to the soil solution in a fractional exchange are mostly sodium ions if the soil has a high exchangeable sodium percentage,

(ii) At the high pH values (sodic soil) usually associated with excess exchangeable sodium in the absence of excess salts, the soil solution contains bicarbonate and carbonate ions that tend to form insoluble precipitates of calcium and magnesium carbonates as follows:

and

(iii) Exclusion of calcium and magnesium from absorption on competitive basis.

(vi) Availability of Plant nutrients. The high pH in alkali or sodic soils decreases the availability of many plant nutrients like P, Ca, N, Mg, Fe, Cu, and Zn.

Relationship between Different Salt Affected Soils and Growth of Different Plants:

A graphic representation showing the relationship among SAR, ESP, different salt affected soils is given in Fig. 15.1