Salt affected soils

Salt affected soils are designated as problematic soils. Salt affected soils are unproductive unless excess salts are reduced or removed. These soils are most extensively found in arid climates, but these soils are also found in coastal areas where soils are inundated by ocean or sea water. Such salt affected soils are categorized into various groups as hereunder.

2.1 Saline soil

Saline soils are defined as soils having electrical conductivity of the saturation extract greater than 4 dSnr1 (0.4 Snr1 or 4 mmlios cm"1) and an exchangeable sodium percentage (ESP) less than 15. The pH is usually less than 8.5. Formerly, these soils were called ‘white alkali soils’ because of surface cmst of white salts. Saline soil is also called as ‘Solonchak’ (Russian term). The process by which saline soils are 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 rocks and minerals generally are percolated downward into the ground water and are transported ultimately by stream 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 aiid 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 further tend 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 high ground water table or low permeability of the soil which causes poor drainage by impeding the downward movement of water.

2.2 Soclic (Alkali) soil

Sodic soil is defined as a soil having electrical conductivity of the saturation extract less than 4 dSnr1 (0.4 Snr1 or 4 minhos cm'1) and an exchangeable sodium percentage (ESP) greater than 15. The pH is usually between 8.5-10.0 (Table 2). Formerly, these soils were called ‘black alkali soils’ and the soils so formed is called ‘solodsoloth or solonetz’ (Russian term). 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 dominant cations found in the soil solutions and on the exchange complex of normal soil 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 absoiption by plants, the solubility limits of calcium phosphate, calcium carbonate and magnesium carbonate 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 and magnesium is replaced by sodium resulting alkali or sodic soils.

2.3 Saline-sodic soil

Saline-sodic soil is defined as a soil having electrical conductivity of the saturation extract greater than 4 dSnr1 (0.4 Snr1 or 4 mmhos cm"1) and an exchangeable sodium percentage (ESP) greater than 15 (Table 2). The pH is variable and usually above 8.5 depending on the relative amount of exchangeable sodium and soluble salts. When soils are dominated by exchangeable sodium, the pH will be more than 8.5 and when soils are dominated by soluble salts, the pH will be less than 8.5. These soils form as a result of 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 exchangeable sodium hydrolyzes and forms sodium hydroxide (NaOH). This may change to sodium carbonate upon reaction with carbon dioxide absorbed from the atmosphere. On extensive leaching, the soil may become strongly alkaline, the particles disperse and the soils become unfavourable 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 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.

Table 2. Basic chemical properties of different salt affected soils.

Soil

EC (dSm'1)

pH

ESP

SAR

Saline soil

>4.0

<8.5

< 15.0

< 13.0

Sodic soil

<4.0

>8.5

> 15.0

> 13.0

Saline-sodic soil

>4.0

<8.5

> 15.0

> 13.0

 
Source
< Prev   CONTENTS   Source   Next >