In this article we will discuss about:- 1. Meaning of Salt Affected Soils 2. Characteristics of Salt Affected Soils 3. Classification.
Meaning of Salt Affected Soils:
Soils of arid and semi-arid regions contain excessive concentration of either soluble salts or exchangeable sodium or both due to inadequate leaching of base forming cations. The pH is usually 7 or above with exchangeable complex dominated by calcium and magnesium ions. Irrigated soils in low rainfall regions also develop soil salinity and alkalinity due to accumulation of salts in surface layers through evaporating water.
The accumulated salts are primarily chlorides and sulfates of calcium, magnesium, sodium and potassium. Weathering of rocks and minerals, rainfall, ground water and irrigation are the sources of these salts.
Salinity is usually expressed in terms of electrical conductivity (EC) which is measured in decisiemens per meter (dS m-1). Formerly it used to be expressed in mmho cm-1. Since 1 S is 1 mho, 1 dS m-1 is mmho cm-1.
Characteristics of Salt Affected Soils:
Sodium status is usually characterised in two ways:
(i) Exchangeable sodium percentage and
(ii) Sodium adsorption ratio.
i. Exchangeable sodium percentage (ESP) indicates the degree of exchangeable complex saturated with sodium:
ii. Sodium adsorption ratio (SAR) indicates comparative concentrations of Na+, Ca2+ and Mg2+ in soil solution:
where, (Na+), (Ca2+) and (Mg2+) are concentrations (C mol kg-1) of sodium, calcium and magnesium ions in soil solution.
Classification of Salt Affected Soils:
Using the EC and ESP characteristics and soil pH, salt affected soils are classified as saline, saline-sodic and sodic (Table 3.3):
1. Saline Soils:
These soils contain neutral soluble salts sufficient to interfere seriously with plant growth. These are also called as white alkali or solonchalk. These soils have white crust of salts on the surface.
Sodium ions may not exceed 50 per cent of the cations in soil solution and hence are not adsorbed to any significant level. Pre-dominant anions usually present are sulphates, chlorides and sometimes nitrates. Bicarbonates may be present in small amounts, but soluble carbonates are absent.
2. Saline-Sodic Soils:
They contain appreciable quantities of neutral soluble salts and adequate sodium ions to affect most plants. These are similar to saline soils in the presence of excess soluble salts. The properties will be similar to sodic soils, if soluble salts are leached down. After leaching, the soil become strongly alkaline and dispersed.
Tillage and water percolation are the major problems. Hence, management of these soil is very difficult. Excess salts as well as exchangeable sodium should be removed from root zone for favourable physical condition of the soil.
3. Sodic Soils:
Leaching of saline-sodic soils in the absence of gypsum in soils or irrigation water leads to formation of sodic soils in arid and semi-arid regions. These soils do not contain appreciable amount of soluble salts. Toxicity of Na+, HCO3– and OH– besides reduced water infiltration and aeration affects the plant growth.
Exchangeable sodium influence the physical and chemical properties of sodic soils. Sodium saturated clay deflocculates (dispersed) leading to unsatisfactory physical condition. Surface of these soils is usually black due to deposition of dispersed humus and hence the name black alkali.
Sodic Soils and Plant Growth:
Saline and saline-sodic soils affect plant growth due to their high soluble salt concentration.
Sodic soils dominated by active sodium exert detrimental effect on plants in four ways:
(1) High concentration of sodium carbonate and bicarbonate leads to cell collapse due to passage of water from cell into concentrated soil solution.
(2) Toxicity of bicarbonate and other anions.
(3) Low micronutrient availability due to high pH of the soil.
(4) Oxygen deficiency in root zone due to breakdown of soil structure (deflocculation).
The limits of tolerance of plants to salts in soil may vary depending on rainfall, physical make-up of the plant and climate. Relative tolerance of crops (Table 3.4) to salinity (saline soils) and sodicity (sodic soils), however, may give some general idea.
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