Salinization

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General aspects on salt-affected soils
Primary salinization occurs naturally where the soil parent material is rich in soluble salts, or in the presence of a shallow saline groundwater table. In arid and semiarid regions, where rainfall is insufficient to leach soluble salts from the soil, or where drainage is restricted, soils with high concentrations of salts (“salt-affected soils”) may be formed. Several geochemical processes can also result in salt-affected soil formation. When an excess of sodium is involved in the salinization process this is referred to as sodification. Secondary salinization occurs when significant amounts of water are provided by irrigation, with no adequate provision of drainage for the leaching and removal of salts, resulting in the soils becoming salty and unproductive. Salt-affected soils reduce both the ability of crops to take up water and the availability of micronutrients. They also concentrate ions toxic to plants and may degrade the soil structure.

Types of salt-affected soils
Salt-affected soils may contain an excess of water soluble salts (saline soils), excess exchangeable sodium (sodic soils) or both an excess of salts and exchangeable sodium (saline-sodic soils). Soil classification is based on electrical conductivity (EC), pH and sodium adsorption rate (SAR) and is given in table 1.

Salt-affected Soil Classification EC
(dS/m)
Soil pH Sodium Adsorption Ratio2 Soil Physical Condition
Saline >4 < 8.5 < 13 Normal
Saline-sodic >4 < 8.5 > 13 Normal
Sodic <4 >8.5 > 13 Poor


Brady, N.C., 2002, The Nature and Properties of Soils, New Jersey, USA, Prentice Hall.

How to recognise salt-affected soils in cultivated areas

  • delayed/reduced germination
  • stunted growth
  • foliar damage
  • salt soil spots
  • salt crusts
  • waterlogging
  • soil monitoring through laboratory analysis

Typical examples of salty soils caused by farmers

  1. Improper irrigation schemes management
    - Insufficient water application
    - Insufficient drainage
    - Irrigation at low efficiency
    - Irrigation with saline or marginal quality water
  2. Poor land levelling
  3. Dry season fallow practices in the presence of a shallow water table
  4. Misuse of heavy machinery leading to soil compaction and poor drainage
  5. Excessive leaching during reclamation techniques on land with insufficient drainage
  6. Use of improper cropping patterns and rotations
  7. Chemical contamination. e.g. as a result of intensive farming, where large amounts of mineral fertilizers have been applied over a long period of time.

Basic measures for preventing salinization and sodification of cultivated soils

  1. Soil
    - Maintenance of satisfactory fertility levels, ph and structure of soils to encourage growth of high yielding crops
    - Maximization of soil surface cover using multiple crop species
    - Mulching exposed ground to help retain soil moisture and reduce erosion
    - Suitable crop selection
    - Using crop rotation, minimum tillage, minimum fallow periods
    - Continuous monitoring of the land by observation and by salinity checks
  2. Water
    - Soil moisture monitoring, accurate determination of water requirements and efficient irrigation of crops
    - Choice of appropriate drainage system
    - Continuous monitoring of water by salinity checks

Reclamation of salt-affected soils
Management of salt-affected soils requires a combination of agronomic practices. For instance, reclamation of saline soils may begin with the provision of effective drainage and good quality irrigation water to lower the levels of soluble salts. Some saline-sodic and sodic soils can be reclaimed by adding soil amendments rich in calcium, followed by leaching out of the sodium. Where salinity is increasing as a problem on an irrigated farm, it may be necessary to select crop varieties that have a greater tolerance to salts. Moreover, where the land is severely salt-affected, it may be more economical to take it out of production and address the negative environmental impacts.

How the salinity affects the plants
Water is taken up by the fine roots of plants through the process of osmosis, which involve the movement of water from regions of low salt1 concentration (such as the soil) to regions of high salt concentration (such as the inside of root cells). When salt concentrations in the soil are high, the movement of water from the soil to the root is slowed down. When the salt concentrations in the soil are higher than inside the root cells, the soil will draw water from the root, and the plant will wilt and die. This is the basic way in which salinization affects plant production.
The damaging effects of salt on plants are caused not only by osmotic forces, but also by toxic levels of sodium and chloride. Fruit crops and woody ornamentals are especially sensitive to high levels of these elements. Also, the high pH value (a measure of the acid/alkaline balance) caused by excess sodium may result in micronutrient deficiencies.
Plants vary in their sensitivity to salt. Some typical examples of plant sensitivity in salts is given in table 2.
Also, based on the soil EC, pH and SAR the most suitable crop in terms of salt sensitivity could be chosen using the Figure 1.

Sensitive
(0-4 dS/m)
Moderately Tolerant
(4-6 dS/m)
Tolerant
(6-8 dS/m)
Highly Tolerant
(8-12 dS/m)
Almond Corn Fig Barley
Bean Grain Sorghum Oats Cotton
Clover Lettuce Pomegranate Olive
Onion Soybean Sunflower Rye
Potato Tomato Wheat Wheatgrass


Brady, N.C., 2002, The Nature and Properties of Soils, New Jersey, USA, Prentice Hall