The salinity of soil refers to the amount of salts in the soil and it can be estimated by measuring the electrical conductivity (EC) of an extracted soil solution.
How does soil salinity affect plant growth?
Salinity can affect plant growth in several ways, directly and indirectly:
Direct soil salinity damages
- Decreased water uptake
High salts concentration results in high osmotic potential of the soil solution, so the plant has to use more energy to absorb water. Under extreme salinity conditions, plants may be unable to absorb water and will wilt, even when the surrounding soil is saturated.
- Ion-specific toxicity
When a plant absorbs water containing ions of harmful salts (e.g. Sodium, Chloride, excess of Boron etc.), visual symptoms might appear, such as stunted plant growth, small leaves, marginal necrosis of leaves or fruit distortions.
Indirect soil salinity damages
- Interference with uptake of essential nutrients
An imbalance in the salts content may result in a competition between elements. This condition is called "antagonism", i.e. an excess of one ion limits the uptake of another ion. For example, excess of chloride reduces the uptake of nitrate, excess of phosphorus reduces the uptake of manganese, and excess of potassium limits the uptake of calcium.
- Sodium effect on soil structure
In saline soils, sodium replaces calcium and magnesium, which are adsorbed to the surface of clay particles in the soil. Thus, aggregation of soil particles is reduced, and the soil will tend to disperse. When wet, a sodic soil tends to seal, its permeability is dramatically reduced, and thus water infiltration capacity is reduced as well. When dry, a sodic soil becomes hard has the tendency to crack. This may result in damages to roots.
It should be noted that salinity by itself actually improves soil structure and eliminates to some degree the negative effect of sodium ions, but of course, salinity cannot be increased without affecting plants growth.
Factors affecting soil salinity
Several factors affect the amount and composition of salts in soils:
Irrigation water quality - The total amount of dissolved salts in the irrigation water, and their composition, influence the soil salinity. Therefore, various parameters, such as source water EC and its minerals content should be tested.
Fertilizers applied - The type and amount of fertilizers applied to soil, affect its salinity. Some fertilizers contain high levels of potentially harmful salts, such as potassium chloride or ammonium sulphate. Overuse and misuse of fertilizers leads to salinity buildup, and should be avoided.
Irrigation regimen and type of irrigation system - The higher the water quantity applied, the closer soil salinity is to irrigation water salts concentration. When the soil dries, the concentration of salts in the soil solution is increased.
Since salts move with the wetting front, the salts accumulate in specific profiles according to the irrigation regimen and the type of irrigation used. For instance, when irrigating using sprinklers, water and salts move deeper, according to the soil's infiltration capacity and the water quantity, until they stop at a certain depth. When using drip irrigation - there is also a lateral movement of water and salts.
Field's characteristics and agricultural history - A poorly drained soil might reach salinity level that is harmful to the plants and to the whole crop. A soil that was not flushed after a previous growing cycle might contain high level of accumulated salts.
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