Salt-Affected Soils

Salt-affected soils are classified based on the soluble salt concentration (ECe expressed as dS/m) in a saturated paste extract, pH, and exchangeable sodium percentage (ESP expressed as %) which is the percentage of total exchangeable cations that are sodium. An alternative to ESP is SAR, which is calculated in the same fashion as for irrigation water; however, since it is from the saturated soil-water paste extract, a different set of values is used to determine whether a soil is sodic and, if so, the the magnitude of the sodium hazrad. The three types of salt-affected soils are shown in the following table:

SOIL TYPE
ECe
pH
ESP
SAR
Saline
>4 dS/m
<8.5
<15%
<12
Sodic
<4 dS/m
>8,5
>15%
>12
Saline-Sodic
>4 dS/m
>8.5
>15%
>12

 

The salinity hazard associated with increasing salinity levels is shown in the following table:

ECe*

HAZARD
<1.5
low
1.6-3.9
moderate
4.0-5.0
high
>5.0
very high

*Soluble salts in ppp ÷ 640 = ECe (dS/m)

 

The sodium hazard associated with increasing ESP and SAR levels is shown in the following table:

ESP
SAR
HAZARD
<3%
<2.1
low
3-9%
2.1-7.0
moderate
9-15%
7.1-12.0
high
>15%
>12
very high

 

Salt-Related Problems

I. Water Deficits

A water-deficit phenomenon in plants, called physiological drought,can be induced by high salt concentrations in the soil solution that limit water absorption, or by high salt accumulations on or in plant leaves causing tissue damage. Within the soil, the osmotic potential (a component of soil-water potential) of the soil solution is inversely related to the salt concentration of the soil solution (ECe); for example, increasing the osmotic potential from 1 dS/m (low salinity) to 6 dS/m (high salinity) results in a decrease in the osmotic potential of the soil solution from -0.36 bar to -2.16 bar (using the formula: osmotic potential [expressed in bars] = -0.36 * ECe [expressed in dS/m]). If the matric potential in this soil were -2 bars, the soil-water potential (i.e., the sum of matric and osmotic potentials) would decrease from -2.36 to -4.16 bars, resulting in a major reduction in soil-water availability. High-salt-induced reductions in soil-water availability cause: increased wilting, desiccation, and respiration, along with reduced growth (as cell expansion requires adequate water), cell turgor, leaf size, carbon dioxide exchange (due to partial stomatal closure), photosynthesis, and transpirational cooling. And because of these effects, plants may be more susceptable to environmental (including wear) stresses.

II. Soil Permeability

Where an abundance of sodium occurs on the exchange sites of clay particles, soil structure deteriorates resulting in the following effects:

The process by which soil permeability declines begins with an increase in the sodium percentage on exchange sites. Sodium displaces divalent cations, such as calcium and magnesium. Because of the increased hydration level associated with monovalent cations, especially sodium, the thickness of the cationic layer between clay particles increases and, as a consequence, the attraction between these particles is reduced. With additional displacement, individual clay particles begin to disperse and soil sructure is destroyed. Even in very sandy soils, dispersed clay particles can migrate with water movement to plug pore channels and reduce water percolation.

III. Ion Toxicity

Many ions can have a direct toxic effect on root and shoot tissues when present at or above specific concentrations in the soil. For example, the accumulation of chlorine ions in leaf tissues can lead to desiccation at 0.3 to 0.5 % dry weight; however, removal of leaf tips by mowing can allow turfgrasses to tolerate chlorine better than most plants.

The accumulation of sodium ions in cell walls can lead to dehydration, reduced turgor, and cell death. Leaf burn typically is evident at the tips or margins, but metabolic activity can be reduced even when leaf injury is not visible. Cell membrane integrity can decline as sodium displaces calcium, causing leakage if cell contents; in affected roots, water and nutrient uptake are significantly affected. With reduced photosynthesis and increased respiration, carbohydrates may be limiting, resulting in reduced growth, vigor, and function. Other adverse effects include reduced protein synthesis and altered hormonal activity.

Boron toxicity occurs as leaf margin or tip chlorosis at levels of 100 to 400 ppm dry weight; however, turfgrasses may tolerate up to 1000 ppm, as leaf tips are removed with mowing.

Excessive bicarbonate ions can inhibit root growth, as well as cytokinin export to shoot tissues, where it promotes chloroplast development and protein synthesis.

And high hydroxide ion concentrations can solubilize certain organic compounds and inhibit root viability. Sodic soils, with pHs from 9.5 to 10.2, may have toxic hydroxide ion cencentrations.

IV. Ion Imbalances

High concentrations of sodium, chloride, and other ions can induce nutrient imbalances, and deficiencies of nitrate, phosphate, potassium, calcium, and magnesium can be induced by high salt concentrations.