Adverse Effects of Na in the Soil-Water Environment

Sodium adsorbs more water molecules per unit (mole) of charge than most other metal ions (K+, Mg2+, Ca2+) commonly found in the soil-water environment. Hence, when brine (NaCl) is discharged in the soil-water environment, clay and organic particles tend to adsorb fully hydrated Na ions. This causes the particles to become waterborne, a process also known as dispersion. Under dispersion, soils become impermeable to water lakes, streams, and rivers experience large increases in suspended solids (clays and organics). Most soil clays undergo dispersion at an ESP of around 15. At this ESP level, soils appear to be toxic because they lose the potential to function as porous media (water infiltration and gas exchange are restricted). 

The phenomenon of soil dispersion with respect to Na+ loads (magnitude of ESP or SAR) appears to be unique to all soils on at least one particular point. As the total salt or Cl- concentration in the water increases, the dispersion index decreases and the saturated hydraulic conductivity increases (Fig. 11.6). When this occurs, the soil-water system becomes toxic to plants and organisms owing to high osmotic pressures. When chloride concentration in solution increases beyond 6000 mg L 1, Na ions near clay surfaces begin to dehydrate because of high osmotic pressure in the surrounding solution. This causes clay particles to flocculate (flocculation is the reverse of dispersion) and, consequently, the saturated hydraulic conductivity of the soil increases. 

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