Effects of salinity and sodicity on physical and hydraulic properties

Accumulation of sodium in the soil solution and the exchange phase can cause clay dispersion, especially for smectitic clays, which can affect soil physical and hydraulic properties by

Reducing structural stability and promoting surface crust formation; increasing bulk density and mechanical resistance resulting in poor soil tilth and soil aeration;
Reducing hydraulic conductivity and infiltration rate, which can cause significant water management problems (see Salinity Management section); and Increasing runoff and erosion potential due to surface sealing and poor infiltration.

When ESP exceeds 15%, the higher level of Na⁺ on the exchangeable sites may cause the soil aggregates to collapse due to dispersion resulting in water penetration and soil compaction problems. While this phenomenon occurs naturally in many soils, the commencement of irrigation with reclaimed wastewater will most likely accelerate soil property deterioration as the reclaimed wastewater is proportionally higher in Na⁺ vs. Ca⁺⁺ + Mg⁺⁺.

In soils that contain very low to moderate amounts of exchangeable sodium (i.e., exchangeable sodium is <0.7 me/100 g) and have sufficiently low cation exchange capacities, the adverse effects on soil properties may be lessened considerably or altogether non-existent. One must recognize that soil sodicity is tied closely to the chemical properties of the irrigation water. Irrigation water which has a high ratio of Na⁺ to Ca⁺⁺ + Mg⁺⁺ and low salinity or through the precipitation of calcium with bicarbonate (HCO₃^-) to form calcium carbonate (CaCO₃) may induce high sodicity.

Likewise, a high SAR can cause soil infiltration problems. Generally, irrigation water with high SAR value (i.e. SAR > 9) can cause severe restrictions on permeability when applied to fine textured clay soils over a period of time. But the sodic (SAR) effect of water is often evaluated together with salinity. At the same SAR level, soil is more susceptible to dispersion in a low salinity water than in a high salinity water (see the following figure). In coarse textured (sandy) soils, restrictions on permeability would be less severe and the water with this magnitude of SAR may be tolerated.