Irrigation Water Quality Assessment: SAR, EC and Soil Permeability Impacts

Why SAR and EC matter together

Soil permeability — the ability of water to move through the soil profile and deliver nutrients to roots — is controlled by the balance of sodium versus calcium and magnesium in the irrigation water. Sodium disperses soil colloids, causing clay swelling and the collapse of soil aggregates that create pore structure. This process, known as sodicity, progressively reduces infiltration rate and eventually creates an impermeable sodic layer. The Sodium Adsorption Ratio (SAR = Na⁺ / √((Ca²⁺ + Mg²⁺)/2), in meq/L) quantifies the relative sodium load.

The SAR–EC interaction

Critically, the risk posed by a given SAR value depends on the EC of the irrigation water. High-EC water carries sufficient soluble calcium to compete with sodium at the soil cation exchange sites, partially mitigating the dispersive effect. Low-EC water (< 0.2 dS/m) can cause soil permeability problems even at modest SAR values (4–8) because there is insufficient calcium to protect aggregate structure. The FAO/Ayers-Westcot framework (1985) addresses this by providing SAR thresholds as a function of EC: a SAR of 6 poses no problem at EC 1.2 dS/m but would cause permeability reduction at EC 0.3 dS/m.

Crop-specific EC tolerance

Crop yield begins to decline when soil EC (ECe, measured in the soil paste extract) exceeds a threshold value specific to the crop. The FAO classification identifies five tolerance categories: sensitive crops (strawberry, ECe threshold ~1.0 dS/m), moderately sensitive (maize, ~1.7 dS/m), moderately tolerant (tomato, ~2.5 dS/m), tolerant (cotton, ~7.7 dS/m) and highly tolerant (date palm, ~4.0 dS/m). The ECe threshold for the irrigation water (ECiw) is roughly half the ECe threshold for the same crop, because soil salinity typically concentrates to about double the irrigation water salinity at the mid-root zone.