Sodium Adsorption Ratio (SAR)

What SAR is

SAR describes the relative proportion of sodium (Na⁺) to calcium (Ca²⁺) and magnesium (Mg²⁺) in water or soil solution.

It is calculated as:

SAR = Na⁺ / √((Ca²⁺ + Mg²⁺) / 2)

(All ions expressed in milliequivalents per litre, meq/L.)

Why SAR matters

SAR predicts the structural effect of sodium, not just salinity.

High SAR water: - Promotes sodium dominance on exchange sites - Displaces calcium and magnesium - Causes soil dispersion - Reduces infiltration, drainage, and aeration

SAR vs EC

• EC describes total salinity (osmotic stress)
• SAR describes structural risk

A water source can have: - High EC but low SAR (salty but structurally safe) - Low EC but high SAR (structurally dangerous despite “low salt”)

Practical thresholds (guidance)

• SAR < 3: low risk
• SAR 3–9: increasing risk, depends on EC and soil type
• SAR > 9: high structural risk in soils

Substrates vs soils

• In substrates, SAR mainly affects cation balance and root uptake
• In soils, SAR directly damages physical structure

Management strategies

• Calcium supply (gypsum, Ca-rich fertilisers)
• Acidification (reduces bicarbonate, improves Ca solubility)
• Blending or treating irrigation water
• Monitoring Na:Ca ratios alongside SAR

Agronomic note

SAR is a diagnostic tool, not a fertiliser recommendation.