Sodium Adsorption Ratio (SAR)
Also called: SAR
Sodium Adsorption Ratio (SAR) quantifies how much sodium dominates a water or soil solution relative to calcium and magnesium, calculated as Na⁺ / √((Ca²⁺ + Mg²⁺) / 2) with all ions in meq/L. SAR values above 9 typically indicate sodicity hazards: excess sodium displaces calcium on clay sites, breaks down soil structure, reduces infiltration, and forms surface crusts. Combined with EC, SAR predicts whether saline irrigation water will also be sodic.
How Sodium Adsorption Ratio (SAR) Works
SAR matters because the *type* of salt is as important as the *amount*. A high-EC water dominated by calcium and magnesium (gypsiferous well water) actually improves soil structure when applied; the same EC dominated by sodium (sea-water intrusion, blackstrap brine) destroys structure within seasons. The combined EC × SAR risk matrix (Ayers & Westcot 1985 Figure 1): low EC + high SAR is the most dangerous combination because rain dilutes the salts but leaves sodium attached to clay, accelerating dispersion.
Field symptoms of high-SAR water: surface sealing after rain, ponding that doesn't infiltrate, white sodium efflorescence in evaporation rings, and slow seedling emergence as crusts harden. Soil tests confirm: ESP (Exchangeable Sodium Percentage) above 15% defines sodic soil; pH typically rises above 8.5 because sodium hydrolyses water to form sodium hydroxide. Reclamation requires gypsum (calcium sulfate) applied at rates calculated from soil-test ESP plus leaching with low-SAR water.
Management strategies: (1) prefer Ca/Mg-dominated water sources when blending well supplies; (2) apply gypsum 1–5 t/ha annually as preventive maintenance on borderline soils; (3) avoid surface flood irrigation that maximizes evaporation and SAR concentration — switch to drip or buried emitters; (4) plant deep-rooted halophytes (date palm, atriplex, distichlis) on heavily sodic plots while reclamation proceeds. WiseYield logs SAR alongside EC in the lab-analyses tab so users see both salinity *and* sodicity risk on the same dashboard.
Sources
- Ayers, R.S. & Westcot, D.W. (1985). Water quality for agriculture. FAO Irrigation and Drainage Paper No. 29 Rev. 1.
- U.S. Salinity Laboratory Staff (1954). Diagnosis and improvement of saline and alkali soils. USDA Agriculture Handbook 60 (Richards 1954).
- Suarez, D.L. & Lebron, I. (1993). Water quality criteria for irrigation with highly saline water.