Irrigation Water SAR & Sodium Hazard, Graded

SAR measures how much sodium dominates over calcium and magnesium in irrigation water. It's defined as SAR = Na ÷ √((Ca+Mg)/2), with all concentrations in milliequivalents per litre (meq/L). A high SAR warns that sodium will displace calcium on soil clays, which damages soil structure over time.

Take the sodium concentration and divide it by the square root of the average of calcium and magnesium: SAR = Na ÷ √((Ca+Mg)/2), all in meq/L. For example Na 8, Ca 3 and Mg 1 meq/L gives √((3+1)/2)=√2≈1.41, so SAR ≈ 8 ÷ 1.41 ≈ 5.7 — a low-sodium S1 water.

Sodium degrades soil structure. When sodium dominates, it disperses clay particles, breaking down the crumbs that give soil its open structure. The dispersed clay seals the surface and clogs pores, which cuts water infiltration, reduces aeration and makes the soil hard and crusty — a sodic soil that's tough to farm.

SAR is graded into four sodium hazard classes: S1 (SAR < 10) low, S2 (10–18) medium, S3 (18–26) high and S4 (> 26) very high. S1 water is safe for most soils and crops; S3 and S4 waters need active management — amendments, drainage and tolerant crops — to avoid building a sodic soil.

Alongside sodium hazard, water is graded for total salt by electrical conductivity into C1 (low) to C4 (very high). Combine the two — for instance C2-S1 or C3-S2 — to judge water overall: salinity affects how much salt builds in the root zone, while SAR affects soil structure. Both matter together.

Add calcium to displace sodium — gypsum is the standard amendment, applied to the soil or dosed into the water. Improve drainage so salts and sodium can leach below the roots, blend high-SAR water with a better source to dilute it, and choose salt- and sodium-tolerant crops. Together these keep a sodic soil from forming.

From a water analysis. A standard irrigation water test from a soil/water lab reports sodium, calcium and magnesium, usually in mg/L or meq/L along with EC and pH. SAR needs them in meq/L; if your report is in mg/L, convert each ion before entering it (Na ÷ 23, Ca ÷ 20, Mg ÷ 12.15).

Gypsum (calcium sulphate) supplies calcium that replaces sodium on the clay and improves structure, so it lowers the effective sodium hazard in the soil even though the water's own SAR is unchanged. It's the workhorse amendment for sodic water and soils; the Gypsum Requirement tool sizes the dose for your situation.

They're standard guideline bands, not hard cliffs. A water just over an S2/S3 boundary on a well-drained, calcium-rich soil may behave better than the class suggests, while a borderline water on a heavy, poorly drained clay can cause trouble sooner. Use the class as a strong signal and weigh it with your soil and drainage.

Yes — borewell, canal, pond or recycled water all work; just enter the Na, Ca and Mg from its analysis in meq/L. SAR and the hazard classes are universal water-quality measures, so the tool gives a consistent read on any irrigation source so you can compare them and plan management.