Irrigation Water SAR & Sodium Hazard, Graded
Grades sodium hazard
Enter sodium, calcium and magnesium in meq/L to get the Sodium Adsorption Ratio, the sodium hazard class S1–S4, the salinity class and management advice.
Enter your water test
Next: follow the advisory — apply gypsum, ensure drainage, blend with better-quality water, and pick salt/sodium-tolerant crops on high-SAR water.
SAR = Na / √((Ca + Mg) / 2) in meq/L; classes per the US Salinity Laboratory — combine with EC (the C1–C4 class) for full suitability.
Irrigation water SAR — key facts
- SAR
- Na ÷ √((Ca+Mg)/2)
- Units
- all ions in meq/L
- S1
- < 10 — low
- S2 / S3
- 10–18 / 18–26
- S4
- > 26 — very high
- Salinity
- EC class C1–C4
- Manage with
- gypsum, drainage, blending
- Privacy
- Runs in your browser; nothing uploaded
Good water can quietly wreck your soil
Irrigation water that looks fine can carry too much sodium relative to calcium and magnesium, and over seasons that sodium disperses clay, seals the surface and chokes infiltration — turning productive ground into hard, sodic soil. The Sodium Adsorption Ratio captures the risk in one number: SAR = Na ÷ √((Ca+Mg)/2). Read alongside the salinity class from EC, it tells you whether your source is safe or needs managing before it does lasting damage.
This tool computes the SAR, the sodium hazard class S1 to S4, the salinity class and a management advisory from your water analysis. Use it to vet a borewell or canal source, compare waters, and decide on gypsum, drainage, blending or tolerant crops. Pair it with the Soil Salinity (EC) and Gypsum Requirement tools to turn the reading into a concrete plan.
Vet your water
Know the sodium hazard before you irrigate.
Read both hazards
Sodium class and salinity class together.
Catch it early
Spot sodic risk before soil structure fails.
Plan the fix
Gypsum, drainage, blending or tolerant crops.
Frequently Asked Questions
What is SAR (Sodium Adsorption Ratio)?+
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.
How is SAR calculated?+
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.
Why is high sodium bad for soil?+
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.
What are the sodium hazard classes S1–S4?+
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.
What is the salinity (EC) class?+
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.
How do I manage high-SAR water?+
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.
Where do I get Na, Ca and Mg values?+
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).
Does adding gypsum lower SAR?+
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.
Are the classes exact thresholds?+
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.
Does it work for any water source?+
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.