What counts as a low or high SAR?
The sodium adsorption ratio (SAR) compares how much sodium is dissolved in a water sample against how much calcium and magnesium are dissolved alongside it. Sodium tends to disperse soil clay particles; calcium and magnesium tend to bind them together. A water with a lot of sodium and very little calcium or magnesium can gradually push soil toward a sodic condition, where clay disperses, pores seal over, and water stops soaking in even though the soil looks the same on the surface.
The classification below comes from the US Salinity Laboratory Staff, the group that introduced the SAR formula in 1954. It sorts water into four sodium hazard classes based on SAR alone.
| Class | SAR range | What it means |
|---|---|---|
| S1 – Low | 0–10 | Usable on almost all soils with little risk of a sodium problem. |
| S2 – Medium | 10–18 | An appreciable risk on fine-textured (clay) soils, especially with limited leaching; less of a concern on coarse, sandy soils. |
| S3 – High | 18–26 | Likely to cause harmful sodium levels in most soils; usually needs gypsum or another calcium source plus good drainage. |
| S4 – Very high | above 26 | Generally unsatisfactory for irrigation except on soils that are already low in salinity. |
SAR alone isn't the whole picture. FAO Irrigation and Drainage Paper 29 (Ayers and Westcot, 1985) evaluates the risk of an infiltration problem from SAR together with the water's salinity (EC), because low-salinity water causes more clay swelling and dispersion than high-salinity water at the same SAR. Treat these classes as a starting screen, not the final word on a specific field.
If your water sample is unusually high in bicarbonate or has a high pH, your lab may also report an adjusted SAR, which corrects the calcium value for the tendency of calcium carbonate to precipitate out of solution. This calculator computes standard SAR only; see the irrigation water quality calculator for a broader look that also weighs salinity.
How this SAR calculator works
The calculator converts whichever concentrations you enter into milliequivalents per liter (meq/L), then applies the standard SAR formula to those converted values.
1. Convert mg/L to meq/L (skipped if you already have meq/L)
Milliequivalents per liter
meq/L = mg/L ÷ equivalent weight
Equivalent weight is each ion's atomic weight divided by its electrical charge: sodium 23.0, calcium 20.0, magnesium 12.15. These are the standard conversion values used on irrigation water test reports.
2. Calculate SAR
Sodium adsorption ratio
SAR = Na ÷ √((Ca + Mg) ÷ 2)
Na, Ca and Mg are all in meq/L. This is the formula introduced by the US Salinity Laboratory Staff (1954) and used throughout irrigation and soil science.
SAR is a dimensionless ratio, so the result is the same number whether you started from mg/L or entered meq/L directly — the conversion step only exists to get every ion onto the same, charge-based footing before the ratio is taken.
Rounding
SAR is shown to 2 decimal places, matching the precision carried through the calculation. The meq/L conversions in the result list are also shown to 2 decimal places, using the exact value you entered rather than a rounded version of it.
When this calculator isn't the right tool
It calculates standard SAR from Na, Ca and Mg only. Use a different tool for:
- A full irrigation water quality assessment, which also weighs salinity (EC) and other measures. See the irrigation water quality calculator.
- Working out how much water it takes to control salt buildup once you know your water's salinity. See the leaching requirement calculator.
- Checking how fast water moves into your specific soil, which sodium affects but doesn't fully determine on its own. See the soil infiltration rate calculator.
SAR calculation example
A well water test reports 200 mg/L sodium, 80 mg/L calcium and 24 mg/L magnesium.
- Convert to meq/L: Na = 200 ÷ 23.0 = 8.70 meq/L. Ca = 80 ÷ 20.0 = 4.00 meq/L. Mg = 24 ÷ 12.15 = 1.98 meq/L.
- Average of Ca and Mg: (4.00 + 1.98) ÷ 2 = 2.99.
- Square root: √2.99 = 1.73.
- SAR: 8.70 ÷ 1.73 = about 5.03.
- Class: 5.03 falls in the S1 – Low range (0–10).
The same calculation starting from meq/L
A soil-paste extract report already gives ions in meq/L: Na = 10, Ca = 4, Mg = 2.
- Average of Ca and Mg: (4 + 2) ÷ 2 = 3.
- Square root: √3 = 1.73.
- SAR: 10 ÷ 1.73 = about 5.77.
- Class: 5.77 also falls in the S1 – Low range (0–10).
Both examples land in the same hazard class, but notice how much the underlying numbers differ — there's no shortcut for converting units correctly, which is exactly what this calculator automates.
Measuring and using your SAR result
Sodium, calcium and magnesium come from a lab water analysis, not from a home test kit. If you're irrigating from a well, ask the lab specifically for these three ions along with electrical conductivity (EC); a basic potability test often skips them. If you're assessing soil rather than water, a soil test lab measures the same three ions from a saturated-paste extract and reports SAR the same way, so you can enter those values here too.
Common mistakes
- Mixing mg/L and meq/L. Plugging mg/L values straight into the SAR formula without converting first produces a number with no real meaning. Always check which unit your lab report uses before choosing the toggle above.
- Confusing SAR with percent sodium. Some reports show "% Na," a different ratio of sodium to total cations. It isn't interchangeable with SAR, even though both describe sodium hazard.
- Treating SAR as the whole answer. A high SAR with high salinity behaves very differently in soil than a high SAR with low salinity, so pair this result with an EC reading before drawing conclusions.
If your SAR comes back high
Gypsum (calcium sulfate) is the standard amendment: it adds calcium, which displaces sodium from clay surfaces so the sodium can be leached out with irrigation or rainfall. How much gypsum a field needs depends on its cation exchange capacity, the depth being treated, and how far you need to bring the SAR down — not on the SAR value alone. Utah State University Extension's Salinity and SAR Management Calculator estimates a gypsum requirement from those soil-specific inputs, and a local extension office or soil test lab can advise on rates for your field.
What changes a water's SAR
SAR isn't a fixed property of a well or canal — it can shift with the season, with the source, and with what's been added to the field.
| Factor | Effect on SAR | What to check |
|---|---|---|
| Evaporation and concentration | Water left standing or applied by flood or furrow can lose water to evaporation, concentrating dissolved minerals and often raising SAR. | Retest later in a dry season rather than relying on a single early sample. |
| Calcium carbonate precipitation | In water high in bicarbonate, calcium can precipitate out as the water evaporates or warms, lowering the calcium available and raising the effective sodium hazard. | Ask your lab whether an adjusted SAR applies to your bicarbonate and pH. |
| Blending water sources | Mixing a high-SAR source with a lower-SAR source lowers the blended SAR, though not in a simple straight-line way since sodium, calcium and magnesium each blend on their own concentration, not on SAR directly. | Recalculate SAR from the blended Na, Ca and Mg, not by averaging two SAR values. |
| Gypsum or other calcium amendments | Adding a soluble calcium source raises calcium in the soil-water and lowers SAR. | Retest after treatment to confirm the change. |
| Greywater and some treated wastewater | These sources are often naturally higher in sodium than fresh well or surface water. | Check greywater guidance in the greywater irrigation calculator before using it long-term on sodium-sensitive soils. |
| Soil clay type | Doesn't change the water's SAR, but changes how much damage a given SAR can cause — swelling clays like montmorillonite are more sensitive than kaolinite. | Check your soil texture alongside your SAR result. |
Because SAR can drift with the season and the source, a single test is a snapshot, not a guarantee. Retesting periodically, especially after a dry spell or a change in water source, catches shifts before they show up as a soil infiltration problem.
Frequently asked questions
What is a good SAR for irrigation water?
The US Salinity Laboratory Staff (1954) classification puts SAR 0-10 in the low sodium hazard class (S1), which can be used on almost any soil. SAR 10-18 is medium (S2), 18-26 is high (S3), and above 26 is very high (S4), where sodium is likely to damage soil structure on most soils. These classes describe sodium hazard alone; the actual risk to a given field also depends on the water's salinity and the soil's clay content.
How do I convert mg/L to meq/L for SAR?
Divide the concentration in mg/L by the ion's equivalent weight: sodium by 23.0, calcium by 20.0, and magnesium by 12.15. These equivalent weights come from each ion's atomic weight divided by its electrical charge. This calculator does the division for you when you choose mg/L as your input unit.
What is the difference between SAR and adjusted SAR?
Standard SAR uses the calcium and magnesium concentrations as measured. Adjusted SAR corrects the calcium value for the tendency of calcium carbonate to precipitate out of water with high bicarbonate, which would otherwise understate the sodium hazard. Colorado State University Extension notes that labs typically calculate an adjusted SAR when bicarbonate is above 200 mg/L or pH is above 8.5. This calculator only computes standard SAR from measured Na, Ca and Mg.
Can I use this calculator for a soil test instead of a water test?
Yes. The same SAR formula is used for water and for the saturated-paste extract from a soil sample, which is how soil test labs report soil SAR. Enter the sodium, calcium and magnesium values from either type of report, in whichever unit the lab used.
Does a high SAR always mean I have a soil problem?
Not by itself. FAO Irrigation and Drainage Paper 29 evaluates infiltration risk from SAR together with the water's salinity (EC), because low-salinity water causes more clay swelling and dispersion than high-salinity water at the same SAR. A high SAR is a warning sign, but a full assessment needs the salinity value too, plus soil clay type and organic matter.
How much gypsum do I need to lower a high SAR?
It depends on your soil's cation exchange capacity, the depth you're treating, and how far you need to bring the SAR down, so there's no fixed rate this calculator can give you. Utah State University Extension's Salinity and SAR Management Calculator estimates a gypsum requirement from your soil's CEC, bulk density and target SAR. A local soil test lab or extension office can also recommend a rate for your specific soil.
Can SAR change over the irrigation season?
Yes. As water evaporates from soil or a reservoir, dissolved minerals become more concentrated, and calcium can precipitate out as calcium carbonate faster than sodium does. Both effects can raise the effective SAR compared with a single water sample, so a well or canal that tests borderline early in the season can test higher after a long, hot, dry spell.
What should I ask my water lab to test for SAR?
Ask for sodium, calcium and magnesium, reported in mg/L or meq/L. If you're concerned about a high pH or an alkaline water source, also ask for bicarbonate, carbonate and pH so a lab can calculate an adjusted SAR if it's needed. Electrical conductivity (EC) is worth requesting too, since it's needed alongside SAR to judge infiltration risk.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- The SAR formula, the mg/L-to-meq/L conversion and both worked examples were verified by hand.
- The sodium hazard classification was checked against the US Salinity Laboratory Staff (1954) source below.
- The infiltration-risk caution follows FAO Irrigation and Drainage Paper 29 (Ayers and Westcot, 1985).
Sources
- US Salinity Laboratory Staff (L.A. Richards, ed.), 1954. Diagnosis and Improvement of Saline and Alkali Soils. USDA Agriculture Handbook No. 60. [VERIFY URL]
- Ayers, R.S. and Westcot, D.W., 1985. Water Quality for Agriculture. FAO Irrigation and Drainage Paper 29 (Rev. 1).
- Bauder, T.A., Waskom, R.M. and Davis, J.G. Colorado State University Extension. Irrigation Water Quality Criteria (Crop Series, Irrigation, no. 0.506).
- Utah State University Extension. Salinity and SAR Management Calculator.