The FAO-29 water quality guideline table
Ayers and Westcot's Water Quality for Agriculture (FAO Irrigation and Drainage Paper 29, Rev. 1) groups common irrigation water problems into three degrees of restriction on use: none, slight to moderate and severe. The table below reproduces the guideline values this calculator applies. It's a general field guide, not a certification: a "severe" rating flags a factor worth managing, not a water source that can never be irrigated with.
| Problem | Units | None | Slight to moderate | Severe |
|---|---|---|---|---|
| Salinity (ECw) | dS/m | < 0.7 | 0.7 – 3.0 | > 3.0 |
| Salinity (TDS) | mg/L | < 450 | 450 – 2,000 | > 2,000 |
| Infiltration (SAR 0–3) | ECw, dS/m | > 0.7 | 0.7 – 0.2 | < 0.2 |
| Infiltration (SAR 3–6) | ECw, dS/m | > 1.2 | 1.2 – 0.3 | < 0.3 |
| Infiltration (SAR 6–12) | ECw, dS/m | > 1.9 | 1.9 – 0.5 | < 0.5 |
| Infiltration (SAR 12–20) | ECw, dS/m | > 2.9 | 2.9 – 1.3 | < 1.3 |
| Infiltration (SAR 20–40) | ECw, dS/m | > 5.0 | 5.0 – 2.9 | < 2.9 |
| Sodium, surface irrigation | SAR | < 3 | 3 – 9 | > 9 |
| Sodium, sprinkler irrigation | meq/L | < 3 | — | > 3 |
| Chloride, surface irrigation | meq/L | < 4 | 4 – 10 | > 10 |
| Chloride, sprinkler irrigation | meq/L | < 3 | — | > 3 |
| Boron | mg/L | < 0.7 | 0.7 – 3.0 | > 3.0 |
| Bicarbonate (HCO₃) | meq/L | < 1.5 | 1.5 – 8.5 | > 8.5 |
Two rows need explaining. The infiltration problem isn't set by SAR or ECw alone: at a given SAR, higher salinity actually helps water soak into the soil, so the table picks the SAR bucket your water falls in, then compares ECw against that bucket's thresholds. And for sodium under surface irrigation, the guideline uses SAR again rather than a sodium concentration, because it's specifically about sodium's effect on sensitive tree and woody crops, not a foliar toxicity.
These are general guidelines, not crop-specific limits. Boron-sensitive crops such as citrus and stone fruit can show injury well inside the "none" range. Check a crop salt-tolerance table for anything you're growing that's known to be sensitive.
How this calculator works
For each value you enter, the calculator finds which of the three columns above it falls into and shows you the rating along with the exact threshold. Nothing is estimated: every rating is a direct comparison against a published number.
1. Converting concentrations to meq/L
Sodium, chloride and bicarbonate are rated in milliequivalents per liter (meq/L) in the FAO-29 table, but many lab reports give milligrams per liter (mg/L). The calculator converts using each ion's equivalent weight:
mg/L to meq/L
meq/L = mg/L ÷ equivalent weight
Equivalent weights used: sodium (Na) 23.0, chloride (Cl) 35.45, bicarbonate (HCO₃) 61.0.
If you choose meq/L in the unit selector, the number you enter is used as-is with no conversion.
2. EC unit conversion
µS/cm to dS/m
dS/m = µS/cm ÷ 1,000
dS/m at 25°C is numerically identical to the older unit mmho/cm.
3. The infiltration check
When you enter both SAR and ECw, the calculator picks the SAR row from the table above (0–3, 3–6, 6–12, 12–20 or 20–40) and compares your ECw to that row's thresholds. A SAR above 40 falls outside the range Ayers and Westcot published, so the calculator flags it instead of extrapolating.
4. Rounding policy
Your own entered numbers are always shown back exactly as you typed them. Converted values (mg/L to meq/L, or µS/cm to dS/m) are shown to 2 decimal places, which is enough precision to sit clearly on one side of a threshold or the other.
When this calculator isn't the right tool
- You only have sodium, calcium and magnesium concentrations, not SAR. Use the sodium adsorption ratio calculator to work out SAR first.
- You need to know how much extra water to apply to flush salts out of the root zone. See the leaching requirement calculator.
- Your water will be used for drip or micro-irrigation. The FAO-29 sodium and chloride rows above are written for surface and sprinkler systems specifically; drip systems avoid direct foliar contact and generally tolerate higher levels, which this table doesn't quantify.
- You need a soil texture reading to plan leaching or drainage. See the soil texture calculator.
Worked examples
A grower irrigates with a sprinkler system. Their lab report shows ECw 1.2 dS/m, SAR 5, sodium 3.5 meq/L, chloride 2.0 meq/L, boron 0.5 mg/L and bicarbonate 2.0 meq/L.
- Salinity: ECw 1.2 dS/m sits between 0.7 and 3.0, so it's slight to moderate.
- Infiltration: SAR 5 falls in the 3–6 bucket, where none requires ECw > 1.2 and severe is ECw < 0.3. At exactly 1.2, that's slight to moderate.
- Sodium (sprinkler): 3.5 meq/L is above the 3 meq/L sprinkler threshold, so it's severe.
- Chloride (sprinkler): 2.0 meq/L is under 3, so it's none.
- Boron: 0.5 mg/L is under 0.7, so it's none.
- Bicarbonate: 2.0 meq/L sits between 1.5 and 8.5, so it's slight to moderate.
- Overall: the worst rating is severe, driven by sodium. That's the rating the calculator shows first.
A surface irrigation example
A different grower uses furrow irrigation. Their report gives TDS 1,600 mg/L, SAR 8 and chloride 12 meq/L, with no ECw reported.
- Salinity (from TDS): 1,600 mg/L sits between 450 and 2,000, so it's slight to moderate.
- Infiltration: can't be rated here, because the guideline's infiltration rows need ECw, not TDS. The calculator notes this rather than guessing.
- Sodium (surface, using SAR): SAR 8 sits between 3 and 9, so it's slight to moderate.
- Chloride (surface): 12 meq/L is above 10, so it's severe.
- Overall: severe, driven by chloride. Adding an ECw reading next time would also let the calculator check infiltration.
Reading your water test and avoiding common mistakes
A standard irrigation water test from an agricultural lab or cooperative extension soil-and-water lab typically reports EC or TDS, SAR (or the ions needed to calculate it), sodium, chloride, boron and bicarbonate, often alongside calcium, magnesium, sulfate, nitrate-nitrogen and pH. Contact a lab through your local extension office if you haven't had your water tested; most publish their own sample-collection instructions, which matter because a sample drawn from standing pipe water rather than flowing water can give misleading results.
Common mistakes
- Mixing up mg/L and meq/L. A sodium reading of 80 mg/L is only 3.5 meq/L; entering 80 directly into a meq/L field would wildly overstate the sodium hazard. Always check which unit your lab used.
- Ignoring irrigation method. Water rated "none" for surface-applied sodium can be "severe" for the same water through a sprinkler. Pick the method you'll actually use.
- Treating "severe" as a hard stop. A severe rating means plan for it (leaching, blending, tolerant crops, better drainage), not necessarily that the water is unusable.
- Skipping EC when you have SAR. Without ECw, the infiltration risk, often the most consequential rating for soil structure, can't be checked at all.
What to do about a restrictive rating
Management options depend on which factor is restrictive. High salinity is usually addressed with extra leaching water, covered in the leaching requirement calculator. A poor infiltration rating (high SAR relative to ECw) is often improved with gypsum or another calcium source added to the water or soil. High sodium or chloride under sprinkler irrigation may call for switching to surface or drip application, night watering to reduce leaf absorption, or blending with a lower-sodium source. In every case, a soil and water specialist familiar with your crop and soil is worth consulting before making a management change based on a single water test.
What changes how much a rating matters
The same FAO-29 rating can mean very different things depending on the crop, soil and how the water is used. These are the main reasons to treat a rating as a starting point rather than the final word.
| Factor | Effect | What to check |
|---|---|---|
| Crop salt tolerance | Sensitive crops (many fruit trees, berries, some vegetables) can show injury below the "severe" threshold; tolerant crops (many grasses, some grains) can handle more. | A crop-specific salinity or boron tolerance table. |
| Soil texture and drainage | Fine-textured or poorly drained soils concentrate salts near the surface and are more sensitive to a poor infiltration rating. | The soil texture calculator and soil infiltration rate calculator. |
| Leaching fraction | Extra water applied beyond crop needs flushes salts below the root zone and can offset a moderate salinity rating over time. | The leaching requirement calculator. |
| Irrigation method | Sprinklers add a foliar-contact hazard for sodium and chloride that surface and drip systems don't. | Match the method selector above to how the water is actually applied. |
| Climate | Hot, dry, windy conditions increase evapotranspiration, concentrating salts in the root zone faster between irrigations. | Water more frequently in peak heat rather than stretching intervals. |
| Water source variability | Wells and surface sources can shift in quality seasonally or with drought, especially near coasts or heavily pumped aquifers. | Retest periodically rather than relying on one old report. |
Frequently asked questions
What is the FAO-29 water quality guideline?
It's a table published by the FAO in Water Quality for Agriculture (Irrigation and Drainage Paper 29, Rev. 1, by Ayers and Westcot), also known as the Ayers and Westcot guidelines. It sorts common irrigation water problems, salinity, infiltration, specific ion toxicity and a few miscellaneous effects, into three restriction levels: none, slight to moderate, and severe. This calculator applies that table to the values you enter.
What do "none", "slight to moderate" and "severe" restriction mean?
They describe how likely that water quality factor is to cause a crop or soil problem under average field conditions, not a pass or fail grade for your water. None means the factor is unlikely to cause a problem. Slight to moderate means it can cause a problem for sensitive crops or soils, often manageable with adjustments such as extra leaching or a soil amendment. Severe means increasingly restrictive management is needed, and many crops or soils will struggle without it.
Why does the irrigation method change the sodium and chloride rating?
Sprinklers wet the leaves directly, so sodium and chloride can be absorbed through the foliage as well as the roots, which is why the FAO-29 table sets a stricter, single threshold for sprinkler-applied water. Surface irrigation (flood, furrow or basin) only exposes the roots, so its threshold is higher, and for sodium it's expressed as SAR rather than a concentration. Select the method you actually use so the calculator applies the right column.
My lab report gives sodium, chloride or bicarbonate in mg/L. How do I get meq/L?
Divide the concentration in mg/L by the ion's equivalent weight: 23.0 for sodium, 35.45 for chloride and 61.0 for bicarbonate. This calculator does that conversion for you: choose mg/L in the unit selector next to the field and enter the number exactly as your lab report shows it.
My SAR is above 40. What does that mean?
The FAO-29 infiltration table only covers SAR up to 40, so a higher value falls outside its published range and the calculator flags it instead of guessing a category. Water that sodic typically needs a site-specific evaluation, gypsum or other amendments, and advice from a soil and water specialist rather than a general guideline.
Does a "severe restriction" mean I can't use this water at all?
Not necessarily. Ayers and Westcot's guidelines describe general field conditions; many growers do use severely restricted water successfully with management such as leaching, blending with a better source, choosing tolerant crops, or improving drainage. A severe rating is a signal to plan for that management, not an automatic barrier to irrigation.
How is this different from the sodium adsorption ratio calculator?
The sodium adsorption ratio calculator works out SAR itself from your sodium, calcium and magnesium concentrations. This calculator assumes you already have a SAR value, EC, or another parameter from a lab report, and tells you what restriction level that value falls into under FAO-29. Use the SAR calculator first if your lab report only gives you the individual ion concentrations.
See the sodium adsorption ratio calculator.
Why isn't pH or nitrate included here?
This calculator focuses on the parameters most lab reports flag as irrigation-specific concerns: salinity, infiltration and specific-ion toxicity. Bicarbonate is included because it's part of the same FAO-29 table. pH and nitrate-nitrogen sit in a separate part of the same guidelines and are planned as their own tools.
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
- Every threshold shown was checked directly against the FAO-29 source table below.
- Both worked examples were traced by hand through the same logic the calculator uses.
- Unit conversions (mg/L to meq/L, µS/cm to dS/m) use standard equivalent weights, checked in both directions.
Sources
- Ayers, R.S. and Westcot, D.W. Water Quality for Agriculture. FAO Irrigation and Drainage Paper 29, Rev. 1. Food and Agriculture Organization of the United Nations, 1985 (reprinted 1989, 1994).
- Scianna, J., Pick, T. and Logar, R. Testing and Interpreting Salt-Affected Water for Tree and Shrub Plantings. USDA NRCS Plant Materials Technical Note No. MT-61, 2007.
- Bauder, T.A., Waskom, R.M. and Davis, J.G. Irrigation Water Quality Criteria. Colorado State University Extension, Fact Sheet 0.506.
- Food and Agriculture Organization of the United Nations. Wastewater Treatment and Use in Agriculture, Chapter 2: Wastewater Quality Guidelines for Agricultural Use.