Water Quality & The Full FAO-29 Verdict
Classifies salinity
Is your water safe to irrigate with, for which crops and soils, and with what restrictions? This runs the full FAO-29 multi-hazard classification — salinity, infiltration, specific-ion toxicity, bicarbonate, RSC, nitrate and pH — to a single suitability verdict and the binding constraint, with the management fix.
Your water test
Runs entirely in your browser — nothing is uploaded. me/L = milli-equivalents per litre (mg/L ÷ equivalent weight). SAR & RSC are computed from your cations.
Next: the binding constraint is Salinity (ECw) — apply a leaching fraction (LR ≈ ECw ÷ (5·ECe_threshold − ECw)) and pick salt-tolerant crops; manage with extra water depth at each irrigation. Re-test after any blend or amendment to confirm the class improves.
FAO-29 (Ayers & Westcot, 1985) Table 1 — degrees of restriction: None / Slight–Moderate / Severe. SAR = Na ÷ √((Ca+Mg)/2); RSC = (HCO₃+CO₃) − (Ca+Mg); TDS ≈ 640 × ECw.
FAO-29 water quality — key facts
- Salinity (ECw)
- none <0.7 · slight 0.7–3.0 · severe >3.0 dS/m
- SAR formula
- Na ÷ √((Ca+Mg)/2), me/L
- Infiltration
- read SAR class × ECw together
- Chloride (surface)
- none <4 · slight 4–10 · severe >10 me/L
- Sodium/Cl (sprinkler)
- severe above ≈3 me/L (foliar)
- RSC
- safe <1.25 · marginal 1.25–2.5 · bad >2.5 me/L
- Boron
- ≈1.0 grape · 1.5 wheat · 6.0 cotton mg/L
- TDS ≈
- 640 × ECw (mg/L)
- Overall class
- = the worst (binding) hazard
- Privacy
- Runs in your browser; nothing uploaded
FAO-29 Table 1 — degree of restriction on use
Guideline thresholds from Ayers & Westcot (1985), FAO Irrigation & Drainage Paper 29.
| Hazard | Units | None | Slight–moderate | Severe |
|---|---|---|---|---|
| Salinity (ECw) | dS/m | < 0.7 | 0.7 – 3.0 | > 3.0 |
| Sodium — SAR (surface) | SAR | < 3 | 3 – 9 | > 9 |
| Sodium (sprinkler) | me/L | < 3 | — | > 3 |
| Chloride (surface) | me/L | < 4 | 4 – 10 | > 10 |
| Chloride (sprinkler) | me/L | < 3 | — | > 3 |
| Boron | mg/L | < crop limit | 1 – 1.5× limit | > 1.5× limit |
| Bicarbonate (sprinkler) | me/L | < 1.5 | 1.5 – 8.5 | > 8.5 |
| Residual sodium carbonate | me/L | < 1.25 | 1.25 – 2.5 | > 2.5 |
| Nitrate (NO₃-N) | mg/L | < 5 | 5 – 30 | > 30 |
| pH | — | 6.5 – 8.4 normal | outside range | — |
Infiltration — SAR × ECw matrix
The minimum ECw (dS/m) needed at each SAR class to avoid a restriction (FAO-29 Table 1, after Rhoades).
| SAR class | ECw ≥ for none | ECw ≥ for slight | Below = severe |
|---|---|---|---|
| 0 – 3 | 0.7 | 0.2 | < 0.2 |
| 3 – 6 | 1.2 | 0.3 | < 0.3 |
| 6 – 12 | 1.9 | 0.5 | < 0.5 |
| 12 – 20 | 2.9 | 1.3 | < 1.3 |
| 20 – 40 | 5.0 | 2.9 | < 2.9 |
Permissible boron by crop (mg/L)
| Sensitive (≤1.0) | Moderate (1.5–3) | Tolerant (4–15) |
|---|---|---|
| Grape · citrus · avocado · apple · walnut · onion · bean (0.5–1.0) | Wheat 1.5 · barley/maize/potato/tomato 2.0 · cabbage 2.5 · lettuce/carrot 3.0 | Oat/sorghum/alfalfa/sugar beet 4.0 · cotton 6.0 · asparagus 15.0 |
One water test, nine hazards, one verdict
Irrigation water rarely fails for one reason. The FAO-29 guidelines split the question into salinity (does the water leave enough moisture for the crop?), infiltration (does the water even enter the soil, which depends on sodium and salinity together?), specific-ion toxicity (sodium, chloride and boron taken up directly), bicarbonate scaling, residual sodium carbonate, nitrate and pH. Each hazard is scored none, slight–moderate or severe against published thresholds, and your water is only as good as its single worst hazard.
Most online tools score one number — an SAR calculator here, an RSC tool there. This one runs all nine hazards at once, identifies the binding constraint that sets your overall class, and names the specific fix: a leaching fraction for salinity, gypsum for sodium and infiltration, acidification for bicarbonate, or blending for boron and chloride. Pair it with the Deficit Irrigation Strategy and Capillary Rise tools for a complete water plan.
How to use it — five steps
- 1
Enter the salts
Type ECw and the cations sodium, calcium and magnesium (me/L) so the tool computes SAR and the infiltration hazard.
- 2
Add the toxic ions
Enter chloride, boron, bicarbonate, nitrate-N and pH from your lab report.
- 3
Pick method and crop
Choose surface/drip or overhead sprinkler, then the crop to set its boron tolerance.
- 4
Read every hazard
Each gauge slides through none → slight–moderate → severe; the binding one is flagged.
- 5
Act on the binding hazard
Apply the named fix — leach, add gypsum, acidify or blend — and re-test to confirm the class improves.
Frequently Asked Questions
How do I know if my water is safe to irrigate with?+
Run every FAO-29 hazard at once: salinity (ECw), the SAR × ECw infiltration pair, specific-ion toxicity (sodium, chloride, boron), bicarbonate, residual sodium carbonate, nitrate and pH. Each scores none, slight–moderate or severe against the FAO-29 Table 1 thresholds, and your water is only as good as its worst hazard. The overall class equals that binding hazard, so a single severe ion can downgrade otherwise good water.
What ECw is safe for irrigation?+
On the FAO-29 salinity scale, ECw below 0.7 dS/m has no restriction, 0.7–3.0 dS/m is a slight-to-moderate restriction, and above 3.0 dS/m is a severe restriction that needs a leaching fraction and salt-tolerant crops. As a rule of thumb, TDS in mg/L is about 640 × ECw, so 1.5 dS/m is roughly 960 mg/L of dissolved salt.
What is SAR and what value is too high?+
The sodium adsorption ratio, SAR = Na ÷ √((Ca + Mg)/2) with all ions in me/L, measures sodium relative to calcium and magnesium. High SAR breaks down soil structure and seals the surface against infiltration. FAO-29 reads SAR together with ECw: at SAR 0–3 you need ECw above 0.7 dS/m to avoid an infiltration problem, while at SAR 12 you need ECw above about 1.9 dS/m, because salinity actually helps water infiltrate.
Why does the tool read SAR and ECw together for infiltration?+
Sodium and salinity pull infiltration in opposite directions. Sodium disperses clay and slows infiltration, but a higher salt concentration (ECw) flocculates the clay and speeds it back up. So the same SAR can be fine in salty water and severe in dilute water — FAO-29 captures this in a SAR-class × ECw matrix, which this calculator applies exactly.
How is boron toxicity judged?+
Boron has the narrowest safe band of any element: the gap between deficiency and toxicity is tiny, and it is hard to leach out. The tool compares your boron in mg/L against the chosen crop's permissible limit from the FAO-29 boron-tolerance table — for example about 1.0 mg/L for grape and sensitive fruit, 1.5 mg/L for wheat, and up to 6 mg/L for cotton. Over the limit means yield loss; the only easy fix is blending with low-boron water.
What is residual sodium carbonate (RSC)?+
RSC = (HCO₃ + CO₃) − (Ca + Mg) in me/L. When carbonates exceed calcium and magnesium they precipitate Ca and Mg out of solution as the water concentrates in the soil, which raises the effective sodium hazard. RSC below 1.25 me/L is safe, 1.25–2.5 is marginal, and above 2.5 me/L is unsuitable without gypsum or acid amendment.
Does the irrigation method change the verdict?+
Yes. With overhead sprinklers, sodium, chloride and bicarbonate can burn or scale the leaves and fruit they wet, so the toxicity thresholds are stricter (sodium and chloride become severe above about 3 me/L). With surface or drip irrigation only root uptake matters, so the thresholds are higher. Switch the method toggle to see how the binding hazard can change.
Is high-nitrate water a problem or a bonus?+
Mostly a bonus — nitrate-N in irrigation water is free fertiliser, and you should credit it against your applied nitrogen. FAO-29 flags slight–moderate above 5 mg/L and severe above 30 mg/L only because excess N can over-stimulate sensitive crops, delay maturity or worsen lodging, and may matter for downstream water quality.
What does 'binding hazard' mean?+
It is the single worst hazard that sets your overall class. Because water can only be as good as its most limiting property, the calculator finds the hazard with the highest restriction and reports it as the binding constraint, then gives the specific management fix for it — leaching for salinity, gypsum for sodium/infiltration, blending for boron, and so on.
Can I fix unsuitable water?+
Often, depending on the binding hazard. Salinity is managed with a leaching fraction and salt-tolerant crops; sodium and infiltration problems with gypsum to supply calcium; bicarbonate and RSC with acidification or gypsum; chloride and boron mainly by blending with cleaner water or changing crops. The tool names the relevant fix once it identifies which hazard binds.
How accurate are these thresholds?+
They are the published FAO-29 guideline values (Ayers & Westcot, 1985, Table 1), which are deliberately conservative planning thresholds rather than hard cut-offs. Local soil, climate, drainage and management shift the real tolerances, so treat the class as a guide, confirm with a full lab report, and re-test after any amendment or blend.