Water Quality & The Full FAO-29 Verdict

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.