A grower looking at a marginal water source — brackish well water, a pond with some tidal influence, runoff with a salt history — will often reach for the same fix: dilute it until it “tastes” weak enough. For a lot of water-quality problems that works. For one of the most important ones in irrigation science, it doesn’t, and the reason is arithmetic you can check yourself.
Two numbers, two different math
Electrical conductivity (EC) is a straightforward measure of total dissolved salts — it’s a sum of everything in the water, so if you dilute the water by a factor of 10, EC drops by a factor of 10. Simple and linear.
The Sodium Adsorption Ratio (SAR) is not a sum — it’s a ratio, defined by the USDA/FAO as SAR = Na / √((Ca + Mg)/2), with every concentration measured in milliequivalents per liter. SAR is what actually predicts whether sodium will degrade soil structure and water infiltration over time, independent of how “strong” the water tastes (Ayers & Westcot, FAO Irrigation and Drainage Paper 29, 1985).
Run the algebra on diluting a water sample by a factor D, and something becomes clear: EC falls as EC₀/D — but SAR falls only as SAR₀/√D. The numerator of the SAR equation (sodium) is linear in concentration; the denominator is a square root. Dilution shrinks the top faster than the bottom can follow. The ratio that actually damages your soil over time doesn’t dilute the way the “saltiness” of the water does. It’s the same reason you can’t fix a lopsided recipe by adding more water to the whole pot — diluting everything equally never changes the ratio between two of the ingredients.
What this looks like with an actual saline source
Full-strength seawater is a convenient worked example because its composition is well documented: roughly 10,760 mg/L sodium, 19,300 mg/L chloride, EC around 50 dS/m, and a computed SAR near 59 (University of Hawai’i oceanography course data; Boyd, 2020).
Dilute it 1-in-30 — a ratio that would look harmless on an EC meter compared to the undiluted source — and chloride still sits at 643 mg/L (18.1 me/L), nearly double FAO’s “severe restriction” threshold of 10 me/L, and SAR is still 10.7, also past FAO’s severe threshold of 9. Both numbers are still in the danger zone at a dilution most people would assume solved the problem.
Push the dilution further, to 1-in-71 — the point where EC finally reads inside FAO’s “no restriction” band (below 0.7 dS/m) — and SAR is still 7.0, squarely inside FAO’s slight-to-moderate infiltration-risk range. Getting SAR itself down to FAO’s “no restriction” threshold of 3 takes a 1-in-384 dilution. By that point calcium, magnesium and potassium are down in the low single-digit milligrams-per-liter range — nutritionally irrelevant, but the water finally passes on sodium hazard.
The nutrient side of the same table
There’s a second, quieter cost to chasing SAR down with dilution alone: by the time the ratio clears, there isn’t much nutrition left to talk about. At 1-in-384, magnesium is down to roughly 3 mg/L and calcium to around 1 mg/L — both far below the 30–50 ppm Mg and 40–100 ppm Ca that extension guidance treats as a desirable range for crop irrigation water. The dilution that makes a saline source pass a sodium-hazard test is the same dilution that makes it agronomically irrelevant as a nutrient source. That’s not a reason to avoid testing — it’s a reason to stop expecting dilution to do double duty as both a safety fix and a feeding strategy.
Why this matters before you commit a field to a water source
This is the same reason extension agronomists push broadacre growers toward a real water test — EC, SAR and chloride read separately — rather than a gut call on “diluted enough.” A field that gets a long-term sodium-structure problem doesn’t show it in this season’s yield; it shows up as declining infiltration and tilth years down the line, after the damage is already baked into the soil’s structure. By the time a grower notices the crust forming on the surface after rain, or water starting to pool instead of soak in, the sodium has usually been accumulating for several seasons already — which is exactly why the test needs to happen before the water goes on the field, not after.
The arithmetic above isn’t a marketing argument about any particular product — it’s the FAO’s own framework, applied honestly to a real saline source. Whatever you’re applying to a field — ours included — the water carrying it is worth testing on its own terms first, with an actual lab reading for EC, SAR and chloride rather than a guess based on taste, color or how diluted the source “feels.” It’s the same water-first discipline behind the water test we’d tell growers to run before ours.