Irrigation

Reading an irrigation water test

An irrigation water report is usually one page, a source ID, a date, and about twenty numbers. This covers what each line means, which of those numbers decide whether the water is fine to use, and two places where a lab's choice of units changes the answer.

Updated September 2026 13 min read

Not the same report as the one from your utility

A municipal water utility's annual quality report is about drinking-water safety, covering things like bacteria, lead, and disinfection byproducts, not irrigation suitability. It usually skips alkalinity, hardness, and sodium in a form useful for irrigation, and it says nothing at all about a private well. An agricultural or "irrigation suitability" panel is a different test, ordered separately, and the one this guide is about.

Which test to order

An irrigation suitability panel, most labs sell one, sometimes labeled "agricultural water" or "greenhouse water", covers pH, EC or TDS, alkalinity, calcium, magnesium, sodium, chloride and sulfate. That base set answers the four questions that decide whether water is fine to use: how salty is it, how much acid will it take to hold pH where you want it, does it carry enough sodium to threaten soil structure over time, and is any single ion concentrated enough to injure a sensitive crop on its own.

Beyond that base set:

Boron, iron, manganese, zinc and copper matter more if you're fertigating through drip or micro-sprinklers, where boron's narrow toxic window and iron's tendency to clog an emitter both carry more weight than they would under overhead irrigation.
Nitrate-N is worth adding on well water. It's the one line on this whole report that's a credit rather than a hazard, but only if you know it's there.
SAR is usually calculated from the sodium, calcium and magnesium already on the report rather than measured on its own, so there's rarely a need to order it separately.
Coliform bacteria if the water touches an edible crop close to harvest, a food-safety question this guide doesn't cover.

Well water is worth retesting periodically. A municipal source is treated to a specification and stays fairly put between draws; a well can shift with the season, the water table, or a neighbor's new irrigation well pulling from the same aquifer.

pH and alkalinity are two different questions

pH is where the water sits right now. Most crops tolerate irrigation water anywhere in 5.0 to 7.0; a little acidic is harmless, and the injectors and drip lines on most systems handle it without issue. Above about 8.0, it's almost always paired with high alkalinity and worth planning around.

Most irrigation water 5.0 to 7.0

Scale 4.05.06.07.08.09.0 pH

Below 5.0, check that the water isn't corroding metal fittings. Above 8.0, alkalinity, below, is almost always the real story.

Alkalinity, reported as ppm CaCO₃, or sometimes as bicarbonate (HCO₃), measures something else: how hard the water will fight a change to its pH, not where that pH currently sits. Every watering with a high-alkalinity source pushes substrate or soil pH up a little, and the water's own pH reading gives almost no warning of how much. Two sources can both read pH 7.2 and carry very different alkalinity, because alkalinity comes from dissolved bicarbonates and carbonates and pH is only a snapshot of the balance between them at the moment it was measured. A pH test alone is not a test of alkalinity, and ordering the one without the other is the single easiest way to misjudge a water source.

How much alkalinity is tolerable depends on what's absorbing it. A plug tray or a hydroponic reservoir has almost no buffering reserve of its own, so incoming alkalinity shows up in the crop within days; a lawn or a field grows in soil with years of buffering capacity behind it, so the same water takes seasons to matter.

Hydroponics / plug trays under 50 ppm Almost no buffering reserve of their own; alkalinity here shows up within days.

Greenhouse / seedlings under 80 ppm Small containers, frequent watering, little reserve to draw on.

General garden irrigation under 120 ppm Field soil buffers some of it, but it's still worth watching over a season.

Lawn / turf under 180 ppm The most tolerant of the four: a deep root zone and a large soil reserve dilute and leach what arrives.

Rough alkalinity targets by what the water is going onto. Extension guidance written for a controlled greenhouse crop puts the ideal band tighter still, around 30 to 100 ppm; treat any of these as a starting point, not a hard line.

How much alkalinity to expect depends heavily on the source.

RO / DI water under 10 ppm

Municipal / city water commonly 50 to 250 ppm

Well water anywhere from near 0 to 400+ ppm

Scale 0100200300400 ppm CaCO₃

Reverse osmosis removes 95 to 99 percent of dissolved solids, alkalinity included, regardless of what fed it. Municipal water is treated and monitored, so it tends to land in a moderate, fairly consistent band. Well water is the wildcard: the same limestone dissolving into groundwater that raises hardness (below) raises alkalinity along with it, so a well drawing from carbonate rock can read high and one drawing from granite a mile away can read near zero.

Bicarbonate and alkalinity aren't quite the same number

A report that gives bicarbonate (HCO₃) in ppm instead of alkalinity as CaCO₃ is naming the same buffering capacity in a different currency. Multiply HCO₃ by about 0.82, the ratio of CaCO₃'s equivalent weight (50) to HCO₃'s (61), to get the CaCO₃-equivalent alkalinity most targets, including the ones above, are written against. Comparing an HCO₃ reading straight to a CaCO₃ target overstates the alkalinity by about a fifth.

Salinity: EC and TDS

EC (electrical conductivity) is the total dissolved salt concentration, every ion together, not any one of them. It's the single best one-number read on whether water is generally usable, because a high reading means trouble even before you know which salt is causing it.

No restriction under 0.75 dS/m Safe for essentially any crop.

Slight to moderate 0.75 to 1.5 dS/m Fine for most crops; leach occasionally and watch sensitive seedlings.

Moderate to high 1.5 to 3.0 dS/m Salt-tolerant crops, regular leaching, or blend with a cleaner source.

Severe above 3.0 dS/m Generally unusable without reverse osmosis or heavy blending.

Salinity hazard by EC, the classification used across most extension irrigation guidance.

TDS (total dissolved solids, in ppm) is usually estimated from EC rather than measured directly: TDS ≈ EC (in dS/m) × 640. If a report gives both and they disagree by more than that, it's worth asking the lab which one was measured.

EC on a report isn't always in dS/m

Labs write electrical conductivity as dS/m, mS/cm, or µS/cm (micromhos and millimhos, the older names for the same units, still show up too). dS/m and mS/cm are the same number; µS/cm is 1,000 times smaller. A source reading "1150" is either a moderately salty 1.15 dS/m, or, read as dS/m directly, a severe 1,150. Check the unit before the number, especially before it drives an acid-injection or blending decision.

Hardness, calcium, and magnesium

Hardness, reported as ppm CaCO₃ like alkalinity, but measuring calcium and magnesium rather than bicarbonate, is how much dissolved Ca and Mg the water is carrying. It isn't a hazard the way salinity or sodium are; it's closer to a nutrient credit with a ceiling. Extension guidance for greenhouse fertigation, where water runs through drip emitters and heaters in a closed loop, puts the sweet spot around 100 to 150 ppm and flags scale buildup above that. Ground irrigation that isn't running through that kind of equipment tolerates more before it's an actual problem, closer to 300 ppm, though the calcium carbonate still shows up as scale on anything it evaporates against.

Below about 50 to 75 ppm, the water is soft enough that it isn't contributing meaningfully, and calcium and magnesium need to come from the fertilizer program instead. Either way, the ppm Ca and Mg already on this report are nutrients the crop is getting with every watering, whether or not they're in the fertilizer mix. Subtract them before sizing a Ca or Mg supplement with the Basic Hydro Fertilizer Calculator, or the program ends up building more than intended.

Calcium (Ca) 25 to 75 ppm Below about 40, plan on supplementing it; above about 100 to 150, it's a heavy source on its own.

Magnesium (Mg) 10 to 30 ppm Below about 25, the same logic applies; rarely a problem at the high end beyond scale.

Target ranges from greenhouse water quality guidance. A field or garden source running lower isn't a defect, just a smaller nutrient credit.

Sodium and the SAR

Sodium (Na) on its own matters two ways: direct leaf injury on sensitive crops under overhead irrigation, and structural damage to soil over years, the slower and more consequential of the two. Above about 50 ppm it's worth watching; past 100 it's a real hazard on sensitive crops and low-calcium soils.

The structural risk is what SAR (sodium adsorption ratio) measures: sodium's concentration relative to the calcium and magnesium competing against it, not sodium alone. Soil clay particles are held together by calcium and magnesium bridging their negative charges; sodium, carrying only a single charge and a larger hydrated radius, does that job poorly and pushes the particles apart instead, collapsing the pore structure water needs to infiltrate through.

Low hazard under 3

Moderate 3 to 6

High 6 to 9

Scale 036912 SAR

Above about 9, sodium is a severe hazard to soil structure on its own. This is the SAR-alone read; the fuller picture also depends on the water's own salinity, below.

SAR alone is a useful rule of thumb, but it isn't the whole picture: a high-EC water partially offsets sodium's structural damage, because the extra dissolved salt keeps clay particles flocculated even with sodium in the mix, while the same SAR in very pure, low-salinity water is more damaging than the number alone suggests. The full infiltration-hazard assessment, from FAO Irrigation and Drainage Paper 29 and most state extension water-quality criteria, reads SAR and EC together rather than SAR on its own. Treat the band above as the first-pass read, and the caveat as the reason a borderline SAR is worth a second look rather than a snap judgment.

The fix for a high SAR is the same shape as for a sodic soil: gypsum (calcium sulfate) to swap calcium onto the exchange sites sodium was occupying, followed by enough leaching water to carry the displaced sodium below the root zone.

Chloride, sulfate, and boron

These three don't act through salinity or soil structure; each is capable of injuring a plant directly once it's concentrated enough, chloride and boron especially through foliar uptake on overhead or sprinkler irrigation rather than through the roots.

Chloride (Cl) under 70 ppm Safe for most crops; 70 to 140 ppm starts marginal leaf-tip burn on sensitive ones, and above 350 ppm it damages most crops outright.

Sulfate (SO₄) under 200 ppm Mostly a sulfur nutrient credit at this level; above 400 ppm it's contributing meaningfully to overall salinity rather than just feeding the crop.

Boron (B) under 0.5 ppm Essential in trace amounts and toxic not far above them; sensitive crops (many fruit trees, beans, citrus) show injury as low as 0.5 to 1.0 ppm.

Chloride and boron have the more standardized thresholds of the three; sulfate's is softer, since it acts more as a salinity contributor and nutrient source than a toxin in its own right.

Boron carries the narrowest safe window of any nutrient on this report, the same distinction it holds on a soil test, essential in trace amounts and toxic only a few ppm above them. Chloride is also the hardest of the three to do anything about once it's in the water. Unlike calcium or bicarbonate, there's no cheap reaction to precipitate it out; blending with a cleaner source or reverse osmosis are close to the only options.

Sulfate: is it reported as S, or as SO₄?

Some labs report sulfur as the whole sulfate ion (SO₄); others report it "as S," the sulfur atom alone, since that's the number a fertilizer program needs. The two aren't close: sulfate's molecular weight (96.1) is about three times sulfur's (32.1), so a report reading "sulfate as S: 7 ppm" works out to about 21 ppm as SO₄, not 7. Multiply S by roughly 3 (2.996, from the ratio of those two weights) to get SO₄, or divide the other way. Comparing an "as S" figure straight against an SO₄ threshold, like the ones above, or the reverse, is off by a factor of three either way.

Iron and nitrate-N

Iron (Fe) is rarely a plant-toxicity problem at the levels found in irrigation water; the real cost is mechanical. Past about 0.2 to 0.3 ppm it starts staining surfaces and feeding the biofilm that fouls a filter; by 1 ppm it will clog a drip emitter outright, especially once it oxidizes and precipitates out of solution on contact with air. Filtering, or letting it aerate and settle before the water reaches the line, handles it either way.

Nitrate, reported as NO₃-N, nitrogen as nitrate, is the one line on this whole report that's a credit rather than a hazard. Clean irrigation water typically carries under 5 ppm N; a well near heavy fertilizer use, livestock, or a septic field can run much higher. Either way, whatever nitrate-N is in the source water is nitrogen the crop is already getting with every irrigation cycle, so subtract it from the fertilizer program rather than counting it twice.

Irrigation Water Quality Calculator Enter every line from this report and it runs the salinity, alkalinity, sodium hazard, and ion checks above against your target use, and sizes an acid-injection rate if alkalinity needs correcting.

Open the calculator

What to take away

pH says where the water is; alkalinity says how hard it will fight a change. Order both, and treat alkalinity as the one that predicts drift.
EC is the single best one-number read on usability. TDS is usually estimated from it (×640), not measured separately.
SAR reads sodium against the calcium and magnesium competing with it, not sodium alone, and a full infiltration-hazard read also depends on the water's own EC.
The Ca, Mg, S and N already on this report are nutrients arriving with every watering. Credit them against the fertilizer program before sizing a supplement.
Two unit traps sit on almost every report: EC in µS/cm read as dS/m overstates salinity a thousandfold, and sulfate "as S" read as SO₄ understates it about threefold.

Sources

General agronomic guidance, drawn from university extension services and the standard FAO reference on irrigation water quality. Lab methods, units, and crop tolerance all vary, which is why this guide gives ranges rather than single cutoffs; a lab's own interpretive notes, when it provides them, are the more precise word on any one of these.

Penn State Extension, "Interpreting Irrigation Water Tests" ↗. Parameter-by-parameter ranges for pH, alkalinity, hardness, Ca, Mg, EC, TDS, boron, chloride, sodium, SAR, nitrate-N and iron, and the note that sulfur is commonly reported as elemental S rather than as sulfate.
University of Massachusetts Amherst Center for Agriculture, Food and the Environment, "Water Quality: pH and Alkalinity" ↗. Why a pH reading alone says nothing about alkalinity, and why plug trays are the most exposed to the combination.
University of Massachusetts Amherst Center for Agriculture, Food and the Environment, "UMass Extension Floriculture Water Quality Project: II. pH, Alkalinity, Calcium, Magnesium and Other Elements" ↗. Target ranges for calcium and magnesium in irrigation water, and crediting them against a fertilizer program.
University of Massachusetts Amherst Center for Agriculture, Food and the Environment, "Water Quality for Crop Production" ↗. Reverse osmosis removing 95 to 99 percent of dissolved solids, and municipal water quality by source.
U.S. Geological Survey, "Hardness of Water" ↗. Why a carbonate-rock aquifer produces harder, more alkaline groundwater than a crystalline-rock one.
Colorado State University Extension, "Irrigation Water Quality Criteria" (Fact Sheet 0.506) ↗. The EC salinity-hazard classification, the combined SAR/EC infiltration-hazard table, and chloride and boron thresholds.
Ayers, R.S. and Westcot, D.W. (1985, rev. 1994), Food and Agriculture Organization of the United Nations, "Water Quality for Agriculture," FAO Irrigation and Drainage Paper 29 ↗. The original combined SAR/EC infiltration-hazard method that state extension criteria are built from, and boron's specific-ion tolerance data.

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