Irrigation Water Quality, pH & EC

Most watering problems start in the water itself, before any fertiliser is added. This page explains the six things worth measuring in your irrigation water, what the numbers mean, and how the water sources available in Thailand — municipal supply, borewell, canal or pond, and harvested rain — actually compare. It is written for growers who have never had a water test done and are not yet sure whether their water is a problem at all.

Every threshold on this page is attributed. Where a figure comes from our own arithmetic on a published number rather than from the source directly, it says so.

What to measure, and why these six

A useful irrigation water report covers six things: pH, EC (electrical conductivity), alkalinity, hardness, sodium and chloride. Two of them you can measure yourself with a handheld meter; the other four need a laboratory.

The split matters, because the two you can measure at home are not the two that most often cause trouble. pH and EC are cheap to read daily. Alkalinity, sodium and chloride are the ones that quietly damage a crop over weeks, and they can only be found by sending a sample away. A grower who checks pH every morning and has never had an alkalinity number is watching the wrong gauge.

Take the sample from the point where water actually enters your system, not from the source. If you draw from a tank, sample the tank. Send it to a laboratory that reports agricultural or irrigation water, and ask for results in the units below.

EC — how salty the water is

Electrical conductivity measures how well water carries an electric current, which rises with the amount of dissolved salt in it. It does not tell you which salts are present — a point that matters later.

The standard unit is deciSiemens per metre (dS/m). FAO notes that dS/m and the older millimhos per centimetre (mmho/cm) are equivalent, so 1 dS/m = 1 mS/cm = 1000 µS/cm.

FAO Irrigation and Drainage Paper 29 classifies irrigation water salinity in three bands:

Restriction on use	None	Slight to moderate	Severe
EC of water (dS/m)	below 0.7	0.7 – 3.0	above 3.0
Total dissolved solids (mg/L)	below 450	450 – 2000	above 2000

Source: Ayers & Westcot (1985), Water Quality for Agriculture, FAO Irrigation and Drainage Paper 29 Rev. 1, Table 1.

Two cautions before you use that table. First, FAO states that its Table 1 is adapted from a 1974 University of California committee report, and that it assumes a semi-arid to arid climate "where rainfall does not play a significant role" in meeting crop water needs, soils from sandy-loam to clay-loam with good drainage, and at least 15 percent of applied water draining below the root zone. In most of Thailand the monsoon supplies a great deal of natural leaching in the field, which generally makes the table conservative for open ground. It applies much more directly to dry-season irrigation, to protected cropping where rain is excluded, and to anything grown in containers.

Second, salt concentrates in the root zone. FAO 29 gives the relationship as soil-water salinity being about three times the salinity of the applied water at a 15–20 percent leaching fraction. Water at 1.0 dS/m is not 1.0 dS/m where the roots are.

Reading ppm without being misled

Many meters display ppm instead of EC. This is the single most common source of confusion among new growers, and it is worth understanding properly, because two meters can read the same water and show numbers 40 percent apart without either being faulty.

An EC meter only ever measures conductivity. Any ppm figure is a back-calculation that assumes which salt is dissolved. There are two common conversion scales, and meter manufacturers document them explicitly: the 500 scale multiplies EC by 500 and is referenced to sodium chloride, while the 700 scale multiplies EC by 700 and is referenced to potassium chloride. So water at EC 1.0 dS/m displays as 500 ppm on one scale and 700 ppm on the other. A manufacturer's own technical note puts it plainly: ppm cannot be measured by an EC meter, and the conversions are offered as a guide only. (meter manufacturer technical documentation)

FAO uses a third multiplier again: mg/L is approximately 640 × EC in dS/m (Pescod 1992, FAO Irrigation and Drainage Paper 47, Table 7 footnote). That sits between the two meter scales, which is why FAO's own Table 1 pairs EC 0.7 with 450 mg/L.

There is also a "442" setting on some instruments. Its manufacturer describes it as a natural-water standard based on 40 percent sodium sulfate, 40 percent sodium bicarbonate and 20 percent sodium chloride, converted on a curve rather than by a single multiplier. It is not simply another name for the 700 scale, and we could not find a published single multiplier for it.

The practical rule: record and discuss EC in dS/m or mS/cm, never in ppm. If someone tells you their feed is "800 ppm", the number is not usable until you know which scale their meter was set to.

pH — a warning light, not a dial

pH is a measure of how acidic or alkaline the water is. Nutrients differ in how readily plant roots can take them up at different pH values, so a root zone sitting far outside the workable range can leave a plant showing a deficiency even when the nutrient is physically present. That is why pH appears in almost every troubleshooting guide.

What surprises most people is how little the pH of the water itself matters. FAO 29 gives the normal range for irrigation water as 6.5 to 8.4 and then states directly that pH "is seldom a problem by itself", and that "the main use of pH in a water analysis is for detecting an abnormal water" — a reading outside the normal range is a signal to investigate further, not a fault in its own right.

University extension sources reach the same conclusion from the greenhouse side. UMass Amherst states that "acidification of water having high pH but low alkalinity is rarely necessary", and Penn State that "higher water pH levels can be tolerated if the water alkalinity is not excessive".

Note that two different pH ranges circulate, and they answer different questions. FAO's 6.5–8.4 is a diagnostic range for water applied to field soil. Greenhouse and container guidance is tighter because the substrate has little buffering of its own: UMass and Penn State suggest roughly 5.0 to 7.0, and Penn State's greenhouse toolkit 5.4 to 7.0 depending on crop. Neither range is wrong; they describe different growing situations.

The terms used on this page are defined in more detail in our measurement and lab testing glossary and fertilizer glossary.

Alkalinity — the parameter that actually moves your root zone

If you take one thing from this page, take this one. Alkalinity is the water's capacity to neutralise acid, carried mostly as bicarbonate. It is not the same as pH, it is not printed on most basic water reports, and it is the parameter most likely to be quietly ruining a container crop.

Three extension sources describe the mechanism the same way. UMass compares high-alkalinity water to "a dilute solution of limestone". Purdue puts it as: "irrigating your crops with water high in alkalinity has the same effect as adding lime to the substrate". The University of Kentucky explains that "these dissolved carbonate and bicarbonate ions neutralize hydrogen ions which increases the pH of the substrate solution".

The key idea is that pH tells you the acidity at this instant, while alkalinity tells you how much liming power you are dosing with every single irrigation. It accumulates. Water at pH 8.0 with very low alkalinity is harmless. Water at pH 7.2 with high alkalinity will push substrate pH upward week after week.

Units are a trap here. A laboratory may report the same water as "bicarbonate 183 mg/L" or "alkalinity 150 mg/L". Always ask which basis was used. The conversions, from North Carolina State University and Purdue: 1 meq/L alkalinity = 50.04 mg/L as calcium carbonate = 61 mg/L as bicarbonate.

meq/L	mg/L as CaCO3	mg/L as HCO3
1.0	50	61
1.5	75	92
2.0	100	122
3.0	150	183
4.0	200	244

The mg/L columns above are our arithmetic using the published 50.04 and 61 factors, not values quoted from a source.

There is no single agreed threshold table for container growing, so here is what the individual sources say rather than a tidy invented one:

Penn State greenhouse toolkit: "the recommended upper limit for alkalinity for both greenhouse and nursery production is 100 mg/L".
Penn State irrigation water tests: "the ideal range for total alkalinity is approximately 30 to 100 mg/L but levels up to 150 mg/L may be suitable for many plants".
UMass: desirable 0 to 100 ppm as calcium carbonate, with 30 to 60 ppm optimum for most plants.
University of Kentucky: problems from 50–75 ppm in plug production and hydroponics; up to 150 ppm manageable in 4-inch and larger containers; 150–300 ppm of increasing concern, especially above pH 7.5; above 300 ppm causing significant problems.
North Carolina State: action above 2.0 meq/L, or above 1.5 meq/L for plug production (100 and 75 mg/L as calcium carbonate).

Read together, these converge on roughly: below 100 mg/L comfortable; 100 to 150 manageable in larger containers; above 150 usually requiring acid injection; above 300 requiring it. That summary is ours, drawn from the sources above — no single publication states it in those three bands.

Acid injection works by destroying bicarbonate rather than by lowering pH as an end in itself: added acid converts bicarbonate to water and carbon dioxide, and the pH drop is simply how you measure that it has happened. North Carolina State gives the titration endpoint at about pH 4.6 and a practical working target near pH 5.8.

Two safety points, both stated directly by the sources. The University of Kentucky: "the acid must always be added to the water", never the reverse. UMass: acid should be "always injected prior to the addition of fertilizer or other chemicals".

One further consideration that beginners rarely hear: nitric and phosphoric acid are also fertilisers. Using North Carolina State's published rates, treating water with 6 meq/L of alkalinity using nitric acid would deliver several hundred ppm of nitrogen along with it — far more than any crop needs. That is the reason sulfuric acid is the usual choice for high-alkalinity water.

Hardness, calcium and magnesium

Hardness is the combined calcium and magnesium content, expressed as an equivalent amount of calcium carbonate. The conversion, from an analytical methods document, is: hardness as CaCO3 = 2.497 × calcium mg/L + 4.118 × magnesium mg/L.

The United States Geological Survey classifies it as: 0–60 mg/L soft; 61–120 moderately hard; 121–180 hard; above 180 very hard.

Hardness and alkalinity usually travel together, but they are not the same measurement, and this is where the practical consequence sits for feeding:

Soft water, rainwater and reverse-osmosis water supply almost no calcium or magnesium. UMass notes that many water-soluble fertilisers do not supply calcium and magnesium either, so on very soft water both must be added deliberately. This is where a calcium-magnesium supplement earns its place — not as a general tonic, but because the water is not bringing any.
Hard water already supplies calcium and magnesium. UMass describes moderately alkaline water as "an important source of Ca and Mg for some greenhouse operators". Adding a full calcium-magnesium dose on top of genuinely hard water is adding what is already there, and it arrives with the bicarbonate load discussed above.

Penn State suggests moderate hardness of 100 to 150 mg/L as suiting plant growth, and that greenhouse and nursery water should not exceed 150 mg/L. We should be straight about one thing: despite searching, we found no extension publication that states in so many words that rain and reverse-osmosis water must be remineralised. The conclusion follows clearly from the sources above, but it is a synthesis, not a quotation.

Sodium, chloride and SAR

Sodium and chloride matter for two separate reasons: direct toxicity to sensitive crops, and damage to soil structure.

FAO 29 Table 1 gives, for surface irrigation, sodium as SAR below 3 (no restriction), 3–9 (slight to moderate), above 9 (severe); and chloride below 4 meq/L, 4–10, above 10. For sprinkler irrigation, where water wets the leaves, both tighten to 3 meq/L. Note that Table 1 does not give mg/L figures for sodium or chloride — any mg/L version you see has been converted by someone. Converting with the standard equivalent weights gives approximately 69 mg/L for sodium at 3 meq/L, and 142 and 355 mg/L for chloride at 4 and 10 meq/L. That arithmetic is ours.

The sodium adsorption ratio is defined in FAO 29 as sodium divided by the square root of half the sum of calcium and magnesium, with all three in meq/L, not mg/L. Using mg/L is the most common error made with this formula. To convert a laboratory report, divide sodium by 23.0, calcium by 20.04 and magnesium by 12.15.

What SAR does is described by FAO 29 directly: "excessive sodium in irrigation water also promotes soil dispersion and structural breakdown but only if sodium exceeds calcium by more than a ratio of about 3:1". A dispersed soil seals at the surface and stops taking water in.

Here is the counter-intuitive part, and it is worth sitting with. For salinity, lower EC is better. For soil infiltration, at any given SAR, higher EC is better, because dissolved salts keep clay particles flocculated. FAO 29's infiltration table puts water below 0.2 dS/m in the severe restriction class even when sodium is almost absent. Pure rainwater and reverse-osmosis water sit exactly there. FAO states it in its own words: "low salinity water tends to leach surface soils free of soluble minerals and salts, especially calcium, reducing their strong stabilizing influence on soil aggregates and soil structure".

For soilless and hydroponic growing, Wageningen University publishes crop-specific limits in mmol/L: tomato sodium 10, chloride 15; cucumber 8 and 10; sweet pepper 8 and 12; lettuce 8 and 15.

Water sources in Thailand

Municipal supply. In the Bangkok, Nonthaburi and Samut Prakan area, the Metropolitan Waterworks Authority publishes measured annual averages. For 2566 (2023): pH 7.37, total hardness 109 mg/L as calcium carbonate, total dissolved solids 209 mg/L, chloride 27 mg/L, free residual chlorine 0.70 mg/L. That is moderately hard water of low salinity — comfortable irrigation water carrying useful calcium and magnesium. The residual chlorine is worth knowing about if you use beneficial micro-organisms; it dissipates on standing or with aeration.

There is one seasonal exception that matters a great deal, because it peaks exactly when irrigation demand peaks. In the dry season, spring tides push seawater up the Chao Phraya to the Samlae intake. MWA's own annual report notes high raw-water salinity in the dry season, and in January 2020 MWA publicly announced chloride exceeding the 250 mg/L standard across around a dozen branch areas for periods of six to twelve hours. Bangkok tap water is not a constant.

Outside MWA's area, the Provincial Waterworks Authority treats to a published standard of pH 6.5–8.5, total dissolved solids not above 600 mg/L, total hardness not above 300 mg/L as calcium carbonate, chloride not above 250 mg/L, and free residual chlorine not below 0.2 mg/L. Note that MWA's own internal ceiling for dissolved solids is 1,000 mg/L while PWA's is 600 — the two authorities do not use the same figure.

Borewell water, and the Isaan salinity question. This is the source most likely to hold an unpleasant surprise, and the Northeast has a specific, well-documented reason. The Khorat Plateau is underlain by the Maha Sarakham Formation, which contains rock salt; groundwater dissolves it and brings the salt upward.

The clearest measured dataset comes from 189 monitoring wells in the central Huai Luang Basin, Udon Thani, sampled in 2014–2015. Total dissolved solids ranged from under 60 to 55,800 mg/L. About 75 percent of samples were below 1,000 mg/L; about 7 percent were above 10,000 mg/L. Water above 1,000 mg/L was of sodium-chloride type. (Pholkern et al. 2019, Water 11(2):241.)

Set against FAO 29's bands, that means roughly three quarters of those wells were usable, a quarter fell into slight-to-moderate or severe restriction, and around 7 percent were at several times the severe threshold — not irrigation water at any concentration. Work in the Khon Kaen Basin and in Nakhon Ratchasima documents the same pattern, with severity tracking underlying geology rather than distance or depth in any simple way.

The practical conclusion is blunt: in the Northeast, a borewell must be tested before it is trusted, and neighbouring wells can differ enormously. The Department of Groundwater Resources confirms the mechanism and notes saline zones in both the Northeast and the lower Central region, but its public material gives depths and causes rather than water-quality numbers, so the figures above come from peer-reviewed studies rather than from the department.

The Central Plain has its own version of the problem. A study of multiple aquifers in Ayutthaya measured EC from 712 to 11,266 µS/cm and chloride from 7.2 to 9,548.8 mg/L, with much of it rated poor or unsuitable for irrigation (Laonamsai et al. 2023, Geosciences 13(7):195).

Canal and pond water. Thailand's surface water standard, issued under Notification of the National Environment Board No. 8 (1994), places agricultural use in Class 3, which permits up to 20,000 total coliform and 4,000 faecal coliform per 100 mL. That is the legal baseline of a Thai irrigation canal: it is not drinking water and was never intended to be. Nationally, the Pollution Control Department's 2563 assessment found 2 percent of monitored water bodies very good, 37 percent good, 43 percent fair and 18 percent degraded.

For irrigation the main issues are suspended solids and organic matter, which clog emitters, and biological load, which builds biofilm. Both are handled by filtration and system maintenance rather than by water treatment — covered on our drip irrigation setup page.

Harvested rainwater. Chemically this is the cleanest source available. Measured at Chiang Mai over 122 daily samples across a full year, Thai rainwater averaged pH 5.5 (range 4.6 to 6.3) with conductivity of 0.65 to 0.69 mS/m — that is about 6.5 to 6.9 µS/cm, a conversion we have done from the published mS/m figures (Chantara & Chunsuk 2008, Atmospheric Environment 42(22)).

Near-zero EC has two consequences, one good and one not. It gives you a blank slate to build a feed on. But as the infiltration discussion above explains, it also sits in FAO's severe class for soil infiltration, and it supplies no calcium or magnesium at all.

Biological quality is the real caveat. A survey of 152 rainwater samples across six Thai regions between November 2022 and February 2024 found around 80 percent acidic and total coliform in around 87 percent of samples. An older study of rain jars in rural Northeast Thailand found jar water significantly cleaner than alternative household sources, and that netting over the jar mouth measurably improved quality. Thailand's Department of Health advises discarding the first rain that washes the roof and gutters, cleaning both thoroughly, and fitting mesh over the container mouth before the lid.

When reverse osmosis is justified

This question deserves a measured answer rather than a general one, so here is what we could and could not find.

The distinction that settles most cases: acid injection and reverse osmosis solve different problems and are not alternatives. Acid removes bicarbonate by converting it to carbon dioxide. It cannot remove sodium, chloride or boron, because those are not affected by acid at all. So:

High alkalinity alone is an acid-injection problem. Verified triggers: above 2.0 meq/L (North Carolina State); acidification desirable above 50 mg/L and sulfuric acid advised above 250 mg/L (Government of Saskatchewan); necessary above 300 ppm (University of Kentucky). Acid is inexpensive, and treating an alkalinity problem with reverse osmosis is spending a great deal of money on the wrong machine.
High sodium, chloride, boron or overall salinity cannot be fixed with acid. This is where treatment or a different water source is the answer.

We found exactly one published source giving a hard numeric trigger for reverse osmosis: the Government of Saskatchewan states that leaching with calcium has been used to manage sodium below 100 mg/L, and "above that rate, reverse-osmosis may be required". The same source treats water above EC 1.0 mS/cm as marginal and above 2.2 mS/cm as not recommended for bedding plants.

We should say plainly what we did not find. No FAO publication recommends reverse osmosis at any threshold — FAO 29 addresses salinity through leaching, crop choice and irrigation method rather than through treatment. And we found no published break-even rule comparing the cost of reverse osmosis against blending or rainwater storage.

For a monsoon climate there is a relevant alternative. Wageningen notes that rainwater is of excellent quality for soilless growing, that depending on rainfall pattern and storage it can cover 20 to 90 percent of a tomato crop's water need, and suggests storage capacity around 1,500 cubic metres per hectare. In Thailand, storing wet-season rain is usually the cheaper answer to a salinity problem than desalination — provided you remember that it brings no calcium or magnesium with it.

Feed EC and runoff EC

Measuring the water going in tells you what you dosed. Measuring the water coming out tells you what the root zone is actually doing, which is the more useful of the two.

The comparison is simple to read once you know the logic. If runoff EC is higher than feed EC, salts are concentrating in the root zone — the plant is taking up water faster than salt, or too little is draining through. If runoff EC is lower than feed EC, you are over-leaching and washing nutrients away. Roughly equal is the steady state to aim for.

For container crops, North Carolina State's pour-through procedure gives a repeatable method: 30 minutes to 2 hours after irrigation, pour 120 mL over a 1-gallon container (or 360 mL over a 3-gallon container) and collect the leachate. Target EC 0.5 to 1.0 dS/m on liquid feed, and not exceeding 2.0 dS/m for most bark-based substrates.

How much should drain is covered on our watering by growing medium page, along with why some drainage is necessary rather than wasteful.

Looking after your meters

A meter that has drifted is worse than no meter, because it produces confident wrong numbers. All the guidance below is from meter manufacturers' own documentation rather than from secondary sources.

Calibration standards. pH meters are calibrated with buffer solutions at pH 4, 7 and 10 (some makers label them 4.01, 7.01 and 10.01, which are the same buffers stated at 25°C). Manufacturers recommend at least a two-point calibration, using pH 7 as the reference plus either pH 4 or pH 10 depending on the range you work in. Conductivity meters are calibrated against a standard solution of known conductivity; 1413 µS/cm is the most widely used, with higher standards available for concentrated solutions.

How often. One manufacturer recommends conductivity calibration about once a month, with pH checked twice a month; another recommends pH recalibration every one to two weeks and conductivity monthly for a daily-use instrument; a third recommends monthly, or more often under heavy use. The consistent picture is EC monthly, pH every one to two weeks. The reason for the difference is that the pH glass electrode is the fragile, drifting part of the instrument.

Storage — this is how most meters die. Every manufacturer we checked says the same thing in capital letters. Store the pH probe wet, in a proper storage solution, saturated potassium chloride, or pH 4 buffer. Never store a pH probe in distilled, deionised or reverse-osmosis water. Pure water leaches ions out of the glass membrane and ruins the electrode. Rinse the probe with clean water after each use, keep it out of oils and anything that coats the glass, and do not touch the bulb with bare fingers. A probe that has dried out can sometimes be recovered by rehydrating for 24 hours in storage solution and recalibrating. Conductivity probes, by contrast, are kept clean and dry.

Downloads

Optimal pH range for crops (PDF, 1.9 MB — Vietnamese)
Ten principles of watering (PDF, 5.5 MB — Vietnamese)

Thai and English editions of both documents are being prepared.

Related guides

Sources

Ayers, R.S. & Westcot, D.W. (1985). Water Quality for Agriculture, FAO Irrigation and Drainage Paper 29 Rev. 1. fao.org
Pescod, M.B. (1992). Wastewater Treatment and Use in Agriculture, FAO Irrigation and Drainage Paper 47. fao.org
Bailey, D. & Bilderback, T. (rev. 1997). Alkalinity Control for Irrigation Water Used in Nurseries and Greenhouses, NCSU Horticulture Information Leaflet 558.
Lopez, R.G., Mickelbart, M.V. & Pasian, C. (2010). Alkalinity Management in Soilless Substrates, Purdue Extension HO-242-W. purdue.edu
Rhea, D. et al. (2025). Interpreting Irrigation Water Tests, Penn State Extension. psu.edu
Boser, S. & Rizzo, D. (2025). A Water Quality Toolkit for Greenhouse and Nursery Production, Penn State Extension. psu.edu
Cox, D. Water Quality: pH and Alkalinity, UMass Amherst. umass.edu
Ingram, D. (undated). Understanding and Managing Irrigation Water Alkalinity, University of Kentucky.
van Os, E. et al. (2016). Water quality and salinity aspects in hydroponic cultivation, Wageningen UR. wur.nl
U.S. Geological Survey. Hardness of Water. usgs.gov
Government of Saskatchewan. Water Quality in Greenhouses.
Bilderback, T.E. (2001). Using the PourThru Procedure for Checking EC and pH for Nursery Crops, NC State Extension. ncsu.edu
Metropolitan Waterworks Authority. Annual Water Quality Report 2566. mwa.co.th
Provincial Waterworks Authority. PWA Water Quality Standards (2024). pwa.co.th
Pholkern, K. et al. (2019). Water 11(2):241. doi.org
Laonamsai, J. et al. (2023). Geosciences 13(7):195. doi.org
Wongsomsak, S. (1986). Salinization in Northeast Thailand, Southeast Asian Studies 24(2).
Chantara, S. & Chunsuk, N. (2008). Atmospheric Environment 42(22):5511. doi.org
Pinfold, J.V. et al. (1993). Water Research 27(2):297.
Pollution Control Department. Surface water quality reports. pcd.go.th