Irrigation Water for Living Soil: Reverse Osmosis, Fluoride, Testing Your Source and Choosing Water in Thailand (Part 2 of 2)

🌱 Cultivation Basic Growing Info
1

Translated and edited from the irrigation-water chapter of the book True Living Organics by The Rev. The research and extension sources used to check the book’s claims are listed under References at the end.

Living soil depends on the water you feed it, but few growers know what their water contains. Part 2 explains how a reverse osmosis filter works and how to use its reject water. It also covers what fluoride does, how to test your own water in Thailand, when Cal-Mag is needed and which water sources suit this region best.

The Rev is the pen name of the American organic grower who wrote True Living Organics. His method, called TLO, builds a soil rich enough that you mostly just water it (how TLO works). This guide says so whenever it relies on his own advice or observations. Wherever a claim could be checked, the research or university extension service is linked. Extension services are the public farm-advice programmes run by US universities.

Part 1 covered three points. Chloramine in tap water does not go away if you leave the water to stand. Hard water builds up salts in the pot over one to two months. Softened water carries sodium and must never be used for irrigation.

This is Part 2. Part 1: Irrigation Water for Living Soil: Chloramine, Hard Water, Softened Water and Why You Should Measure Your Own

5. How a reverse osmosis unit works, and why its reject water is useful

A reverse osmosis (RO) unit pushes water through a very fine membrane. Clean water passes through, and most dissolved minerals stay behind. Those minerals leave the unit in a second stream, called reject water or “waste” water.

A typical benchtop unit has three stages: carbon, then the membrane, then carbon again. These units are inexpensive. On The Rev’s own tap water, the RO output reads about 10 PPM. PPM means parts per million, roughly milligrams per litre.

Maintenance:

  • The Rev changes his carbon cartridges at least once a year and gets several years out of a membrane.
  • A TDS meter on the output line shows when the membrane is failing, because the reading starts to rise. A TDS meter is a cheap pen-shaped meter that shows dissolved minerals as PPM.
  • Output is slow, measured in litres per hour rather than litres per minute. Plan a storage tank instead of filtering water as you need it.

The reject water is neither waste nor toxic. It is your source water in concentrated form, usually 1.3–2 times as strong as what went in. The exact figure depends on the unit’s recovery ratio, which is the share of incoming water that comes out as clean water. Ways to use it:

  • Plants growing directly in the ground outdoors. The large volume of soil absorbs the extra minerals. See also Outdoor Living Soil.
  • Tap water that has already lost its disinfectant. If your unit has a carbon stage before the membrane, the reject stream from tap water has already lost its chlorine or chloramine. It still contains fluoride and other dissolved minerals.
  • Household jobs such as cleaning or boiling vegetables.
  • Blending. If you use well or spring water and know its mineral profile, The Rev blends reject water with filtered water to raise irrigation water to about 70–90 PPM.

Where water costs money and sometimes runs short, setting this up properly is worth it. A second tank and a short length of pipe recover a large volume of water.

Translator’s note: If your area has frequent water or power cuts, keep the RO storage tank full so a cut does not stop irrigation. At 30–35 °C, keep both tanks covered and out of direct sun, and clean them regularly.

What to do: Measure how many litres of reject water your unit produces per day. Then plan where that water will go, whether to outdoor beds, cleaning or blending, before it goes down the drain.

6. Fluoride: a real effect, but smaller than the book claims

The book makes four claims about fluoride:

  • Fluoride interferes with more than thirty biological processes in plants.
  • It will certainly reduce germination rates.
  • Carbon filters do not remove it.
  • RO does remove it.

What extension sources confirm: Fluoride can genuinely damage plants. The Pacific Northwest Plant Disease Handbook notes that many municipal supplies add fluoride at about 1 ppm. Irrigating with that water can cause toxicity symptoms on sensitive species. Its examples include Dracaena, spider plant, gladiolus, lily, tulip and yucca, along with some fruit trees and conifers.

Michigan State University Extension notes that monocots such as spider plant, lilies and dracaena are among the more susceptible plants. For sensitive crops it suggests reverse osmosis as a long-term fix, or collecting rainwater to use alone or to dilute city water. It also suggests keeping the pH of the growing medium at about 6.0–6.8 so less fluoride is available, if the crop tolerates that range.

What could not be confirmed: No source supports the figure of “more than thirty biological processes”, or the claim that fluoride reduces germination in cannabis. Cannabis does not appear on published lists of fluoride-sensitive species. No study of fluoride effects on cannabis was found. Treat the book’s fluoride claim as unproven for this crop.

Translator’s note: No source was found showing that Thai tap water is fluoridated the way many US supplies are. Natural fluoride in groundwater is a separate matter. A survey of wells in Chiang Mai and Lamphun in northern Thailand found that a substantial share contained more than 1.5 mg/L. The fluoride was linked to geothermal and granite sources.

What to do: If you use well water in northern Thailand, include fluoride in your water test instead of assuming it is low. If the level is high, RO is the fix. Carbon will not remove it.

7. Mineral readings: why you should not copy the book’s numbers

The Rev gives rough bands for PPM readings from his own water:

  • A few tens of PPM: he only has to deal with chloramine and hardly thinks about pH.
  • Above about 80 PPM: pH starts to matter much more.
  • About 150–170 PPM: he treats the water as hard and fits a softener before the RO unit.

He then undercuts his own bands. He reports healthy TLO plants on 150 PPM well water and poor results on 80 PPM well water. He concludes that what is in the water matters, not only how much.

That conclusion is correct, and it is why this guide does not present his bands as rules for this region. A TDS reading (total dissolved solids) is a single number for the total amount dissolved in the water. It tells you nothing about which minerals are there. Water at 120 PPM that is mostly calcium bicarbonate is completely different from water at 120 PPM that carries sodium and manganese.

A TDS meter actually measures EC (electrical conductivity), which rises as more minerals dissolve in the water. It then converts EC into an estimated PPM figure. Meters use different conversion factors, so compare readings only from the same meter, or record the EC figure itself.

What to do instead:

  1. Buy a TDS meter. It costs very little and is the cheapest useful instrument in a grow room. Measure your source water, then measure it again in a different season.
  2. Get a full water analysis if you run anything bigger than a small hobby setup. Ask your water supplier for its water-quality report. A commercial lab test costs a fraction of one crop’s value. Ask for calcium, magnesium, sodium, iron, manganese, sulphate, bicarbonate, fluoride (for well water) and the type of disinfectant.
  3. Treat the analysis as part of your fertiliser plan. Hard water may already supply some of the calcium and magnesium you would otherwise add. Only the analysis can tell you how much (see section 8).
  4. Measure again every season. In a monsoon climate, groundwater chemistry changes through the year.

For measuring and adjusting pH and EC, see pH & EC: The Nutritional Foundation Every Grower Needs to Know. Related threads on water quality are collected in General Nutrients: pH, EC, Water Quality, Mixing and Biostimulants.

Translator’s note: In Thailand, a water analysis can come from three places:

  • Your utility. PWA regional offices post annual water-quality reports for their branches online. MWA shows real-time residual chlorine at its monitoring points. If you cannot find a report for your area, ask MWA, PWA or your local water system for one.
  • A commercial laboratory. For wells, send a sample to a water-testing laboratory. Before you send it, confirm that the lab tests sodium, manganese and fluoride. To hear which labs other members have used, ask in the B2B Services section.
  • Test strips. Aquarium hardness strips give a rough idea of calcium and magnesium hardness between lab tests. They do not replace a lab test.

8. Calcium and magnesium (Cal-Mag): when your water calls for it

“Cal-Mag” is the common name for supplements that supply calcium and magnesium. Whether you need one depends on your source water and your growing system:

  • Hard water. It may already supply part of the calcium and magnesium you would otherwise add with dolomite (a ground rock containing both) or oyster shell. Only the calcium and magnesium figures from a water analysis tell you how much. Without them, you risk adding more on top of what is already there.
  • RO water, rainwater and dehumidifier condensate. All three contain very few dissolved minerals: RO output was about 10 PPM in The Rev’s case, and condensate is close to distilled water. In living soil, the calcium and magnesium must then come from your amendments, such as dolomite or oyster shell. With a bottled programme, follow the nutrient maker’s label for low-mineral water. This guide gives no dose, because the original gives none.
  • Softened water. The softener has already replaced the calcium and magnesium with sodium. Adding Cal-Mag does not remove the sodium. Do not use this water (see Part 1).
  • Calcium problems with good water. In The Rev’s experience, calcium problems often follow uneven watering that leaves dry pockets in the pot. Fix the watering first (section 10) before you add more calcium.

What to do: Before buying a Cal-Mag product, get the calcium and magnesium figures for your source water. Compare them with what your soil recipe or nutrient label already supplies, and add more only if there is a real gap.

9. Water sources ranked for this region

1. Rainwater. This is the book’s first choice. In Southeast Asia it has the strongest practical case: rain is plentiful for most of the year and naturally low in dissolved minerals. The Rev gives three conditions:

  • Check your roof material, because many materials leach substances into the runoff.
  • Store the water in tanks.
  • Aerate stored water by bubbling air through it.

There are two additions for this region. Where air pollution is significant, fit a first-flush diverter, a device that sends the first, dirtiest rain off the roof to waste. Also cover stored water so mosquitoes cannot breed in it.

Translator’s note: Where the dry season is long, rain alone may not last all year. Plan a second source, such as RO or condensate, for the dry months.

2. RO water. RO is reliable and controllable, and it is the book’s everyday choice. Section 5 covers setup and maintenance.

3. Dehumidifier condensate. This is close to distilled water and safe to use. In a humid climate with a dehumidifier running, it is free water you are already producing. Keep the collection tray clean.

4. Aquarium water. The Rev adds a small dose of freshwater aquarium water to RO water. He uses a little more during vegetative growth than in flower, because aquarium water contains nitrogen. It raises the PPM only slightly and adds living organisms. It only works if the tank runs on RO or very low-mineral water and has never been treated with dechlorinating chemicals. This is a hobby technique, not a production method.

5. Municipal tap water. Tap water is workable with carbon filtration or neutralisation (see Part 1), but only if two conditions hold. Your reading must be genuinely low, and you must know which disinfectant your supplier uses. No published TDS data on Thai tap water was found that would allow general statements. Measure yours, and do not assume it is low.

6. Well and spring water. These vary the most. Sometimes they are excellent. Sometimes they carry damaging levels of iron, manganese, sulphur or fluoride, and they change with the seasons. Test before relying on them, then test again.

What to do: Choose your main source and a backup source from this list. Measure both with your TDS meter before you start the next cycle.

10. Two watering techniques that depend on water quality

Double watering. Water slowly until water starts to drain from the bottom (runoff). Wait at least 30 minutes; The Rev often waits several hours. Then water to runoff again. This stops water from channelling, which means running down a few paths through the pot and leaving dry pockets. In his experience, dry pockets often come before calcium problems. That is his observation, not measured data.

Bottom-tray reabsorption. Here the runoff collects in a tray and wicks back up into the pot. It helps on hot days and with large plants in small pots. Do not leave the pot standing in water for more than about 30–40 minutes.

Both techniques assume low-PPM water. With hard water, reabsorption puts minerals back into the pot and speeds up the salt build-up described in Part 1. If your water is hard, skip bottom-tray reabsorption and fix the water instead. Tray and wicking systems are covered in Self-Watering Pots for Living Soil.

Translator’s note: On 30–35 °C days, trays are tempting because pots dry out fast. Measure your water first. Use a tray only if the reading is low, and empty it after 30–40 minutes.

What to do: For the next two weeks, use double watering at every irrigation. Check by hand or by pot weight whether the whole pot is evenly moist.

Share your data below this topic. Please post three things:

  • the TDS reading of your source water;
  • the season you measured it in;
  • whether you have ever had a full lab analysis.

Very few growers in the region seem to have a full analysis. A lot of unexplained decline in the middle of a cycle may trace back to the water. If you have a lab report for your well or tap water, post the sodium and manganese figures. These are the two numbers most worth collecting across the region.

Legal note: This article gives technical horticultural information for readers acting legally where they live. Cannabis is regulated differently in every country and is still illegal in several countries in this region. In Thailand, cannabis flower is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568, in force since 26 June 2025. Activities involving it are subject to Ministry of Public Health licensing. Flower may be sold only against a PT 33 (ภ.ท.33) prescription, for a maximum of 30 days. Advertising and online sale of flower are prohibited, and recreational use is not permitted. The Cannabis Control Act is still a draft. The Cabinet approved it on 22 September 2026 and sent it to the House. Check your local law before growing. Nothing here is legal advice.

References

Further reading on Asiannabis