The pH of water for cannabis should be 6.0–7.0 in soil (ideally 6.2–6.8) and 5.5–6.5 in coco and hydroponics (ideally 5.8–6.2). Always measure and adjust it after the nutrients have been mixed in, using a calibrated pH meter and small doses of pH down or pH up. Outside these ranges the nutrients stay in the growing medium, but the plant absorbs them poorly and shows deficiencies even when the feeding is right.
In brief
- Soil: pH 6.0–7.0; coco, rockwool and hydroponics: pH 5.5–6.5.
- The right order: water, nutrients, stir, measure, adjust.
- Calibrate a digital pH meter every 2–4 weeks with pH 7.0 and 4.0 buffer solutions.
- Tap water in Ticino is usable: what matters is its hardness in °fH, which varies from one municipality to the next.
- Runoff pH shows whether the growing medium is drifting: check it every 1–2 weeks.
pH levels for soil, coco and hydroponics
The right range depends on the growing medium. Soil contains organic matter and minerals that act as a buffer, so the pH of the water can vary a little without consequences. Coco, rockwool and hydroponic solutions buffer very little: the pH you give is almost exactly what the roots get.
| Growing medium | Range | Recommended value | Check |
|---|---|---|---|
| Soil and potting mix | 6.0–7.0 | 6.2–6.8 | every batch of nutrient solution |
| Coco coir | 5.5–6.5 | 5.8–6.2 | every watering |
| Rockwool | 5.5–6.5 | 5.8–6.2 | every watering |
| Hydroponics (DWC, NFT, recirculating) | 5.5–6.5 | 5.8–6.2 | daily in the reservoir |
Why pH matters: nutrient availability
pH does not feed the plant, but it determines the form in which nutrients are present in the water and the growing medium. If the pH is too high (above 7.0 in soil, above 6.5 in coco and hydro), iron, manganese, zinc and phosphorus become less available: new leaves turn yellow between the veins and growth slows. If the pH is too low (below 6.0 in soil, below 5.5 in coco and hydro), calcium and magnesium uptake drops and spots and yellowing appear on middle and older leaves.
That is why many apparent deficiencies are really pH lockouts: measure before adding more fertiliser. The symptoms for each nutrient are covered in the guide to cannabis nutrient deficiencies.
How to measure pH: drops, strips or a meter
- Indicator drops: cheap and fine for a rough check; the colour is compared against a chart in steps of about 0.5. Dark nutrient solutions make them hard to read.
- Test strips: only useful as a rough check, because the colours between 5.5 and 6.5 are hard to tell apart.
- Digital pH meter: 0.1 resolution and a quick reading; the right tool for coco and hydro, provided it is calibrated and the electrode is kept moist.
For a digital meter: calibrate every 2–4 weeks with pH 7.0 and 4.0 buffer solutions (more often with daily use), rinse the electrode after every reading and store it in storage solution (KCl), never in distilled water. An electrode usually lasts 1–2 years: if the reading stays slow or unstable after calibration, replace it. Combined meters also measure EC, the concentration of nutrients; you will find them among the pH and EC meters and controllers.
Adjusting pH: after the nutrients, in small steps
- Fill the container and let the water reach 18–22 °C.
- Add the nutrients one at a time (any Cal-Mag first), stirring after each one.
- Measure the pH and EC of the finished solution.
- Add pH down or pH up in small doses, stir, wait 1–2 minutes and measure again.
- Write down how much adjuster you used: with the same water and recipe, you will need a similar amount next time.
pH down usually contains phosphoric or nitric acid, pH up potassium hydroxide or potassium carbonate: both are concentrated, corrosive products. Wear gloves and goggles, always add the adjuster to the water and never the other way round, and keep it out of children's reach. Mineral nutrients usually lower the pH on their own, so you often need less pH down than expected; pH up is mainly needed with very soft or reverse osmosis water. For choosing and dosing fertilisers, see the guide to cannabis nutrients.
Tap water in Ticino: hardness, limescale and chlorine
Swiss mains water usually has a pH between 7 and 8 and is fine for growing. More important than the starting pH is hardness, meaning how much calcium and bicarbonate it contains: hard water buffers, needs more pH down and pushes the pH back up; very soft water changes pH easily and supplies little calcium and magnesium.
In Ticino hardness varies a lot between municipalities: in areas of crystalline rock, which covers much of the Sopraceneri, the water is often soft, while it is harder where limestone dominates. The exact figure, in French degrees (°fH), is published by your municipality's water supplier.
| Hardness | °fH | What to do |
|---|---|---|
| Soft | below 15 | pH down in small doses; Cal-Mag often needed in coco and under LED |
| Medium | 15–25 | normal adjustment with pH down |
| Fairly hard to hard | above 25 | more pH down, check that the pH does not creep back up; consider mixing with reverse osmosis water |
In Switzerland mains water often contains no chlorine, or only minimal amounts. If you can smell it, leave the water in an open container for 24 hours, which also brings it to room temperature. Reverse osmosis water is almost free of salts and has an unstable pH: always remineralise it with Cal-Mag or blend it with tap water.
Runoff and growing medium pH
The pH you give is not always the pH around the roots. Every 1–2 weeks, collect the water that drains from the bottom of the pot after a normal watering and measure its pH and EC.
- Runoff within 0.5 points of the input pH: all is well.
- Runoff consistently higher (for example above 7.0 in soil): for a few waterings, bring the input towards the lower limit, 6.0–6.2, then check again.
- Runoff consistently lower (for example below 5.8 in soil): bring the input towards 6.6–6.8 and check the EC; if it is much higher than the input, consider flushing the roots.
For a more accurate check of the growing medium, use the 1:2 method: one part growing medium to two parts distilled water, stir, wait at least 30 minutes and measure the filtered liquid. Always correct gradually: sudden pH swings stress the roots more than the original problem. How much and how often to water is explained in the guide on how often to water cannabis.
Frequently asked questions
What pH should water be for soil?
Between 6.0 and 7.0, ideally 6.2–6.8. A good potting mix buffers small deviations: avoiding values that stay out of range matters more than hitting the exact decimal.
Should pH be adjusted before or after adding nutrients?
After. Nutrients change the pH of the water, usually lowering it: measure the finished solution and adjust only at the end, in small steps.
Is tap water OK?
In most Swiss municipalities, yes. Ask your water supplier for the hardness (°fH), measure pH and EC and adjust accordingly: hard water needs more pH down, very soft water often needs Cal-Mag.
What happens if the pH is too high?
Iron, manganese, zinc and phosphorus become poorly available: new leaves yellow between the veins and growth slows. Bring the pH back into range gradually and judge only the leaves that grow after the correction.
Containers, pumps and accessories for mixing your solution are in the Gro.w.LaB Water and irrigation collection.
Updated on 13 October 2026.