If you learn one number as a cannabis grower, make it pH. More plant problems are caused by incorrect pH than by incorrect nutrients, light, temperature, or watering combined. And the cruelest part: a pH problem looks like a nutrient deficiency, so growers add more nutrients (making it worse) instead of fixing the pH (the actual problem).
What pH is
pH measures how acidic or alkaline (basic) a solution is, on a scale from 0 to 14:
- pH 0–6.9: acidic (lower = more acidic)
- pH 7.0: neutral (pure water)
- pH 7.1–14: alkaline / basic (higher = more alkaline)
Cannabis likes slightly acidic conditions. The optimal range depends on your growing medium:
| Medium | Optimal pH range |
|---|---|
| Soil | 6.0–6.8 (sweet spot: 6.3–6.5) |
| Coco coir | 5.5–6.5 (sweet spot: 5.8–6.2) |
| Hydro (DWC) | 5.5–6.0 (sweet spot: 5.8) |
Why pH matters
Nutrients dissolve in water as ions. The availability of each ion depends on pH — at certain pH levels, some nutrients become chemically "locked out," meaning they're present in the solution but the roots can't absorb them.
- Below pH 5.5: calcium, magnesium, and phosphorus lock out
- Above pH 7.0: iron, manganese, zinc, and boron lock out
- At pH 6.0–6.5 (soil): all essential nutrients are available simultaneously
When pH is out of range, the plant shows deficiency symptoms even though the nutrients are there. Adding more nutrients doesn't fix it — it just increases salt concentration while the lockout persists. The ONLY fix is correcting the pH.
See our pH lockout guide for the detailed science.
How to measure pH
pH pen (digital meter)
The most practical tool for cannabis growers. Dip the probe in your nutrient solution, read the number on the screen.
Good pens:
- Apera PH20 (~$45) — reliable, accurate, long-lasting probe
- Bluelab pH Pen (~$80) — professional quality, replaceable probe
- Milwaukee pH600 (~$30) — budget, works fine if calibrated regularly
Calibration: calibrate your pH pen every 2–4 weeks using calibration solution (pH 4.0 and pH 7.0 buffer solutions). An uncalibrated pen drifts over time and gives false readings — the worst kind of wrong, because you trust it.
pH drops (liquid test kit)
A plastic vial with drops that change color based on pH. Cheap ($5), no calibration needed, but less precise (accuracy ±0.5 pH vs. ±0.1 for a good pen). Fine for beginners.
pH test strips
Paper strips you dip in the solution. Cheap but imprecise (color changes are hard to read accurately). Not recommended — spend the $30 on a digital pen.
What to measure
1. Feed pH (input)
After mixing your nutrient solution, measure the pH before watering the plant. Adjust up or down to hit the target range for your medium.
2. Runoff pH (output)
After watering, collect the runoff (water that drains from the bottom of the pot) and measure its pH. Runoff pH tells you what's happening inside the root zone.
| Runoff vs. feed | What it means |
|---|---|
| Runoff pH ≈ feed pH | Root zone is stable |
| Runoff pH lower than feed | Root zone is acidifying (common as nutrients break down) |
| Runoff pH higher than feed | Root zone is alkalinizing (common in some soils with lime) |
| Runoff pH below 5.5 | Lockout zone — flush with pH 6.5 water |
| Runoff pH above 7.0 | Lockout zone — flush with pH 6.0 water |
How to adjust pH
pH down (too high → lower it)
Products:
- General Hydroponics pH Down — phosphoric acid. The standard. A few drops per gallon is usually enough
- Citric acid — organic option. Works but degrades faster in the solution
Method:
- Mix your nutrients in the water first
- Measure pH
- Add 1–2 drops of pH Down per gallon, stir
- Measure again
- Repeat until in range
pH up (too low → raise it)
Products:
- General Hydroponics pH Up — potassium hydroxide. The standard
- Baking soda — sodium bicarbonate. Works in a pinch but adds sodium (not ideal long-term)
Method: same as above — add tiny amounts, stir, re-measure.
How much to add
pH adjusters are concentrated. Start with 1 drop (from a dropper) per gallon and work up. It's easy to overshoot — going from pH 7.0 to pH 5.0 with too much pH Down is worse than not adjusting at all.
Mixing order matters
Always add nutrients to water FIRST, then adjust pH LAST. Nutrients change the pH of the solution — if you pH first and then add nutrients, the pH will shift and you'll need to adjust again.
Correct order:
- Water (at room temperature)
- Silica supplement (if using — highly alkaline, add first)
- Base nutrients
- Cal-mag and other supplements
- pH adjustment (last)
Common pH problems
pH drift in reservoirs (hydro)
In DWC and recirculating hydro, pH naturally drifts upward over time as the plant absorbs acidic ions. Check reservoir pH daily and adjust as needed.
Soil buffering
Soil has buffering capacity — it resists pH changes. This is usually good (it protects against small errors) but means that if pH gets out of range in soil, it's harder to correct. You may need to water with adjusted pH water several times before runoff pH reaches the target.
Coco has no buffer
Coco coir has almost no buffering capacity. pH swings happen fast and hit the plant immediately. Always pH every feeding in coco — skipping once can cause problems.
Tap water pH
Tap water pH varies by location (6.5–8.5+). Always measure your tap water pH before adding nutrients, and measure again after mixing. Your tap water's pH has no direct bearing on the feed pH — the nutrients change it significantly.
The daily routine
For consistent pH management:
- Mix nutrients in water
- Measure pH
- Adjust to target range (6.3–6.5 for soil, 5.8–6.0 for coco/hydro)
- Water the plant
- Check runoff pH once per week (or more in coco)
Total time: 2 minutes. This single habit prevents more problems than any other practice in cannabis growing.
Common mistakes
- Not checking pH at all — "I just use tap water and nutrients, it's probably fine." It's probably not. Tap water ranges from pH 6.5–8.5+, and nutrients shift it further
- Checking pH before adding nutrients — nutrients change the pH of the solution. Always add nutrients first, pH adjust last
- Chasing exact numbers — pH 6.3 and pH 6.5 produce the same results. Don't over-adjust. Anywhere in the target range is fine
- Using litmus strips or color drops for precision — these tools give a rough estimate (±0.5). A digital pen gives ±0.1. The difference matters when the target range is only 0.8 units wide
- Not calibrating the pH pen — an uncalibrated pen can read 0.5+ off. Calibrate monthly with pH 4.0 and 7.0 buffer solutions. A wrong reading is worse than no reading — it sends you in the wrong direction
For the science of pH lockout and nutrient availability, see our pH lockout guide. For water quality beyond pH, see our water quality guide. For nutrient management, see our feeding guide.