Short answer. Feed water at pH 6.2–6.8 for soil, 5.8–6.2 for coco and 5.6–6.0 for hydro. The accepted wider ranges are about 6.0–7.0 for soil and 5.5–6.5 for soilless and hydro, and drift within them is normal. Calibrate your pH pen with pH 7 then pH 4 buffers at least weekly, add nutrients before you adjust, change pH in small drops, and check runoff weekly. Wear gloves and eye protection with acids.
pH is a measure of how acidic or alkaline a solution is, on a 0–14 scale where 7 is neutral and each whole step is ten times stronger. For a grower it matters for one reason: root-zone pH controls how available each nutrient is [3]. These ranges are broadly agreed, so this guide doesn't hedge: get into the range, then stop fiddling.
What do you need to manage pH?
Managing pH needs a meter you trust and a way to move the number. That's it.
- [ ] A digital pH meter or pen with a replaceable glass probe, reading to 0.1 or 0.01
- [ ] Fresh pH 7.0 and pH 4.0 calibration buffers (pH 10 only if you work above 7)
- [ ] Probe storage solution (or pH 4 buffer) for the cap
- [ ] pH down: a dilute grower's acid, usually phosphoric or nitric
- [ ] pH up: usually potassium hydroxide or potassium carbonate
- [ ] A dropper or 1 mL syringe, a stirring rod, and a measuring jug or bucket marked in litres
- [ ] Nitrile gloves and safety glasses
- [ ] A small cup to catch runoff, and your grow journal
- [ ] Optional: your water supplier's water report, for alkalinity (water quality)
What pH should you aim for in each medium?
Aim for the sweet spot of your medium and accept anything inside the wider range. Soil buffers pH strongly, so it tolerates a higher, wider band; coco and hydro have little buffer, so they run lower and need closer checks [1][2].
| Medium | Sweet spot (feed water) | Accepted range | Check runoff | Notes |
|---|---|---|---|---|
| Soil / potting mix | 6.2–6.8 | 6.0–7.0 | Weekly | Organic soils self-buffer; adjust gently |
| Living soil | Often unadjusted water | 6.0–7.0 | Every 2 weeks | The soil does the work (living soil) |
| Coco coir | 5.8–6.2 | 5.5–6.5 | Weekly | Buffer new coco (buffering coco) |
| Rockwool | 5.8–6.2 | 5.5–6.5 | Weekly | Pre-soak at pH ~5.5 |
| DWC / hydro | 5.6–6.0 | 5.5–6.5 | Reservoir daily | Let it ride 5.5–6.2 (pH drift) |
How do you calibrate a pH meter, step by step?
Calibrate a pH meter in seven steps, at least weekly and before any session where a decision depends on the number [4].
- Rinse the probe with distilled or RO water and blot gently with tissue. Don't wipe the glass bulb.
- Pour a little fresh buffer into a clean cup. Never put the probe into the buffer bottle, and never pour used buffer back.
- Calibrate at pH 7 first. Put the probe in, wait until the reading is steady (usually 30–60 seconds) and press calibrate.
- Rinse again, then calibrate at pH 4. This two-point calibration sets the slope for the acidic range you work in.
- Check it. Put the probe back in pH 7: it should read 6.9–7.1. If not, repeat.
- Store it wet. Cap the probe with storage solution or pH 4 buffer. Never store it dry or in RO water, which ruins the glass.
- Log the date. If the meter won't adjust to the buffers or drifts during a reading, the probe is worn; most glass probes last one to two years (meter calibration).
How do you adjust pH, step by step?
Adjust pH in eight steps. The rule that saves most beginners: nutrients first, pH last.
- Fill the jug or reservoir with water at room temperature.
- Add all nutrients and supplements first, stirring well between each. Nutrients themselves change pH, often lowering it.
- Measure the pH after 30 seconds of stirring.
- Put on gloves and glasses before opening the acid or base.
- Add pH down or up a few drops at a time. A typical start is about 0.5–1 mL per 10 L (2.6 gal) of concentrated grower's acid; dilute products need more. Add acid to water, never water to acid.
- Stir for 30 seconds and re-measure. Repeat in small steps. Overshooting and correcting back adds salts and wastes time.
- Stop inside the target range. pH 6.3 in soil or 5.9 in coco is perfect; there's no prize for 6.00.
- Log the result, along with the EC (feeding basics). Next time you'll know roughly how much to add.
Safety. Concentrated phosphoric and nitric acids and potassium hydroxide burn skin and eyes. Wear nitrile gloves and safety glasses, work over a sink, keep them in labelled, closed containers out of reach of children and pets, and never mix acids with bleach or other cleaners. If you get any in your eye, rinse with running water for 15 minutes and seek medical help.
Why does pH drift, and when should you worry?
pH drifts because plants and water change it, and a slow drift within the range is normal. When plants take up nitrate, they release hydroxide or bicarbonate to stay electrically balanced, so reservoir and root-zone pH usually rises a few tenths a day in a healthy hydro system [5]. Let it ride within 5.5–6.2 rather than chasing one number.
Alkalinity, not water pH, is what pushes the root zone up over time. Bicarbonate in tap water acts as a buffer: water above about 100–150 mg/L alkalinity as CaCO3 keeps raising root-zone pH for weeks after you adjust the feed [1]. A tap water at pH 8 with low alkalinity is easy to fix; a pH 7.5 water with high alkalinity is not. Get your water report and see water quality.
Worry when:
- pH falls fast with no explanation, or rises with brown, slimy roots (root trouble; see root browning in hydro).
- Runoff pH sits outside the accepted range for two weeks running.
- New growth turns pale or yellow while the feed is complete, a classic sign of high-pH iron or manganese lockout (yellow new growth).
How do you check runoff pH?
Check runoff pH weekly in soil and coco with the pour-through method, because what drains out is closer to what the roots sit in than what you poured in [6]. Water normally, wait 30 minutes to 2 hours, pour about 0.3–0.5 L of pH-adjusted water on top, catch the first 50 mL that drains, and measure. Runoff within about 0.5 of your input pH is fine; a steady trend away from it over 2–3 weeks means you need to change the feed pH or the water (runoff testing).
How will you know it's working?
You'll know your pH management is working when the readings stay boring and the new growth stays green:
- Meter: reads 6.9–7.1 in pH 7 buffer after a week.
- Input pH: inside your target band every time, with similar acid doses from week to week.
- Runoff pH: within about 0.5 of input, with no trend over several weeks.
- Reservoir (hydro): drifts up a few tenths a day and you correct it once a day or less.
- Plant: new growth evenly green; no interveinal yellowing on top leaves; no rust spots appearing across many leaves.
What mistakes do beginners make with pH?
Beginners make pH harder than it is, mostly by trusting a meter that isn't calibrated.
- Never calibrating, or calibrating with old, contaminated buffer.
- Storing the probe dry or in RO water.
- Adjusting pH before adding nutrients, then finding it has moved.
- Chasing a single exact number and adjusting the reservoir several times a day.
- Adding too much acid at once, overshooting, then adding base, and building up salts.
- Blaming nutrients first. Check pH and the root zone before buying a supplement (how to diagnose any problem).
- Ignoring alkalinity in hard tap water.
- Using test drops or strips for decisions. They're only good to 0.5 units.
- Handling acids without gloves and glasses.
Evidence
- Accepted pH ranges for soil and soilless media: Established in extension and horticultural texts [1][2][3]; house position: broadly agreed.
- pH effects on nutrient availability: Established [3]; cannabis-specific nutrient uptake data is Supported [7].
- Two-point calibration and wet probe storage: Established [4].
- Nitrate uptake raising hydro pH; letting it drift in range: Established [5].
- Alkalinity as the driver of rising root-zone pH: Established [1].
- Pour-through runoff checks: Established [6].
