How to Check and Adjust pH for Cannabis

3 min read

A grower testing the pH of nutrient solution with a digital pH pen

The Locksmith did everything right. Textbook feed, perfect ratios, premium nutrients to the millilitre. The plant showed magnesium deficiency, so he added magnesium. It got worse. Then it looked like calcium too, so he added CalMag. Worse again. He was feeding a balanced diet and the plant was starving in front of him — because he never once checked pH. The front door was locked and pH is the key.

The short version:

  • Most “deficiencies” are pH problems, not missing nutrients
  • Soil sweet spot: pH 6.0–7.0 (ideally 6.2–6.8). Coco/hydro: 5.5–6.5 (ideally 5.8–6.2)
  • Test your feed after mixing nutrients, and test the runoff
  • Adjust in small steps — you’re staying in a range, not hitting one number
  • Wrong pH locks nutrients out even when they’re right there in the pot

Want the full breakdown? Keep scrolling.

Why does pH matter so much?

Every nutrient has a pH range where the roots can actually absorb it. Drift outside that range and the nutrient binds up chemically and becomes unavailable — that’s lockout. The food is in the medium; the plant just can’t reach it. Worse, if you misread lockout as a deficiency and feed more, it accumulates as salt and starts locking out other nutrients too. That’s the Locksmith’s spiral. In soil, below 6.0 you lose calcium, magnesium and phosphorus; above 7.0 you lose iron, manganese and zinc. The 6.2–6.8 band is where everything overlaps.

How do I test pH?

Get a pH pen — DIG stock the Bluelab ones, they’re reliable and they last. Calibrate it monthly with buffer solution; an uncalibrated pen lies to you confidently. Two readings matter: your input water after you’ve added nutrients (nutrients shift pH, so test last), and the runoff that comes out the bottom of the pot. The gap between them tells you what’s happening in the root zone. Input at 6.5 and runoff at 5.2 means the root zone has drifted acidic — useful information, not an instant panic.

How do I adjust pH?

pH down is phosphoric acid; pH up is potassium hydroxide. A few drops at a time, stir, test again — small moves. You’re not chasing a precise figure, you’re staying in the band. A feed that goes in at 6.5 today and 6.3 tomorrow is fine; that gentle drift actually helps, because different nutrients peak in availability at different points in the range. Bounce gently between 6.2 and 6.8 in soil and everything gets its turn. DIG stock the CANNA pH+ and pH- if you need them. And the golden rule: before you diagnose any deficiency, check pH. Fix pH first, then wait — the problem often corrects itself.

FAQ

What pH should cannabis be in soil? 6.0–7.0, with 6.2–6.8 the sweet spot. In coco or hydro it’s lower: 5.5–6.5, ideally 5.8–6.2.

Do I test pH before or after adding nutrients? After. Nutrients change the pH of the water, so mix first, then test and adjust the final solution before it goes in.

My runoff pH is lower than my input — is that bad? It tells you the root zone is drifting acidic. Not an emergency on its own, but worth watching — persistent drift means it’s time to correct and possibly flush.

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