Cannabis pH: what the research actually says

The short answer: aim for a substrate pH of 6.5. That’s the number that produced the most growth in the only controlled study of substrate pH in Cannabis sativa. Anywhere from 5.5 to 6.5 is fine. Below 5.0 the plant slows down; at 7.1 root mass drops off.

That is a wider, more forgiving range than almost any growing guide will tell you — and the reason those guides give tight windows turns out not to be supported by the research.

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The number everyone repeats, and where it came from

Search for cannabis pH and you’ll get confident, narrow ranges: soil 6.2–6.8, coco 5.8–6.2, hydro 5.5–6.0. Drift outside and you’re warned about nutrient lockout — specifically that low pH makes iron and manganese over-available and poisons the plant.

Most of those pages cite nothing. Several cite a “Bevan et al. (2021)” study from NC State. We went looking for it and couldn’t find it; none of the sites citing it link to it. If it exists, it isn’t where they say it is.

What does exist is a 2025 paper from the same university, and it is the first published work on this question.

There is currently no published literature on micronutrient rates or the impact of substrate pH on the micronutrient uptake of C. sativa.
— Veazie et al., 2025, stating the gap their study was designed to fill

Everything written about cannabis pH before February 2025 was extrapolated from other greenhouse crops, or from growers comparing notes. Some of it holds up. One central piece of it doesn’t.

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What the study found

Veazie, Cockson, Smith, Schulker, Jackson, Hicks & Whipker (2025). “Impact of substrate pH and micronutrient fertility rates on Cannabis sativa.” Agrosystems, Geosciences & Environment 8(1), e70044. Open access. No conflicts of interest declared.

Three experiments in 80% sphagnum peat / 20% perlite, pH adjusted with dolomitic and hydrated lime.

Substrate pH	What happened
2.97	Shoot and root growth clearly reduced
4.17	Still reduced relative to ≥5.0
4.94	Growth recovers — the floor is around here
5.93	Good
6.5	Greatest plant growth
6.97 / 7.1	Root mass drops off again

Their stated recommendation: 5.5–6.5, with 6.5 performing best.

The part that contradicts the internet

The standard warning is that acidic conditions make micronutrients over-available and cause iron and manganese toxicity. That’s true of greenhouse crops generally. It appears not to be true of cannabis at normal feed rates:

“C. sativa does not accumulate micronutrients to toxic levels at low pH when micros are applied within normal growing ranges, but growth is inhibited at substrate pH < 5.0.”

Leaf tissue micronutrient concentrations were highest at the lowest pH — the chemistry works as expected — but the plants didn’t reach toxic levels. The researchers pushed harder: at pH 3.8 with four times the normal micronutrient rate, one cultivar showed toxicity symptoms on lower foliage and the other didn’t.

So the problem with low pH is real, but it isn’t poisoning. It’s that the plant simply grows less. And the susceptibility that does exist varies by cultivar, which is why one grower’s horror story doesn’t transfer to your tent.

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Three things this study does not tell you

Being straight about the limits is the difference between a source and a blog post.

1. It was conducted on hemp, not drug-type cannabis. The cultivars were Baox and T1 (experiment 1) and Cherry Wine and Sweetened (experiment 3) — all ≤0.3% THC by legal definition. Same species, same physiology, so the findings should carry. They have not been directly tested on high-THC cultivars.

2. It was peat-based substrate only. No coco, no hydro, no living soil. The familiar “coco 5.8–6.2, hydro 5.5–6.0” figures are not from this study, and no equivalent study exists for those media. They remain grower convention — which isn’t worthless, but isn’t evidence either.

3. It measures substrate pH, not your input pH. See below, because this is the most common mistake in the whole topic.

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Substrate pH is not the pH of what you pour in

Most guides blur this, and it causes more confusion than any other point.

• Input pH is the pH of your water or nutrient solution going in.
• Substrate pH is the pH in the root zone, which is what the plant actually experiences — and what the research above measured.

They are not the same number. Peat is naturally acidic and buffered with lime; coco holds cations and releases them; organic matter resists change. The medium transforms your input.

Practically: in peat or soil, pH your input to roughly 6.0–6.5 and check what the root zone is actually doing. Measure runoff, or better, use the pour-through method — collect what drains after a normal watering and test that. A single input reading tells you what you did, not what the plant got.

In inert media (coco, rockwool, pure hydro) the gap narrows because there’s little buffering, which is why those systems need more frequent checking rather than more precise numbers.

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What to actually do

1. Target 6.0–6.5 at the root zone. Based on the only controlled data available, 6.5 is where growth peaked.
2. Stop panicking about small drift. The research floor is around 5.0 and the ceiling around 7.0. A reading of 6.8 is not an emergency.
3. Check runoff, not just input. Input pH tells you what you added. Runoff tells you what’s happening.
4. If you’re below 5.0, raise it — but expect slowed growth, not burnt leaves. The symptom is a plant that isn’t doing much, not visible toxicity.
5. Don’t chase a deficiency with more feed. Low pH already raises tissue micronutrient levels. Adding more of something the plant can’t use is how you turn a pH problem into two problems.
6. Expect cultivar variation. The study found two cultivars responding differently under identical conditions.

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