# pH

> pH measures how acidic or alkaline a solution is on a 0–14 scale; in growing it controls nutrient uptake, with soil best near 6.0–7.0 and hydro 5.5–6.5.

- Canonical: https://genetixbay.com/knowledge/glossary/ph.html
- Also written: ph level, potential of hydrogen, acidity, pH value
- Glossary category: pH, EC and water · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

pH is the negative base-10 logarithm of the hydrogen ion activity in a solution. Low pH is acidic, high pH is alkaline, and 7 is neutral at 25 °C. In the [root zone](https://genetixbay.com/knowledge/glossary/root-zone.md), pH controls nutrient solubility and uptake: iron, [manganese](https://genetixbay.com/knowledge/glossary/manganese.md) and zinc become less available as pH rises, while calcium, magnesium and [molybdenum](https://genetixbay.com/knowledge/glossary/molybdenum.md) are less available when pH falls too low.

## In practice

Accepted ranges, which GenetixBay treats as settled practice:

| Medium | pH |
|---|---|
| Soil | about 6.0–7.0 |
| [Coco](https://genetixbay.com/knowledge/glossary/coco-coir.md), [rockwool](https://genetixbay.com/knowledge/glossary/rockwool.md) and hydro | about 5.5–6.5 |

Drift within the range is normal and even helpful: a solution that wanders from 5.6 to 6.2 over a few days exposes roots to the best uptake window for different elements. Correct only when it leaves the range. *Established*.

Measure pH of your input solution after adding nutrients, because fertilisers shift it. In soil, the medium buffers pH strongly, so the input matters less than in hydro. Check [runoff](https://genetixbay.com/knowledge/glossary/runoff.md) occasionally to see what the root zone is doing.

## Units and formula

pH = −log₁₀[H⁺]. A change of 1 unit means a 10-fold change in hydrogen ion concentration.

## Don't confuse with

- **Alkalinity**: the water's capacity to resist pH change, not the pH itself.
- **EC**: total dissolved salts; unrelated to acidity.

## Myth check

"pH must be held at exactly 5.8" is a *Myth*. A range is the target; see [pH and nutrient availability](https://genetixbay.com/knowledge/ph-ec/ph-nutrient-availability.md).

## Related terms

[pH scale](https://genetixbay.com/knowledge/glossary/ph-scale.md), [pH drift](https://genetixbay.com/knowledge/glossary/ph-drift.md), [Nutrient lockout](https://genetixbay.com/knowledge/glossary/nutrient-lockout.md), [pH meter](https://genetixbay.com/knowledge/glossary/ph-meter.md), [Alkalinity](https://genetixbay.com/knowledge/glossary/alkalinity.md)

## Read more

- [pH and nutrient availability: what pH really controls, by medium](https://genetixbay.com/knowledge/ph-ec/ph-nutrient-availability.md)
- [pH drift in hydro: why reservoir pH rises (and sometimes falls)](https://genetixbay.com/knowledge/ph-ec/ph-drift-hydro.md)

## Sources

1. Marschner P (ed.). 2012. Marschner's Mineral Nutrition of Higher Plants, 3rd ed. https://doi.org/10.1016/C2009-0-63043-9
2. Bugbee B. 2004. Nutrient management in recirculating hydroponic culture. Acta Horticulturae 648:99-112. https://www.ishs.org/ishs-article/648_12