# pH scale

> The pH scale runs from 0 (strongly acidic) to 14 (strongly alkaline) with 7 neutral; it is logarithmic, so each whole unit is a tenfold change in acidity.

- Canonical: https://genetixbay.com/knowledge/glossary/ph-scale.html
- Also written: pH range, 0-14 scale, acid-base scale, logarithmic pH scale
- 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"

The pH scale expresses [acidity](https://genetixbay.com/knowledge/glossary/ph.md) and [alkalinity](https://genetixbay.com/knowledge/glossary/alkalinity.md) of water-based solutions, usually from 0 to 14 at 25 °C. Values below 7 are acidic, 7 is neutral and above 7 is alkaline. The scale is logarithmic: pH 5 has ten times the hydrogen ion activity of pH 6, and a hundred times that of pH 7.

## In practice

The logarithm is why small numbers matter. Moving a hydro reservoir from 6.0 to 7.0 is not a small nudge; it is a tenfold drop in hydrogen ions, enough to make iron and [manganese](https://genetixbay.com/knowledge/glossary/manganese.md) markedly less available. It is also why "a little" pH down can overshoot in unbuffered water: [RO water](https://genetixbay.com/knowledge/glossary/ro-water.md) has almost nothing to resist the change.

Reference points: lemon juice about 2, rainwater about 5.6, pure water at 25 °C 7.0, many [tap waters](https://genetixbay.com/knowledge/glossary/tap-water.md) 7–8.5, baking soda solution about 8.3, household ammonia about 11. Cannabis [root zones](https://genetixbay.com/knowledge/glossary/root-zone.md) are kept at roughly 5.5–7.0 depending on medium.

## Units and formula

pH = −log₁₀[H⁺]; [H⁺] = 10^(−pH) mol/L.

## Don't confuse with

- **pH**: the measured value; the scale is the system.
- **Buffering capacity**: how hard it is to move along the scale.

## Related terms

[pH](https://genetixbay.com/knowledge/glossary/ph.md), [pH meter](https://genetixbay.com/knowledge/glossary/ph-meter.md), [Alkalinity](https://genetixbay.com/knowledge/glossary/alkalinity.md), [pH buffer](https://genetixbay.com/knowledge/glossary/ph-buffer.md)

## Read more

- [pH and nutrient availability: what pH really controls, by medium](https://genetixbay.com/knowledge/ph-ec/ph-nutrient-availability.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