# Cation exchange capacity

> Cation exchange capacity (CEC) is a medium's ability to hold positively charged nutrients such as calcium, magnesium and potassium on its surfaces and release them later.

- Canonical: https://genetixbay.com/knowledge/glossary/cation-exchange-capacity.html
- Also written: CEC, cation exchange, exchange capacity, cation-exchange capacity
- Glossary category: Systems and media · 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"

Cation exchange capacity is the total amount of exchangeable positive ions (cations) that a medium can hold on negatively charged sites on clay, organic matter and organic fibres. Held cations, such as Ca²⁺, Mg²⁺, K⁺, NH₄⁺ and Na⁺, are not washed straight out; they swap with others in the soil solution, giving a nutrient reserve and buffering against sudden change.

## In practice

High-CEC media (clay soils, compost-rich soil, peat, [vermiculite](https://genetixbay.com/knowledge/glossary/vermiculite.md)) are forgiving: they hold nutrients between feeds and resist [pH](https://genetixbay.com/knowledge/glossary/ph.md) swings. Low-CEC media ([rockwool](https://genetixbay.com/knowledge/glossary/rockwool.md), [perlite](https://genetixbay.com/knowledge/glossary/perlite.md), [clay pebbles](https://genetixbay.com/knowledge/glossary/clay-pebbles.md), sand) hold almost nothing, so the plant sees exactly what is in the solution. [Coco](https://genetixbay.com/knowledge/glossary/coco-coir.md) sits in between and has a characteristic preference: its sites load with potassium and [sodium](https://genetixbay.com/knowledge/glossary/sodium.md) during processing and tend to grab calcium and magnesium from feed, which is why coco is buffered with calcium before use.

## Units and formula

CEC is measured in centimoles of charge per kilogram (cmol(+)/kg), equivalent to milliequivalents per 100 g (meq/100 g). Sandy soils are often under 10; clay- and humus-rich soils 20-40 or more.

## Don't confuse with

- **Buffering (pH)**: resistance to pH change; related but not the same quantity.
- **EC**: dissolved salts in solution; CEC is nutrients held on solids.

## Related terms

[Coco coir](https://genetixbay.com/knowledge/glossary/coco-coir.md), [Buffering capacity](https://genetixbay.com/knowledge/glossary/buffering-capacity.md), [Peat](https://genetixbay.com/knowledge/glossary/peat.md), [Soil](https://genetixbay.com/knowledge/glossary/soil.md), [Vermiculite](https://genetixbay.com/knowledge/glossary/vermiculite.md)

## Read more

- [Buffering coco coir: why it matters and how to do it](https://genetixbay.com/knowledge/ph-ec/buffering-coco.md)
- [Growing in soil (potting mix with bottled feed)](https://genetixbay.com/knowledge/systems/soil.md)

## Sources

1. Brady NC, Weil RR, 2017. The Nature and Properties of Soils, 15th ed. Pearson.
2. Raviv M, Lieth JH, Bar-Tal A (eds), 2019. Soilless Culture: Theory and Practice, 2nd ed. Elsevier. https://doi.org/10.1016/C2015-0-01807-5