# Air-filled porosity

> Air-filled porosity is the share of a container's volume that holds air rather than water after it has drained, a key factor in root oxygen and overwatering risk.

- Canonical: https://genetixbay.com/knowledge/glossary/air-filled-porosity.html
- Also written: AFP, air porosity, air space, aeration porosity, air-filled pore space
- 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"

Air-filled porosity is the percentage of a medium's total volume occupied by air at [container capacity](https://genetixbay.com/knowledge/glossary/field-capacity.md), after free [drainage](https://genetixbay.com/knowledge/glossary/runoff.md). It sits alongside water-holding capacity: the pores in a pot are shared between water and air, and roots need both. Coarse materials such as [perlite](https://genetixbay.com/knowledge/glossary/perlite.md), bark, [coco chips](https://genetixbay.com/knowledge/glossary/coco-coir.md) and [clay pebbles](https://genetixbay.com/knowledge/glossary/clay-pebbles.md) raise it; fine peat and fine coco lower it.

## In practice

Container media for most crops perform well at roughly 10-30% air-filled porosity after drainage. Below about 10% roots are prone to oxygen shortage and [root rot](https://genetixbay.com/knowledge/glossary/root-rot.md); very high values dry quickly and need frequent irrigation. Pot height matters too: a short, wide pot holds a deeper saturated layer at its base relative to its volume, so the same mix has less air in a shallow pot than a tall one.

This is why adding perlite helps [overwatered](https://genetixbay.com/knowledge/glossary/overwatering.md) soil grows and why coco and [rockwool](https://genetixbay.com/knowledge/glossary/rockwool.md) can be irrigated many times a day.

## Don't confuse with

- **Total porosity**: all pore space, air plus water.
- **Water-holding capacity**: the water side of the same split.

## Related terms

[Field capacity](https://genetixbay.com/knowledge/glossary/field-capacity.md), [Perlite](https://genetixbay.com/knowledge/glossary/perlite.md), [Overwatering](https://genetixbay.com/knowledge/glossary/overwatering.md), [Coco coir](https://genetixbay.com/knowledge/glossary/coco-coir.md), [Rockwool](https://genetixbay.com/knowledge/glossary/rockwool.md)

## Read more

- [Growing in soil (potting mix with bottled feed)](https://genetixbay.com/knowledge/systems/soil.md)
- [Growing in coco coir](https://genetixbay.com/knowledge/systems/coco.md)

## Sources

1. 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