# Rockwool

> Rockwool is an inert growing medium spun from melted basalt rock into fibres, sold as plugs, cubes and slabs, that holds water and air in fixed proportions.

- Canonical: https://genetixbay.com/knowledge/glossary/rockwool.html
- Also written: rock wool, stonewool, stone wool, mineral wool, rockwool cubes
- 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"

Rockwool (stone wool) is made by melting basalt and chalk and spinning the melt into fine fibres, bound and formed into plugs, starter cubes, blocks and slabs. It is sterile when new, holds no nutrients, and has a very high pore volume, so it can be saturated and still hold a good fraction of air. That makes [water content](https://genetixbay.com/knowledge/glossary/moisture-content.md) highly controllable, which is why it dominates commercial crop steering.

## In practice

New rockwool tends to be alkaline. Pre-soak it in pH-adjusted nutrient solution (about [pH](https://genetixbay.com/knowledge/glossary/ph.md) 5.5-5.8 is common) before use. Plants start in small plugs, move to 10 cm (4 in) blocks, and finish on slabs. Feed with every irrigation, watch [dryback](https://genetixbay.com/knowledge/glossary/dryback.md) by weight or sensor, and avoid letting it dry right out, as rewetting unevenly creates dry pockets.

Fibres irritate skin and lungs; wear gloves and a dust mask when handling dry material.

## Don't confuse with

- **Slab**: the large rockwool bag a mature plant finishes on; one product format.
- **Insulation wool**: similar material, different density and binders; not for growing.

## Related terms

[Rockwool slab](https://genetixbay.com/knowledge/glossary/rockwool-slab.md), [Crop steering](https://genetixbay.com/knowledge/glossary/crop-steering.md), [Dryback](https://genetixbay.com/knowledge/glossary/dryback.md), [Drain-to-waste](https://genetixbay.com/knowledge/glossary/drain-to-waste.md), [Soilless](https://genetixbay.com/knowledge/glossary/soilless.md)

## Read more

- [Growing in rockwool (stonewool cubes and slabs)](https://genetixbay.com/knowledge/systems/rockwool.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