Pumice

Pumice is a porous volcanic rock, light enough to float briefly in water, that bonsai growers mix into soil to open up air space around the roots. Its rough, pitted surface gives roots something to grip and branch into, and because it does not break down as quickly as some clay components, it helps a mix keep its structure between repottings.

In a typical bonsai soil blend, pumice is combined with akadama and lava rock in proportions that shift with species and local climate. Conifers and trees prone to root rot in humid conditions often get a pumice-heavy mix, since it drains fast and dries out between waterings, while trees that dislike drying out get less of it relative to more water-retentive components.

Pumice also plays a role during root pruning and repotting, since a well-aerated mix encourages the fine, fibrous root growth that is easier to work with than a few thick roots grown in dense, compacted soil. Screening pumice to a consistent particle size before use, rather than using it straight from the bag, keeps a mix from clogging with dust-sized fragments.