What are perlite, vermiculite and grit for?

All three are inert additives mixed into compost to change its physical structure, and they do genuinely different things — which is why substituting one for another sometimes fails.

Perlite is volcanic glass heated until it pops like popcorn, producing lightweight white granules full of sealed air pockets.

What it does: improves drainage and aeration. It holds very little water internally, instead creating air spaces between compost particles so water drains through and roots get oxygen.

Use it for: cactus and succulent mixes, cuttings, seed compost, and any container mix that stays too wet. Typically 10–30% by volume.

Note: it floats to the surface over time, and the dust is an irritant — dampen it before use.

Vermiculite is a heated mica mineral that expands into layered, spongy flakes.

What it does: retains water and nutrients. It is the opposite of perlite in this respect — it absorbs several times its weight in water and releases it gradually, and it has cation exchange capacity, meaning it holds nutrients rather than letting them leach.

Use it for: seed sowing — a thin layer over seeds keeps them evenly moist while still admitting light, which is why it is the standard covering for surface-sown seed. Also for moisture-loving plants and for propagation.

Grit — horticultural grit or coarse sand — is heavy, inert stone.

What it does: drainage plus weight. It sheds water fast and adds ballast, which perlite cannot. A tall pot with a top-heavy plant in a light peat-free mix blows over; grit prevents it.

Use it for: alpines, Mediterranean plants, bulb pots, and as a surface mulch to keep plant collars dry and deter fungus gnats.

Important: use horticultural grit, not builders' sand, which is fine, often contains lime and salt, and clogs rather than drains.

The common mistake is using vermiculite where drainage is needed — it makes the mix wetter.