Pumice
Also known as Pumice stone
In short
Pumice is a light, frothy volcanic rock full of gas bubbles. Crushed into small grains it is a popular, long-lasting mineral additive for succulents, cacti and bonsai because it drains freely yet holds some water in its pores.
How it looks
Light-coloured grains or stones, usually white, cream or grey, sometimes greenish-brown or darker, with a rough, bubbly texture. Porosity can be 64–85% by volume. Dry pumice is light enough to float, though it eventually waterlogs and sinks. Horticultural grades are commonly about 3–10 mm (1/8 to 3/8 inch).
Signs
Mixes with enough pumice drain quickly and dry evenly; too little leaves a heavy, slow-drying mix. Because it is mineral, it does not break down, so a mix rich in pumice keeps its structure between repottings.
What favours it
Pumice forms when gas-rich lava is blasted from a volcano and cools quickly, freezing the bubbles in place. It is chemically about pH-neutral.
Prevention
- Rinse dusty pumice before use.
- Use grain sizes suited to the pot: finer grains for small containers, larger grains for beds.
- Match the proportion to the plant: more pumice for cacti and other water-storing succulents, less for thin-leaved plants.
Treatment
Start with the first step and move down only if the problem persists.
- To speed drying in a heavy succulent mix, repot into a mix with a larger share of pumice or similar grit.
Pros and cons
Pros
- Does not decompose or compact.
- Heavier than perlite, so it is less likely to float to the surface.
- Holds some water in its pores; one succulent grower reports it staying moist about twice as long as bark.
- pH-neutral and suitable for many plants.
Cons
- Mined, non-renewable material that can be hard to find or expensive in regions without local deposits.
- Heavier than perlite, adding weight to large pots.
- Holds no nutrients of its own, so plants need feeding.
Look-alikes
- Perlite
- Perlite is bright white, lighter and crumbles between the fingers.
- Lava rock (scoria)
- Scoria is dark red, brown or black, heavier and sinks in water.
Typical uses
Cacti, succulents, bonsai and other plants that need fast drainage; also used as a general mix additive.
Related
Sources
- Pumice — Wikipedia
- About Pumice: The Ideal Soil Amendment for Succulents — Debra Lee Baldwin
- Bonsai cultivation and care — Wikipedia
Written in our own words from these sources. Check a product label before using any treatment.