Q: About perlite, pumice, lava (scoria), and gravel.


Aggregates A: In addition to the standards of sand, peat, and Sphagnum, some people include perlite, pumice, and gravel in their planting mixes. These are called aggregates. They all serve the same purpose as sand of providing drainage, but have slightly different attributes.

Perlite is a light, crumbly, white rocky material. It is very nice if you want to provide good drainage to a mix--it does not retain water. A pot full of perlite is very light! It is white, so reflects sun well---a useful consideration if you have pots in full sun and they are overheating.

In contrast, pumice is denser and is a nice alternative to perlite if you want some ballast in the pot. This is important if you are growing a tall plant like Heliamphora which might topple a pot filled with a light perlite mix.

Lava is similar to pumice, and is often red or black. From a geologist's perspective, this is better called scoria, and has slightly different compositional differences from pumice. I do not know if it has any long-term consequences in a mix, as compared to pumice. However, experiments I have done on it indicate that the mass of a pot of pure dry pumice is less than the mass of dry scoria. Meanwhile, when fully wetted and then allowed to drain, the pot of moistened pumice holds more water than moistened scoria.

Gravel is denser still, and makes a very heavy medium if it constitutes a significant fraction of the soil mix.

You should wash any of these products before using them to remove the fines (tiny particulates). See my instructions on washing sand. Do not breathe the dust from these products as they could cause lung ailments.

Page citations: Rice, B.A. 2006a; personal observation.