Vermiculite is an aluminum-iron-magnesium silicate that resembles mica in appearance. For use in horticultural applications, vermiculite is heated to expand the particles. This expansion enables it to absorb moisture when used as a potting medium. Vermiculite can soak up 3 to 4 times its volume in water. It also attracts plant nutrients such as potassium, magnesium, calcium and phosphorus.
Perlite and vermiculite are both good at retaining water, but vermiculite acts more like a sponge, holding much more water than perlite and offering less aeration for the plant roots.