Perlite

Perlite is crushed volcanic rock, which has been heated until it explodes into small white pieces. It has a medium capacity to hold water, which still allows for very good drainage, and is therefore used in soil mediums where very good drainage is necessary, such as for cactus soil or soil-less propagation mediums. Perlite also improves insulation of plant roots against extreme fluctuations of temperatures. Its does not do a good job of retaining nutrients.