Perlite is used as a standalone growth medium, as well as a way to enhance soil mixes. Made from an amorphous volcanic glass, it has a relatively high water content.
 
The tiny channels suck water and nutrients up from a reservoir below into the upper layers of the medium, whilst maintaining drainage to keep air around the roots.
 
Lightweight, sterile and incredibly effective as a growing medium.
Also for mixing in soil or other composts to improve drainage.
Often mixed together with vermiculite to make a cheap seedling and cutting medium