Perlite, to improve aeration in compost
Perlite is a natural mineral that is heated to very high temperatures, until it expands and pops and ends up as light, white, porous beads that looks much like styrofoam. I mix it in my sowing compost to make it extra light and airy. I use one third perlite to two thirds compost.
The airy cavities provided by the perlite helps drain the compost and supply air to the roots.