Perlite

Perlite is volcanic glass expanded with heat into light, white granules full of air pockets. Mixed into soil it does one job extremely well: it keeps the mix open so water drains away and air can reach the roots.

It is what stops a potting mix from compacting into a dense block by the end of the season, and it is the standard fix for houseplants that keep rotting off at the base. Cuttings root faster in it too, because the airy structure encourages new roots without holding excess water.

A common blend is one part perlite to three or four parts potting mix, and more for cactus, succulents and anything that dislikes wet feet. It is sterile, pH neutral, and does not break down over time.