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Perlite
A form of volcanic ore
Perlite is a volcanic ore that, when heated, pops open like pop corn. Perlite doesn't absorb water (it holds it on its surface) so it hurts your mixes total water retention.
What it does do, however, is make more air-filled pores, so you can be sure roots are getting enough oxygen. It's light. It's nice. And you can use it as mulch `cause it's pretty.