...is just basically volcanic rock that has been expanded (and made sponge-like) with heat. Use it to help 'fluff-up' normal potting soil--our favourite Weed likes aerated soil...and wet it beforehand to help keep the dust down. The dust is Not Good For Your Lungs.

I use it with both soil applications, as well as hydroponic ones (though I try to separate out and use the coarser stuff in those cases--you don't want tiny little bits of volcanic glass going thru your pumps on a regular basis).

Shultz Horticultural Perlite... trusted by Gardeners for a Mazillion Years...
(only five bux per bag at WM!)