medium means, the stuff where you grow your plants in: soil, rockwool, ..... You can mix perlite in your soil, it is wellknow that perlite makes your soil lighter and it holds water
Perlite is used to prevent soil from compacting.
It doesn't hold water, that's vermiculite's job.
Most potting soil comes with perlite added. It's those small white "rocks" in the dirt.
huhum, perlite hodls water, why do you think they use it for growing mushroom? cause it hols water, just like vermiculite, by the way, vermiculite is asbeste, so can cause cancer.
Perlite's job is to enable the plants' roots to access oxygen easily.
And it's recommended that you use gloves and a particulate/painter's mask when handling vermiculite AND perlite.