1 Liter Perlite Perlite is a naturally occurring type of glass formed by volcanic processies. Its porous structure and ability to retain water make it a suitable medium to grow shrooms or cannabis plants The main reason why perlite is mixed into the soil or substrate mixture is to maintain humidity. perlite can also be a filter aid. Shayanashop also recommends the use of perlite in other soil mixtures, not just for mushrooms. It has proven very useful in cactus or cannabis soil mixtures, not just mushrooms.