wich ppromix do you mean=? the plagron promix, is just peatmoss
and yes you have to ferttilise or mix your stuff into,,, peatmoss by itself is like a" swamp"
and has no nutrients
I did grow into and it worked welll, but i lke it more comfortabel, so i mixed it with my soilmix ( for fluffyness and drainage)
and soem dry amendment called " easy boost organic nutrition)
other wisee you have to grow into like soilless, ( coco) that means adding the nutes