Mix your pro mix with compost, I usually do something like 2 parts promix, 1 part commercial compost, 1 part mushroom compost. Add in your dry organic nutrients. I've ben using the same soil for about 12 yrs now, once a year I'll put it all in a kiddie pool and add in new compost/amendments. Seems to work for me... I also use sub irrigated containers and never let the soil get dry, even when not in use. I also have extra containers so soil can 'rest' between cycles.