There are lots of answers, but one of my favorite products to use is some form of humic+fulvic acid, and some form of microbe product. I'm really, really partial to real growers recharge and have been using it for almost a decade at this point, back when scotty was making everything in his garage. The whole idea behind adding some form of microbe is to unlock minerals and nutrients that may not be plant available, and the amino acids, humic and fulvic acids, on top of the mycorrhiza really work together to help use whats left and work towards the finish line. Im also really partial to KNF additives, but my personal favorite for working in inert media is Fermented Plant Extracts. What ive noticed with FPE is delicious flavor development and a kind of frostiness ive never seen before. and you use so little of it its really cost effective.