Right- worm castings are an additive, but cannot be used as a soil base. Start with a good base like potting mix or even promix, and you can add low levels of additives like worm castings or kelp compost or whatever, but if you intend to run chemmy ferts, I'd stick with a very plain soil and just add a little extra perlite to it, and remember to only pot up to slightly larger pots, and do it on a regualr schedule every 4-6 weeks.