The original soil will only last around four weeks to five weeks before suplimental feeding is needed. You then need tomaintain the soil to keep it alive and healthy, alive with microbes and the like.
earthworm casts, compost teas, insect frass and fish emulshion will do this, along with things like comfrey liquid, borage liquid ,nettle tea and one of the best things for organic growth is animal shit liquid, where you half fill a hessian sack with animal manure and let it soak in a 55gallon drum of water.
once you get the feeding and general maintainance of the soil down to a t, then you will get great results.
but starting with smaller pots and working up to larger ones will also help you keep the soil nice n healthy.
yet another thing to remember is a bit of space in the top of the pot for topdressing..
but look into compost tea s and teas in general. Hope this helps a little