I use miracle grow for one and also osmocoat (I may have spelled that wrong lol) but one thing my ferns respond well to is Epsom salt. I usually just sprinkle it on them then water it in. Also as far as a planting medium I add about three bags of the special orchid bark blend generally to one large ( 2 cubic foot) bag of a good potting mix. This keeps the ferns well drained and happy. Here in SC during the summer I let them hang out under the trees and they do really well for me. I hope this helps
