Two potting mixtures generally recommended for ferns:
1 part sterilized fibrous soil
1 part medium grade peat moss (lite colored type), ground tree bark, or leaf mold
(Leaf mold is made of decayed leaves, and is rich in organic matter. Like soil, leaf mold has bacterial activity in it, and therefore contains some nutreients. Beech and oak leaf molds are excellent... pine needles are useful for lending texture to the mix)
1 part coarse sand or perilite
Add a balance granular fertilizer according to package instructions to the mix.
Best mix for ferns...High-humus peat based mixture;
3 parts coarse peat moss (not ground or dark in color)
3 parts leaf mold
2 parts coarse sand or perilite
Add 1 cup of charcoal granuals ( helps keep the mixture from going "sour") to 2 quarts of your mix, and follow fertilizer package instructions.
Generaly, most packaged potting soils available at your garden center is good and you can add leaf mold and or soil to it.
Most ferns are not deep rooted. The few that do have deep roots (large blechums (Brazil tree fern, indoor), or Interupted fern, outdoor, if potting them for example) need to be potted in standard containers, but shallow-rooted ferns do much better in shallow pans (bulb pot, or a mum pot... a diameter 2-3 times its depth) or half containers, where there is room for their shallow roots to spread out. In shallow pots, moreover, there is less chance for the potting mix to become compacted thus blocking good drainage.
Repot a fern only when the roots have completely filled the pot or when a creeping rhizomes (surface roots) has covered the potting mix and is begining to spread beyond the rim of the pot. Under ideal conditions of light, warmth and watering, and feeding, ferns may require repotting every six to seven months. In less than ideal conditions they can remain contented in the same pot for several years. If repotting becomes neccessary, try to do it in the spring or early summer months, and repot into a container only one size bigger. It is never a good idea to overpot since it causes overwatering and the root system may suffer.
After reaching maximum convenient pot size for you and your fern, the plant is best divided or used in some other way for propagation. Your fern can be kept intact, however, if the root ball is trimmed by a third or half. The fern can then be returned to its pot...Which has been cleaned and filled with fresh potting mix.