Ever wonder how the ferns in those hanging baskets became so large? and quickly? Fertilizer; and frequent applications. If you have a large fern and it appears healthy with adequate moisture, there is probably no need for fertilizer. A fern guru we know grows many of those and he uses a fertilizer with a 3:1:2(NPK) ratio, usually 18-6-12. I use the same fertilizer formula of Osmocote(TM)to grow potted liners or 4" pots on to gal.(and larger) size. Once they attain the desired size, I discontinue the fertilizer regimen. I once used the same fertilizer as an annual application for my Hostas and companion plants, but recent warmer than usual winters, with excessive moisture and the high Nitrogen content promoted the growth of a type of fungus that causes crown rot, often called Southern Blight disease.
Bottom line, if size is acceptable and it is healthy, I wouldn't bother with applying fertilizer. It probably doesn't need it.
Rb