Fern plants aren't able to store nutrients like most other plants, so its best to keep them on continuous feed. Also when feeding the fern plant, its best to feed them sparingly and with a slow-release liquid food. Fern plants need more nutrients when they begin to have new fronds uncurling from the center of the plant.
A good start to feeding a fern plant is to mix 100 parts per million of nitrogen. Look for something that uses ratios close to 15 parts to 5 parts to 15 parts when purchasing fern food. Its always best when reading labels to watch the ammonia and nitrogen levels to make sure that they are equal. Another good choice is a slow-release pellet such as Osmocote, which is simply sprinkled around the dirt toward the base of the plant. Aluminum sulfate also is a beneficial nutrient for fern plants and other garden plants as well.
Fern plants love any type of organic compost. The most beneficial way of feeding a fern with compost is to layer the organic mulch compost on top of the soil and spread it out evenly. This is the most affective way of supplying the fern with all of the nutrients that it needs to build its root base and enable it to start growing new fronds.
There are a few essential things that ferns need in addition to proper nutrients. Ferns need shade, humidity and moist soil at all times. Ferns also prefer alkaline soil; if it's difficult to provide this for the fern plant, then mix crushed limestone, oyster shell grit or cement rubble into the soil, which will provide a constant source of lime for the fern plant to feed off of.