Ferns are graceful plants most often associated with woodland or shade gardens, but several varieties tolerate sun as well. Almost 12,000 species of ferns lend their distinctive foliage to dark corners, forest floors and sunny spaces. These ancient plants have been traced back to the Paleozoic era over 225 million years ago, when much of the Earth's surface was tropical rainforest.
Ferns evolved in a tropical environment and many still grow in the understory of tropical rainforests. However, ferns such as the cinnamon and maidenhair adapted to temperate areas as the Earth cooled. They became what is called the hardy and half-hardy ferns. Most prefer the soil of the forest floor, rich in organic matter from decaying plants. Forest soil's organic matter is light, and annual layering makes it friable, meaning it drains well.
Duplicating the Forest Floor
One way to duplicate the friable, highly organic soil of the fern's natural environment is to dig up and cook forest soil, sterilising it and killing any travelling fungus. A more convenient method is to mix equal parts of loam, leaf mould, peat moss and coarse sand; for houseplants, use a soilless potting mix with at least 50 per cent peat moss, or simply mix 75 per cent peat moss with 25 per cent sand. The common element in all these mixes is peat moss.
Soil for ferns contains heavy percentages of organic matter in peat moss and leaf mould which provide slow-release nitrogen. Nitrogen is the element that feeds leaf growth. Ferns do not flower, so they require little phosphorus and potassium, the other two major plant nutrients. Fern roots are very fine, giving them a large surface area to absorb soil nutrients. As a result, ferns need little additional fertiliser. In fact, their leaves burn when fertilised too heavily. Liquid houseplant fertiliser applied at 1/2 the rate recommended on the package is all most houseplant ferns need. Outdoor ferns seldom need fertilising.
Peat, the essential ingredient in fern soil, has a pH (a measure of acidity) of 3.5 to 4.5, according to the Canadian Peat Moss Association. When mixed with other soil ingredients, the soil recommended for ferns should reach a pH of 6.0 to 7.0. The University of Arkansas recommends adding lime, an alkali, if the pH registers below 5.5. Neutral soil registers a pH of 7.0, and numbers below 7.0 are acidic. Instructions for planting ferns recommend slightly acidic soil like that found on the forest floor.