Preparing the Soil Most ferns prefer a well-prepared, humus-rich and acidic (pH 5.5 to 6.5) soil. The ideal fern bed composition is soil/sharp sand/peat moss in a ratio of 1:1:2. Beds should be dug to about 18" - 24' deep if practical. The top 6" of soil should have copious amounts of compost and other organic matter worked into it. Sphagnum moss, coniferous tree needles and oak leaves are ideal additions to a fern garden soil. To make the soil more acidic, add powdered sulphur, superphosphate, or ammonium sulphate. However, do this gradually (use a pH test kit to monitor the changing pH) to not destroy the soil micro-organisms from pH-change shock. Place a decaying log in the fern bed and use it as a backdrop for ferns!