Acidifying organic materials include peat or sphagnum peat moss. Aggregates are generally "Sphagnum" (peat moss), sand, and perlite or vermiculite. They are frequently covered in ericaceous shrubs rooted in the sphagnum moss and peat. Sphagnum mosses also often grow in these wetlands. Most soils derived from Sphagnum mosses are Fibrisols. Peat swamp associates include "Sphagnum cuspidatum" and various sedges. For example, Sphagnum mosses actively secrete tannins, which preserve organic material. Sphagnum moss bogs are common where there is abundant rain and poor drainage. In the wettest areas of Bodmin Moor, sphagnum or bog moss can be found. "Nepenthes" will grow in orchid compost or in pure "Sphagnum" moss. Acidic flushes (areas of water seepage) with their carpets of sphagnum mosses are common. In the Southern Hemisphere, however, peat landscapes may contain many moss species other than "Sphagnum". The plant can often be found growing on hummocks of "Sphagnum" mosses. The U.S. gets up to 80% of sphagnum peat moss it uses from Canada. Many species may be found in the fissures between patches of sphagnum moss in marshes.