peat

Partially decomposed plant material. It builds up in areas with poor drainage, namely bogs and fens. Acid bog peat (or peat moss) is composed primarily of the remains of bog plants, such as Sphagnum mosses and sedges. It is used as a mulch and to improve soil and is also a constituent of potting composts. Fen peat contains considerably more nutrients than bog peat and if drained provides fertile agricultural land.