The mushroom obtains almost all of its nutrients from the compost layer, but both compost and casing layers contribute to the high water requirement of the crop. The characteristics of casing materials are different from those of most other growing media, which contain inorganic plant nutrients and are usually kept at lower moisture levels. To obtain casing for producing high yields of quality mushrooms, the industry is reliant on supplies of wet, deep-dug peat and sugar beet lime (SBL). Mushroom casing accounts for about 2.5% of the four million cubic metres of peat used annually in the UK, and as with other horticultural uses, there is pressure to find alternatives. The decline in sugar beet processing in Britain and Ireland has affected the supply of SBL. Research at EMR has shown that several recycled materials have the potential to replace proportions of the peat and SBL used in casing, providing that the alternatives have a sufficiently high water holding capacity. Alternative materials include inorganic and composted and uncomposted organic by-products from other industries. The research has identified key air and water holding characteristics of casing materials that should enable them to be optimised for mushroom production. Research has also shown that by measuring the water status of the casing and compost layers using electronic sensors, the reliability and quality of mushroom cropping can be improved.