Maher, M., Teagasc,Kinsealy Research Centre, Dublin 17, Ireland
Prasad, M., Research Centre,Bord na Mona Horticulture, Main Street Newbridge, Co. Kildare, Ireland
Raviv, M., Agricultural Research Organization, Institute of Plant Sciences, Newe Ya'ar Research Center, P.O.B 1021, Ramat Yishay, 30095, Ireland

This chapter deals with the organic materials used in soilless production: peat, coir, bark, wood products, and compost. It describes their physical and chemical properties and their effect on plant performance. It also discusses the composting process and reviews the biological stability of growing media and disease suppression. Peat has long been used as a component of potting mixes and has become the most widely used growing medium for containers as a complete growing medium by itself. However, the use of peat in horticulture has recently been questioned from an environmental standpoint, since peat is a non-renewable resource and since it plays a major role in atmospheric CO2 sequestration. Alternative organic substrates in organic--inorganic media mixes include waste organic by-products, such as wood industry wastes, urban wastes, cork, wood fibers, livestock manure composts, coconut wastes, etc. While some of these have been in use for a long time, others, such as coir and coconut wastes, have been tried more recently, sometimes with promising results. Alternative organic substrates that are well characterized and corrected by suitable blending with inorganic components make it possible to produce high-quality horticultural plants and contribute to the reduction of overexploitation of natural peat-lands. Chief among replacement materials has been bark and wood fiber from forestry and the wood industry. There is also increasing interest in coconut fiber (coir dust), green waste, and other plant and animal residues. © 2008 Elsevier B.V. All rights reserved.