Raviv, M., Department of Environmental Horticulture, Newe ya'Ar Research Center, Agricultural Research Organization, POB 1021, Ramat Yishay 30095, Israel

Horticulturists have long used various organic materials as components of potting substrates. Since the 1950s, peat moss replaced most of these materials due to its superior physical properties. More recently the trend has changed again as a result of the need to recycle organic wastes in an environmentally-sensitive manner, the rising cost of peat and the understanding of the crucial role that peat bogs play in the global carbon cycle. Most organic materials require controlled composting before they can be used as growing medium components. The composting process should be aerobic and the compost should be exposed to thermophilic conditions. The composting process should achieve a number of important objectives. It should eliminate phytotoxicity, pathogens and weed seeds, and stabilise the material with respect to N and oxygen demand of micro-organisms. The end product is an easy-tohandle material that can be used as a component of growing media. Compost maturity is an important characteristic in relation to its use in growing media. If active decomposition continues after incorporation into a growing medium, it will negatively affect plant growth due to medium shrinkage, reduced oxygen level, nitrogen immobilization and the presence of phytotoxic compounds. Sources for composts as ingredients of growing media include bark, sawdust, plant wastes (spent mushroom compost, grape marc, composted green wastes, rice hulls, etc.), animal manures, bio-waste and others. Limitations to the use of composts as ingredients of growing media are their physical properties (high bulk density and low amount of easily available water), salinity, residual phytotoxicity, high biological oxygen demand, pH and rate of residual degradation with time. As a result, normally the fraction of the compost in the mixture should not exceed 50%, although some exceptions exist. Advantages of composts as ingredients of growing media include their low cost, nutritional contribution and suppressiveness against soil-borne diseases. The above-mentioned considerations suggest a bright future for the use of composts in growing media. Required future research is highlighted. It is of paramount importance to study the linkage between composting techniques and compost characteristics and predicted performance. The effect of compost storage on the shelf life of its desirable properties should also be studied.