Raviv, M., Agricultural Research Organization, Newe Ya'ar Research Center, POB 1021, Ramat Yishay, 30095, Israel

Peat moss serves as the main organic component of growing media (GM) due to its relative homogeneity and excellent physical properties. Yet, there is a rising trend of replacing the use of peat in GM, driven by the need to recycle organic wastes in an environmentally-sensitive manner, by the lower cost of peat alternatives and due to the understanding of the role of peat bogs in the global carbon cycle. Also, frequently peat is conducive to soil-borne diseases, while some peat substitutes may suppress these diseases. Many feedstocks can be composted, to be used later in GM. This includes coir, bark, sawdust and other plant wastes, animal manures and others. Limitations to the use of most of the composts as ingredients of GM are related to their physical properties (high bulk density and low amount of easily available water). In some cases also salinity, residual phytotoxicity, high pH, and high biological oxygen demand resulting in potential N immobilization and substrate shrinkage with time may be problematic. All the above-mentioned limitations lead to the use of only mature composts as components of GM, dictating a relatively long and well-controlled composting. The composting process should be aerobic to prevent formation of phytotoxic compounds. The compost should be exposed to thermophilic conditions in order to eradicate human, zoonotic and plant pathogens and weed seeds. Major composting methods are described. The end product should be an easy-to-handle material of relatively low bulk density and high porosity. Normally, the fraction of the compost in the mixture is about 50%, although in some cases composts serve as stand-alone substrates. Advantages of composts as ingredients of GM include their low cost, as compared to horticultural peat, their nutritional contribution and suppressiveness against soil-borne diseases. The abovementioned considerations suggest a continuing expansion of the use of composts in GM. Required future research includes the study of the linkage between feedstocks, 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. Reuse of compost-containing GM is another subject for future research.