Stability is an important but often ill-defined aspect of compost quality. Compost stability should identify the point reached in the process of organic matter decomposition, i.e. the extent to which readily biodegradable organic matter has been consumed (FCQAO, 1994; Lasaridi and Stentiford, 1998a). The term stability is often used interchangeably with the term maturity. However, maturity is a more general and somewhat elusive concept, relating more to the agricultural value of the compost and the plants response to its application. Stability, on the other hand, is more process oriented as it specifically refers to the microbial activity in compost, which is directly related to the remaining amount of readily biodegradable substances (Herrmann and Shan, 1993; Iannotti et al., 1993, 1994; Morel and Nicolardot, 1986; Morel et al., 1986).
Assessment of compost stability is important for compost producers and end users, as they both need reliable measures for compost quality. Unstable composts can sustain high microbial activities; therefore they may become anaerobic during storage and transport, enhance pathogen regrowth (Finstein et al., 1987), develop unpleasant odours (Miller, 1993) and phytotoxic compounds such as low chain organic acids (Zucconi et al., 1985; Inbar and Chen, 1993). When used in growing media or applied to soil, unstable composts may also immobilise nitrogen and reduce nitrogen availability to plants (Forster et al., 1993). Moreover, local authorities, compost producers and plant operators need to evaluate different composting systems and monitor the process performance, in objective terms of organic matter decomposition achieved in a specific period (Stentiford, 1993), in the same way as BOD5 removal is a measure of wastewater treatment processes performance.