An experiment was performed to evaluate the importance of pre-incubation on the stabilization of a soil-mix composed of coal fly-ash and sewage sludge. Sludge was amended with ash at rates of 0, 5, 10, 35 and 50% (w/w). Each ash-sludge mixture was mixed with a loamy soil at either 1:1 or 1:5 ash-sludge mixture:soil (v/v) and then incubated under greenhouse conditions for 42 days. Ash amendment resulted in a decrease in EC, NH4+, PO43- and Zn, but an increase in pH and B for most of the incubation period especially for soil with 35 and 50% ash-sludge mixture added at 1:1 (v/v). Ammonium increased and then decreased while PO43- and NO3- remained relatively unchanged, in the first 14-21 days and then increased until the end of the incubation period. Soluble Cu of all treatments decreased at the end of incubation except for soil with 0% ash-sludge mixture at 1:5 v/v. With an increase in ash amendment rate, both seed germination and root growth of Agropyron elongatum in the soil extracts increased at each incubation period indicating the beneficial effect of the fly-ash-sludge mixture. However; an incubation period of 21 days would be needed to obtain an optimum physico-chemical condition for plant growth. (C) 1997 Elsevier Science Ltd.