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Oil shale ash contains mineral ingredients of agricultural importance and could be used for treatment of soils. However, the ash contains several hazardous ingredients, i.e. toxic trace metals which could be mobile and create environmental concerns. Beneficial use of the oil shale ash in agriculture and forestry requires better knowledge of leaching characteristics of soil-ash systems. In this study two different ash samples have been added to two types of soil, particularly Rendzic Leptosol and Podzolic Gleysols, in order to investigate the leaching characteristics of soil/ash mixtures using the traditional leaching scheme and a scheme modified by pretreatment (incubation) of solid in wet conditions. The pH, conductivity and concentration of Cd, Pb, Cr, Zn and Ni in leachates are determined. The results revealed that transport of hazard components to the water phase was highly dependent on the type of soil and ash and leaching method used. Throughout the experiment, concentrations of heavy metals in leachates of any combination of soil and fly ash are low. The most mobile metals are found to be nickel, zinc and chromium. Application of oil shale ash to soil might increase mobile fraction of Ni in soil.For the safe recycling of oil shale wastes in agriculture, it is recommended to take into account the leaching behavior of trace elements as ingredients in combustion ash.