Rehabilitation is an important stage in mining operations for environmental conservation. However, the shortage of topsoil makes it difficult to achieve rehabilitation in open-cast coal mines. Securing topsoil by mixing soil with fly ash (FA), which is treated as an industrial waste, is expected to solve this issue in coal mines. While mixing soil with FA makes it possible to secure the topsoil and treat industrial waste simultaneously, the high alkalinity of FA and the dissolution of heavy metals from FA may inhibit plant growth. This study investigated the effects of FA in the topsoil on plant growth via vegetation tests with simulated topsoil mixed with FA using Acacia mangium, a species of flowering tree: the FA mixing ratios were set to 0%, 20%, 40%, 60%, and 100%. The growth of Acacia mangium was inhibited with increasing FA mixing ratio, especially from 60% to 80%. However, the growth rate of Acacia mangium in an FA mixing ratio of 100% was nearly comparable to that in a mixing ratio of 40%. Furthermore, there were no effects of the physical characteristics and pH conditions in the topsoil on the plant growth at any of the mixing ratios; meanwhile, the accumulated concentration of Al in the plant body increased significantly at an FA mixing ratio of 60%–80%. This suggests that the accumulation of Al, which inhibits plant growth, including root growth and its functions, in the plant body inhibited the growth of Acacia mangium. Therefore, the most important aspect in terms of rehabilitation concerning the use of FA for securing topsoil is not the mixing ratio of FA but the amount of Al in the FA and the accumulation of Al in the plant body.
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