To find the latent effect of paper mill sludges (PMS) with and without alum, and alum paper mill sludge compost(PMSC) on aluminum toxicity of crops, the pH buffer capacity and the acidification resistance index (ARI) for PMSs and PMSC were investigated, comparing to upland soil. Buffer capacities of PMS with and without alum, and PMSC were significantly higher than 2.5 cmol^+ ㎏^(-1) of check soil as 137, 250, and 147 cmol^+ ㎏^(-1), respectively. Owing to such high buffer capacity of PMS and PMSC, they showed to be well-resisted to acidification.