This study, through an incubation experiment, evaluates the effect that different biomass ashes exert on the extractability of nutrients and some chemical parameters and dehydrogenase activity of a slightly acid soil. Three types of as were selected: two new ashes generated by the dry combustion of olive cake or by gasification of greenhouse vegetable wastes, and a third produced by wood combustion. The ash significantly increased the pH, electrical conductivity, and dehydrogenase activity of the soil. Dry olive-cake ash was the most effective in raising the levels of soil AB-DTPA extractable P, K, and Cu. By contrast, wood ash caused the greatest increases in soil AB-DTPA-extractable Zn. The three as he showed little effectiveness for increasing soil ABDTPA extractable Fe and Mn. Ash from dry olive cake could be used as a low-cost potassium fertilizer.