Wood ashes can raise pH, but are not recommended. There is no way to determine accurately the volume to add for the desired change. They are very messy and leach out of the soil so rapidly that they are ineffective. If pH is too alkaline, add sulfur to lower the pH. Chelated iron sulfate or aluminum sulfate has an acid reaction and is commonly added to correct chlorosis or yellowing of leaves. Overly alkaline soil interferes with plant uptake of iron.
Iron sulfate reduces alkalinity and adds iron.
Specific plants affected by chlorosis include evergreen shrubs, raspberries, roses, mountain ash, pin oak and currants.