When mixed with other components in the compost heap, the resulting alkaline compost can be used as a mulch around most ornamental plants and vegetables unless, like raspberries, rhododendrons and roses, they require an acidic soil. Fruit too performs best in slightly acid soil so wood ashes are unsuited for use in the fruit garden.
Vegetables grow best in soil with a pH of 6.5, so testing the level before adding the compost is recommended so as not to raise the pH too much (greater than pH7.0). However, where club root is present, wood ash can be used to raise the pH to as much as 7.5 to inhibit this disease.
Wood ash may contain useful levels of potassium (about three percent), a major plant nutrient associated with flowering and fruiting. However, the levels will vary depending on the age of the wood that was burnt; young wood such as from pruning will have higher potassium content than older, thicker branches.