Wood ash is alkaline based, which means that it has a high number of hydrogen ions within its solution. Alkaline is a term applied that means the opposite to acidic, and when applied to soil various plants thrive better in one rather than the other. For example, plants including rhododendrons, blueberries, and azaleas prefer soils that are slightly acidic or neutral when compared to alkaline soil.
Therefore, wood ash is a great way to condition and enhance the soil that your plant sits on. In addition, it is a much more cost effective way to condition your soil and much better for the environment then choosing to use lime or commercial fertiliser.
Lime, also known by its formal name of calcium carbonate, can also be used to help neutralise the soil and its acidity. A soil test is a great place to start to see how much you should add. The test can be conducted by scooping up small samples of soil from different areas in the garden. Once you have the samples blend them together and test the nutrient and pH levels. The results will give you your baseline for adding lime.
If you choose to use ash instead you can figure out how much by multiplying how much lime you would use by about 1.5. This figure will give you a rough estimate of the weight of wood ash that you will need in order to neutralise your soil.
Roughly you can estimate that you will need to apply one half cup of ash for every 3×3 surface area. The ash that will remain after burning wood often has potassium carbonate and other trace minerals in it that will help to neutralise the soil and make it healthier for your plants.