Wood ash is the inevitable result of owning a wood stove or fireplace. Adding ashes to the garden is one way to get rid of ashes while adding useful nutrients to your soil at the same time. But because all gardens have different soil types and characteristics, wood ash cannot be recommended for all soils.
Soil pH indicates the acidity or alkalinity of your garden. A low pH (6.5 or below) indicates acidic soil, while a high pH (7.4 or higher) indicates alkaline soil. The in-between range is considered neutral. Wood ash tends to be quite alkaline, so it raises pH when added to soil. It makes soil less acidic, not more.
Adding wood ash raises pH, but it also adds much-needed nutrients to the soil. Wood ash contains potash, phosphate, iron, manganese, boron, copper and zinc in levels that vary depending on what type of wood the ash came from.
If you have acidic soil in your garden, adding wood ash is a great way to raise the pH and neutralize the acid. Most plants grow best in neutral or near-neutral soil. But if you have alkaline soil in your garden, adding wood ash may do more harm than good, making the soil even more alkaline.