Wood ash contains many inorganic elements that are important to plant growth and can in most cases be put back on the land. Wood ash consists mostly of calcium, potassium, phosphorus, and magnesium. Bark ash may also contain relatively high levels of silicon and aluminum. The potassium, phosphorus, magnesium, and calcium are important elements in plant growth. They aid in making strong plants and also helps a plant's resistance to cold and heat stress and diseases. The calcium raises the soil pH, which makes nutrients available to plants that prefer alkaline soil. Softwood (conifer) ash in general has about a third of the nutrients found in hardwood ash. Potassium (aka potash; it's the K in N-P-K) is the key nutrient found in wood ash. It does not easily move through soil in solution as nitrogen does. For this reason, the ash must be incorporated into the soil. The right amount to apply should be based on a soil test. Pound for pound, ash has less than half the calcium that lime does, so apply up to twice the rate recommended for lime.
Ash from chemically treated or painted wood should not be used as a soil amendment unless it has been tested to insure it does not contain any heavy metals or other contaminants that could be a concern.
As with any soil amendment method it is wise to test the soil first to make sure the elements being added will be beneficial.
