I want to use wood ashes from my stove in my garden. How much should I apply each year?
Wood ash is a good source of plant-available potassium and other mineral nutrients, BUT it is very alkaline. Because wood ash can raise your garden soil pH too much, it is very important to begin with a soil test before applying this material. Only apply wood ash when your garden soil pH is low or when it is desirable to raise the soil pH level. A soil pH of 6.5 is near the optimum range for most garden vegetables. Wood ash is not recommended for use with garden soils that have a pH greater than 6.5, or when the soil test potassium level is very high. Only use wood ash on acid soils and low-potassium soils that can benefit from its application. Never apply more than 25 pounds of wood ash per 1,000 square feet in a given year. After applying ashes, have your soil tested again before applying any more wood ash. While wood ash is a good fertilizer and liming agent, excessive use of wood ash in the garden can be harmful to soil fertility balance.
— Joseph Heckman, Ph.D., Extension Specialist in Soil Fertility, Department of Plant Biology & Pathology, Rutgers, The State University of New Jersey