With the increase in the use of wood stoves for home heating, questions have arisen concerning the feasibility of applying wood ashes to the home garden to supply plant nutrients. The nutrient content of wood ashes is variable, but in general they contain approximately 2% phosphate, 5% potash, and small amounts of boron and other elements. In addition to supplying nutrients, wood ashes have about half the acid neutralizing power of limestone. (20 pounds of wood ashes equal 10 pounds of limestone)
Wood ashes may be applied to the home garden to supply the above needed plant nutrients and reduce soil acidity. Application should be made only if the pH of the soil is 7.0 or lower, based on soil test results. Suggested rates are 10-20 pounds of wood ashes per 1000 square feet per year. A 10 quart pail, filled to within 2 inches of the top, will contain about 5 pounds of wood ashes, so 2-4 pails may be used per 1000 square feet per year. Higher rates should be avoided because of potential plant toxicity problems. The soil should be tested every two years to check for shanges in pH, phosphorous , and potassium levels to determine if further wood ash applications should be made.
Wood ashes should not be applied to germinating seedlings or plant roots. This could result in salt burns from the potash in the ash. For this reason, wood ashes should be incorporated into the soil with a rototiller or plow prior to planting.
One word about coal ashes-these should not be used because they contain no nutrient value or acid neutralizing power.
Additional notes--The reason not to apply more than 20 pounds per 1000 square feet per year is that wood ashes contain a relatively high amount of boron. Boron is an essential plant nutrient, but can be highly toxic to plants at relatively low levels. At levels higher than 20 pounds per 1000 square feet, boron toxicity problems can become a problem.
Wood ashes may also be applied to lawns. Make sure any chunks of charcoal have been removed. Maximum rate of application is about 15 pounds per 1000 square feet per year. If the turf is actively growing at the time of application, the wood ashes should be watered in.