After a few cold winter months of snuggling up in front a warm fire, your fireplace or wood burning stove may be full of wood ashes. An average cord of wood, depending on the type of wood, may yield nearly 20 pounds of ashes or the equivalent of one five-gallon pail. If you burn wood frequently throughout the winter, a lot of ashes will accumulate. Spreading the ashes over your lawn and garden may or may not be the best means of disposal. Wood ash is somewhat beneficial to the soil because it contains essential plant nutrients. Depending on the type of wood, the ash may contain five to eight percent potash, one percent phosphate and trace amounts of micro-nutrients such as iron, manganese, boron, copper and zinc.
On the other hand, wood ash may be detrimental to some soils because of its effect on the soil chemistry. When applied to the soil, it acts much like limestone by raising the pH or alkalinity of the soil. Unlike limestone, however, wood ash has high water solubility and works more quickly to change the soil pH. This may cause a problem in many Iowa gardens that tend to have a naturally high soil pH. Wood ashes should not be applied around acid-loving plants, such as azaleas, rhododendrons, and blueberries. Although a soil test is the best way to determine whether it is advisable to apply wood ashes to garden soil, an annual application of no more than 20 pounds sprinkled over 1,000 square feet of the lawn or garden in the winter or early spring should be safe to turf and garden plants. Avoid applying wood ashes if a soil test reveals a pH above 7.0. Soil testing information and a soil sample information sheet for horticulture crops (PDF) can be downloaded.
This article originally appeared in the 2/23/2005 issue.