Wood ash acts on the soil much like limestone in that it raises the pH or alkalinity of the soil. Consequently, many wood stove burners dump the ashes on their garden site with the thought that they are improving the soil condition of their garden. Yet unlike limestone, which can take six months or more to take effect, wood ash has high water solubility and quickly changes the soil pH. This can cause a problem with raising the soil pH over the optimum level of 6.5 to 7.0 if we spread too many ashes in the same area. A soil pH over the optimum level can affect plants as adversely as a pH that is too low. High pH will limit the uptake of important soil nutrients needed by the plant such as phosphorous, iron, and magnesium.
A safe rate of wood ash application for a garden or lawn area would be twenty pounds per thousand square feet or a five-gallon pail full of wood ash. Twenty pounds of wood ash is equivalent to six pounds of ground limestone per thousand square feet. If the soil is in the proper pH range, this rate of application is considered appropriate for yearly treatments. After wood ash application, we should need no additional lime, with nitrogen and possibly phosphorous being the other plant nutrient requirements. The wood application will also supply potassium. We should mix the application of wood ash to the garden soil well.
The UNH soil-testing laboratory, in Durham, has recorded very high pH values from soil samples that we have treated with wood ash. Having a soil test taken once every two years from your garden and lawn is wise. The recommendations for soil treatment, including any adjustment of soil pH, will give one a more accurate means to care for their garden and lawn.
We should make careful consideration when applying wood ash to lawn and garden sites. A measured application can be beneficial to increase soil pH. Over applications of wood ash will increase the likelihood of soil related problems.
George W. Hamilton, Extension Educator, Agricultural Resources, University of New Hampshire Cooperative Extension-- Hillsborough County
This information was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.
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