by Tim Baker, Northwest Region Horticulture Specialist, University of Missouri Extension
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A question I am occasionally asked is if it is ok to use wood ashes in the garden. The homeowner typically has a fireplace or wood stove which produces a large amount of wood ashes each winter. This can become a disposal problem. Is it acceptable to put them on the garden?
I always answer that with a guarded “Yes.” Wood ashes can benefit your garden. But overuse will cause problems which are not easily corrected. Caution is in order.
First, the good news. If you have an acid soil, you may use some wood ashes without problems.
Wood ashes are alkaline, and will raise your soil’s pH. Some wood ashes have a neutralizing equivalent up to around half of that of limestone. Thus, if a soil test called for five pounds of limestone per 100 square feet, you could use around ten pounds of wood ashes to get the same effect.
But this is only approximate. If you use wood ashes regularly, you will want to keep a close eye on your soil’s pH, since you don’t want to raise it too high.
Once you get up to 6.5, you need to be careful, and will probably need to discontinue adding wood ashes until the pH starts to decline again.
And that’s the bad news… you can actually apply too many wood ashes to your soil, and raise its pH to a level which is too high for most plants. Sometimes I get a soil test back with a very high pH level.
The first question I ask is, “Did you apply wood ashes?” Quite often, they have. If it’s too high, they may have to apply sulfur to bring the pH back down to an acceptable level.
Wood ashes do contain some nutrient value. So as long as your soil is acid, they can be beneficial. Typically, wood ashes contain from one to two percent phosphate, and from four to ten percent potassium. They also contain calcium. The levels will depend on the type of wood being burned, and the completeness of combustion. If wood ashes have been rained on before spreading on your garden, many of the nutrients may have been leached out. So should you apply wood ashes to your garden? First, get a soil test, to see what your soil’s pH is. If it is acid, feel free to add a light sprinkling of wood ashes. Avoid heavy applications in small areas.
But by using a reasonable amount under the right conditions, you’ll be adding some nutrient value and eliminating the need for lime.