By The Old House Web


Considerable quantities of wood ashes are now being produced due to the increasing use of wood as a fuel source for home heating, creating the problem of disposal of ashes. One practice of disposal of wood ashes is to scatter them over garden soil, a method which gives rise to questions regarding the effects of wood ash on soil.

Reputable references indicate that the composition of wood is variable, dependent not only on the nature of the original wood, but also on the completeness of the wood combustion. Also affecting the composition is any leaching that has taken place due to exposure to the elements and the degree to which the ashes may have been mixed with impurities.

The nutrient composition of unleached wood ashes will generally be 0 percent nitrogen, 1.0-2.0 percent phosphate and 4.0-10.0 percent potash. The lime in wood ashes is originally in an oxide form, but is converted to the hydroxide and carbonate forms when exposed to the weather. Wood ashes as dumped on the garden soil may be 20 to 50 percent lime, as expressed on a carbon carbonate basis. The ash of hardwoods such as maple, elm, oak, and beech contains one third more calcium than the ash of softwoods, while coal ashes have no value as liming material.

In composition, it would require 4,000 to 5,000 pounds of wood ashes to be equivalent to one ton of calcium carbonate. Due to the light weight of dry ashes and their total neutralizing power, it would appear that considerable quantities of ash would need to be applied to eventually make the soil too alkaline for good crop production. It is recommended that gardeners applying large amounts of wood ashes sample and test their soil every two years to monitor soil PH. Due to the fine structure of wood ashes, it seems that the material would have little value as a soil conditioning agent. Also, weathered wood ash have practically no fertilizing or liming value.


Landscape Facts-Cooperative Extension Service, Ohio State University LF-CP 24-79 "Effect of Wood Ashes On Garden Soil" James D. Utzinger, William B. Brooks and Robert D. Touse, The Ohio State University

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