Wood ash is the residue powder left after the combustion of wood, such as burning wood in a home fireplace or an industrial power plant. It is used traditionally by gardeners as a good source of potash for domestic gardens or any garden.
Variability in assessment
Many studies have been conducted regarding the chemical composition of wood ash, with widely varying results. Some quote calcium carbonate as the major constituent, others find no carbonate at all, but calcium oxide instead. Some show as much as twelve percent iron oxide while others show none.
There are several factors which have a major impact on the composition:
- Fly ash: some studies include the solids escaping via the flue during combustion, others do not.
- Temperature of combustion  carries two direct effects:
- Dissociation: conversion of carbonates, sulfides, etc. to oxides results in no carbon, sulfur, carbonates, or sulfides. Some metallic oxides (e.g. mercuric oxide) even dissociate to elemental state and/or vaporize completely at wood fire temperatures.
- Volatilization: in studies where the fly ash is not measured, some combustion products may not be present at all.
- Experimental process: If the ashes are exposed to the environment between combustion and the analysis, oxides may convert back to carbonates via carbon dioxide in the air.
- Type, age, and growing environment of the wood stock impact the composition of the wood, and thus the ash.
Typically between 0.43 and 1.82 percent of the mass of burned wood (dry basis) results in ash. Also the conditions of the combustion affect the composition and amount of the residue ash, thus higher temperature will reduce ash yield.
Much wood ash contains calcium carbonate as its major component, representing 25 or even 45 percent Less than 10 percent is potash, and less than 1 percent phosphate; there are trace elements of iron, manganese, zinc, copper and some heavy metals. However these numbers vary as combustion temperature is an important variable in determining wood ash composition. All of these are, primarily, in the form of oxides.
For a long time wood ash has been used in agricultural soil applications as it recycles nutrients back to the land. Wood ash has some value as a fertilizer, but does not contain nitrogen. Because of the presence of calcium carbonate it acts as a liming agent and will deacidify the soil increasing its pH.
Potassium hydroxide can be indirectly made from wood ash by the addition of calcium hydroxide, and in this form is known as caustic potash or lye. Because of this property, wood ash has also traditionally been used to make wood-ash soap.
Wood ash with a high char content has also proven to be effective as an odor control agent, especially in composting operations.
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