Wood ash

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.

Many studies have been conducted regarding the chemical composition of wood ash, with widely varying results. Some quote calcium carbonate (CaCO₃) as the major constituent, others find no carbonate at all, but calcium oxide (CaO) instead. Some show as much as twelve percent iron oxide while others show none, though iron oxide is often introduced through contamination with soil. A comprehensive set of analyses of wood ash composition from many tree species has been carried out by Emil Wolff, among others.

Several factors have a major impact on the composition:

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.

Wood ash can be used as an organic fertilizer used to enrich agricultural soil nutrition. In this role, wood ash serves a source of potassium and calcium carbonate, the latter acting as a liming agent to neutralize acidic soils.

...
Wikipedia