Hello Clasher,
I hope you find the following useful, if you are think of what to do with the wood ash from a log fire.
Wood ash is sometimes recommended as a source of potash (in the development of flowers and fruit). It contains potassium in the form of potassium carbonate and traces of other elements. The percentage of potash may vary from 4-15% depending on the source material. Ashes from young sappy material may contain up to 15% potash, where as heartwood, wood from thicker limbs and from sawdust may contain as little as 4%.
Wood ash is alkaline and tends to raise the pH level of soils. Do not use it among roses, fruit and vegetable crops, rhododendrons and other plants needing acid soil conditions.
Use wood ash as a general dressing to soils known to be deficient in potassium. All wood ash will provide some liming effect, but only the fresh material will also supply potassium.
Alternatively it can be added to the compost heap, in place of lime. A suitable rate of application would be 800g/sq.m of compost heap area, repeated in layers 15-20cm apart.
Happy gardening
The Organic Gardener