Posted: 01/04/2014 at 12:50
It depends on the wood - if it's firewood logs it won't do any harm to use it as has been described above. If it's constructional timber which has been treated or painted I wouldn't use it on the garden.
However, there's not a lot of benefit to be gained from using the ash of large logs - the most potash is produced by twiggy growth rather than heartwood. That is why the ash from a garden bonfire using old pea sticks, prunings etc is so useful.