Not coal ash, no, dispose of that away from your garden soil. Wood ash can be distributed around the garden, but only lightly, and not near any acid loving plants. As to how much use the wood ash is, very little, in reality. In theory, it contains varying levels, albeit very low, of potash, but it depends on the wood burnt, and whether the ash got damp at any point, as to how much there is.

We put quite a lot of wood ash on our quite clayey soil, and it seems, slightly, to improve the texture, breaking up the lumps and making it drier and more friable.

Will change the ph balance though, Gattina, if you use a fair bit.

You're right, Bamboo, and although I haven't tested it properly, I don't think it's having too adverse an effect. The vegetable garden does quite well. I intend making an acidophile bed later this year, and the wood ash will be going nowhere near it.

I think the woodash is great for onions - not because of the potash, but because of its alkalinising effect.