Yes it is a source of Potash. But it's so alkaline that in Adelaide it's a better herbicide than it is a fertiliser. If you must use it do it this way. Tip the wood ash out over some dirt you want to kill - like a driveway. Do that all winter, let the rain wash the ash into the dirt.
Use a plastic rake to rake out the little bits of leftoover charcoal and pile them up. Then put THOSE little black marbles into your compost heap. THAT will add value, and the composting should balance the pH. But putting ash straight on your garden is death to plants.
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