Wood ash is not particularly soluble in water, but it’s not quite that simple.
Solubility in water is simply a measure of how much of a solid can be dissolved in water under specified conditions. It’s complicated for wood ashes because ash isn’t a simple solid. It has a quite course charcoal-like component – the product of incomplete combustion of the wood, and this component isn’t very soluble at all. It just floats on water. The grey powdery component of the ashes is also a mixture of minerals. It’s about 9% potassium (K) minerals, for example, and these are very soluble in water. There is also a small component of phosphorus (P), and it is quite insoluble, likely a good thing as excess supply of P to watersheds could have unintended consequences. But what we care most about is the roughly 30% of the ash formed of minerals of calcium (Ca), mainly calcium carbonate. Calcium carbonate is somewhat soluble in water, much less than the potassium component and much more than the phosphorus competent, again likely a good thing. With every rain, a little more of the Ca in the ash will dissolve and percolate into the soil. Because of this modest water solubility, wood ashes should provide a long-term source of the missing Ca in soils, the long-term goal of ASHMuskoka.