FWIW (as a chemistry teacher), ashes are mostly KOH. Potassium hydroxide (KOH) is extremely soluble in water (all compounds with potassium, a group 1 metal are). That being said, it's possible that with enough throughput of moisture any OH- that's at a harmful level will be diluted and/or washed deeper into the soil horizon: more easily in sandy/loamy soil (clay has less water permeability, hence clay pots, clay-bottom ponds etc). Considering the snow many of us have had, that water is going to help dissipate the concentrated KOH from ashes. I'd ditto what others have said to spreading thin: it's easier to dissolve a spoonful of sugar when it's spread out rather than in a lump in the bottom of the cup. In a perfect world we'd also add some bicarbonate salt of some kind since it buffers really nicely at near neutral pH. Don't use the baking soda though, the sodium ions can have toxicity as well: you'll keep chasing your tail there.