I've read that also about the wood ash leaching quickly and not being a long-term pH influence, but being potent in the short-term. True that...
Good idea on testing runoff pH before using! I'll be doing that.

I want to try the wood ash more for the micronutrients than anything else. The reasoning is roughly this:

A piece of wood contains all the minerals necessary for sustaining plant life. The existence of that piece of wood pretty much proves that. Burning the wood completely removes H and O as water vapor, drives off some carbon as carbon dioxide and what's left is practically every element (less C,H and O)that was needed by that piece of wood to become a piece of wood.

Sounds like a good source of micronute to me...as long as I don't scald my plants and make them squeal like pigs! (Wood ash and boiling water - old time pig scalding recipe)