OP here's some info on the use of agricultural wood ash in soil. It's not cannabis but uses all the same minerals just different amounts. The ash of burned trees still contains the same the minerals inside it, it did during growth that's why it can be used as fertilizer. If these minerals can't be burned out what's putting a few extra gallons of water through your soil gonna do? If you've ever heard the term "potash" it comes from soaking ashes in a pot of water for it's potassium content.
http://extension.uga.edu/publications/detail.html?number=B1142
http://extension.uga.edu/publications/detail.html?number=B1142