We have used a lot of woodash from a local papermill's cogen plant. Effective as both a liming agent and a potash source. We use a pushgate solid manure spreader, typical application is 6-10 tons/acre. I don't know why or how you would want to bother to try to variable rate it.

In Ontario it is classed as a industrial waste, proper permits need to be acquired, it is routinely tested for heavy metals ect., the permits are a pain but they keep the mill honest. Our local research station has done a lot of work with woodash, let me know if you want me to dig some info up for you.

Bernie