I live in Costa Rica, land of many volcanoes which means lots of different types of volcanic soil. There are countless varieties of volcanic ash, some good, some not so good and some which won't grow anything. Volcanic ashe tends to be very clay based and depending on the type would depend on how much to use in your soil mix. I would suggest mixing varying amounts in a mix and running some clones to determine what amount in the mix will give you the best results.