I haven't dealt with snails and so can't add any thoughts about what might work, but I'd be cautious about using ashes where they can get into the soil. I imagine they are probably helpful in the correct amounts, but I nearly killed a persimmon tree by dumping ashes underneath it, thinking they'd be great fertilizer. Bad idea. Most of the tree died, but a few new sprouts appeared and after about six years it is finally a little bigger than before the disaster.