I've found a non chemical preservative to use on the wood of my raised beds. It's called 'Procol'. I don't know if anyone else has tried it? I'll now have some raised beds made with untreated wood, and some treated with this - so we'll see if it makes any difference!

http://www.recycleworks.co.uk/non-toxic-wood-preservative-chestnut-c-273-p-1-pr-16149.html