In reply to Reach>Talent:
I would suggest a "micro emulsion wood preservative" purely because you can spray it easily and over-paint it quickly. As for soil-contact, solvent-based products would probably be more effective, but check the manufacturer's recommendations. If you google that phrase I put in parentheses, you'll come up with several products.
Personally though, I would use creosote substitute because it's cheap if you shop around and penetrates reasonably well. You can let it weather for a year and then over-paint with water-based outdoor paint. If you want to be eco-friendly, you should first scrape away the nearby topsoil and protect the ground with polythene.