I would raise them off the ground. I don't know of any preservative that's non toxic. We have a raised flowerbed made of cedar and lined it with heavy plastic leaving the bottom open for drainage.

The beds will be directly on the ground. The timbers/posts are just a border 9-12" high off the ground to keep the soil mix contained and to provide visual separation from the lawn. So there will be no bottom. I won't be using any preservative, just thinking of a moisture barrier.

My choice so far:

Plain grey bricks 2"x3.5"x7.5" are .44 each at local cement supplier, and black plastic lawn edging $11.00 for 50 feet.
Lay bricks down end-to-end, drive rebar posts 18" deep on either side of bricks, lay fence posts horizontally on bricks between rebar posts, black lawn edging between inner rebar posts and wood fence posts.

Brick underneath, and black plastic lawn edging on inner, dirt side of wood should minimize moisture in wood.

I may have to use two layers of lawn edging because bed may be taller than 1 layer of lawn edging. If I could find taller lawn edging locally that would be better. Will look for some recycled material (possibly aluminum sheet or fiberglass).

Soil in itself isn't the enemy of wood. The killer is moisture that promotes rot, that plastic by it's nature holds in, and why I prefer to use heavyweight weed blocker fabric to line my raised beds. It prevents any soil from sifting through the most minute spaces between the timbers, yet allows excess water to drain out.

It also allows oxygen to get to the roots of the plants and why many of us still prefer unglazed clay to plastic pots. It does mean watering more often, but the plants don't suffer root rot either.

Thanks birdwidow. You're right, moisture not soil is the enemy of wood. The lawn edging I was considering is stiff and flat while my timbers are round fence posts contained by rebar stakes so they would only contact at one point, (a tangent), leaving room for air circulation. But heavyweight weed blocker fabric could be even better, offering aeration to the soil like clay pot.

It seems that with a deep bed, the wood would stay wet most of the time, with regular watering. Is your experience that your wood is dry enough with the fabric?

doug4: To me honest, I wouldn't use old fence posts to build a raised bed. For me, there would be too much work involved to construct with old wood already near the end of its usefulness. But free is still free, so I do understand why you plan to do so and the flexibility of the fabric would make it easy to fit it around the odd, rounded shapes of the posts, then to staple it in place.

To answer your question: Yes, as the soil dries, so will the wood, as the fabric allows moisture to eventually evaporate, but as you would not want the soil in a bed containing live plants to get totally dry, it still means damp wood- but not soaking wet.

Thanks for more good feedback.

Completed: 1 layer brick set and leveled on washed garden sand.

Next: rebar stakes on either side of brick, horizontal wood fence posts laid between rebar stakes, and 6 inch plastic lawn edging between inner stakes and fence posts. I think this will work well.

The fence posts are not that much work for me, and the look definitely attracts me (and my partner!) to the garden, much as the discrete 4 foot square with 1 foot grid markings of "square foot gardening" motivates discrete care and prompt replanting. Although I usually have most of my attention on function and little on aesthetics, the clean, discrete look is showing great impetus for continued development.

My very small town has no large supplier of bulk landscape materials, only lawn centers with bags, e.g. Wal-mart, etc. My cement plant is a great, cheap source of sand (50 cents for 5 gal pail loosely calculated); also brick (44 cents a brick) and rebar (~24 cents a foot, with free use of their rebar cutter/bender - I bent a couple rebar into U shape frames, 3.5-4 feet across for netting trellises). On the other hand, small town means I am surrounded by farms and have easily found free year-aged horse manure, and cheap truckloads of compost.