The secret to a long lived fence post is not so much the material, but how it is put in the ground. Most people who set posts, dig a hole, put the post in, level it, plumb it and then fill the hole partway with dirt - tamp the dirt then pour in dry quick set concrete, fill will water and let it set up. These leaves the lowest end of the post with dirt to wood contact. Here in California the termites just come up from below and get all fat and sassy eating the posts away - or root sets in. I set posts differently. I get concrete brick which is used to support rebar for concrete projects - its basically a small square of concrete with a bit of wire in it. I set that in the bottom of the hole. Pour in a handful of two of wet concrete to cover the bottom of the hole and the brick slightly. Then set the post on the brick (the brick supports it) and pour in a little more concrete - plumb and level my post and fill the hole the rest of the way with concrete. Most fences I end the concrete about 1/2 inch above the soil, I round it off slightly sort of like a cone, to where water will run off the concrete away from the post. In cases where I have a mow strip/curb, running the length of the fence line (a strip of concrete beneath the grass plate of the fence) I round the curb slightly into a bump, promoting run off from the middle of the curbing. A wood fence should have a grass plate - that is a 1 x 12 that runs horizontally at the base of the fence it should not be touching the earth. It should be screwed in and grass plates should be inspected yearly for termites and rot. If there are termites it should be removed immediately, if there is rot one should keep and eye on it to make certain that the rot does not reach the bottom 2x4. Grass plates should be screwed on not nailed on since its easier to remove a screw (relatively speaking). When it comes to material. Pressure treated should be used for posts. Except in places where you plan on raising food. The chemicals in pressure treated wood will leech out of the wood over the time and contaminate the near soil - food crops will pick up many of those chemicals and deposit them on your plate.