There are a couple of companies making hard plastic sleeves that slip over pole barn posts. One company offers a post protector that actually has a bottom in it. So the post drops into the square tube and the entire assembly goes into the ground. Has some ribs on it to help prevent it from pulling out of the ground. Ribs on the inside for it to "air out". They come in different sizes. I suppose it would come out of the ground 16" to 24". Another one I saw is just a sleeve that protects the post maybe a foot or two into the soil and a foot or two out of the soil. Right where the wood rots. Looks like a good idea to me. Any one have experience with these or installed them?? Thanks.