How long will metal pipe last in earth and concrete
You could build big concrete posts, maybe 12" in diameter, up about 12" above ground level, and use metal post bases to keep the wood off the concrete.  Be sure to treat the end grain and base of the post with the best preservative you have available, and it ought to hold up pretty well.  

In the comments you mention you're in the mid Pacific.  Without pressure treated available, keep in mind that some species are far more rot resistant than others.  The people selling lumber may be able to help suggest what will work best.  Farmers usually know, too.  Some of the most rot resistant woods are tropicals that would be very expensive exotics where I am, but they may be affordable where you are.  

Sometimes small utility poles are at a decent price, they are treated for direct embedment in post holes.  Setting a pole is doable with nothing more than a shovel if you can hand dig the necessary depth.