But for ground contact I would just buy quality treated posts and be done with it.
My father quit keeping dairy cows to focus on pigs in 1979. He started horse breeding back then, to put the land to use.
Somewhere in 1990 we replaced all of our old barbed wire fencing with double, smooth wire fencing, on 4 foot instead of 3 foot posts.
Most of those posts have already rotten off at ground level, these were creosote (tar) treated pine posts, 12 cm in diameter (4 inch)
Couple of summers ago, we removed some overgrown fence near the bush, which my grandfather has placed over 40 years ago. He used untreated oak posts, all he did was remove the bark with an axe. they were getting thin, but the core was so hard that it would still bend nails. We had to pull out out with a tractor.
I rebuilt my muck spreader 5 years ago with boards from a big oak tree, cut on the WoodMizer bandsaw of a local agricultural contractor.
After 5 years in the sh?t, no singn of wood rot yet...
Oaks are so tough that they hardly require any treatment... I just rub them in with old engine oil.
If i ever have enough time, i want to build a band saw. We have a lot of overgrown woodlots that havent been cut for the last 30 years, ever since we got central heating

Looking at lumber prices today, it would be such a waste to just burn them heavy logs in the stove or shop heater...