wood preservative for yellow pine posts

I have various projects pending that involve setting posts.  Our farm has a surplus of second growth yellow pine, nice and straight.

Looking for the best option for treating the wood.  We live in the humid southeast so softwood posts won't last long without a preservative.  I know all the standard DIY methods but we want something less toxic

I would think a combination of charring and setting in gravel to assist drainage will give decent results.