When I use steel posts, I set them in concrete about three feet deep, unless using pipe over six inches in diameter. I never pour concrete around wood posts or railroad ties. Water gets between the post and the concrete and the post rots out in a relatively short time. For fence posts I tamp down native soil removed from the hole. For structures using wood posts I pour a concrete base above grade and set a post base in it. For a pole barn I would use telephone poles and tamp dirt around the post. There's a reason utility companies don't pour concrete around their poles. Frost is not a problem in the southwest desert.
