Perhaps a few details about what you are trying to accomplish would help. Is the post supposed to hold up your building? Where are you located? What kind of soil are you installing the post in? Will there be any lateral load on the post? Do you need a permit? You get the picture, we can't read minds.
definitely don't just stick a post in the ground for an actual room above it.
here in MA they'll let you build a porch or deck that way, but why would you want to? once you have the hole dug might as well spend a few bucks on a tube and some concrete and fill it up. insert J-bolt while wet, then the post anchor bracket on top of the pier. i like my piers just at grade.