The color of the wood shows it is not rated as underground.

http://www.popularmechanics.com/home/how-to-plans/how-to/a1715/4217636/

This will not pass inspection in my city.

You still can use this wood. you have to dig 3' hole I cannot rember the diameter that code required.

fill the hole with concrete. make sure the conrete is a little bit high than the groud.

put a post anchor