We moved in a house at Dallas area and the wood post at my deck has a big crack in it, I read some post that it is normal, but if I don't treat it, will it keep cracking. Thanks a lot of any suggestions.
It is normal for that type of post in the south. To treat the wood and not affect the color use a linseed based sealer.
The applicator I've linked to is only intended as a reference as there are several manufacturers and styles of rough surface applicators.