Unscrew the hinge from the post using a screwdriver. Set the hinge and screws aside.
Open a split that is tightly closed by wedging a flathead screwdriver into the crack. This gives you room to apply an adhesive.
Scoop some exterior-grade wood putty on a small putty knife and work it into the back of the split. Layer the putty into the crack so it is completely full.
Remove the screwdriver.
Attach a clamp around the post so the split is securely shut. Wipe up any excess putty that oozes out with a damp cloth.
Allow the putty to cure according to manufacturer's instructions -- at least eight hours.
Reinstall the hinge, screwing back into the now hardened putty.