Q&A for How to Protect a Wooden Post from Rotting in the Ground

Yes. This will stop the moisture transfer from the ground to the lumber, which could be the most common reason for wood rotting that has ground contact. The wood should still be treated against fungal attack (pressure treated is good).

That's not necessary. Paint the entire wooden fence with a waterproofing sealant intended for exterior wood (such as decks and fences). You'll also paint the portion of the fence post that will be beneath the ground. You can use a garden sprayer to spray the wood sealant if preferred

Definitely, don’t use bleach, chlordane, or insecticide. Regardless, if the post is already set, there’s nothing you can pour on the exposed part that would help in preservation. It would only preserve the above ground section of post that you apply it to.

If you can excavate to expose, then you could apply your treatment or sealant. The most vulnerable part will be the very bottom of the posts. You can install temporary supports for your stairs using a screw jack and a few 4x4, 4x6 or 6x6 timbers or three 2x4’s bolted together: screw jack and timber upon it where you want to lift, carefully turn the screw until the the weight is taken off your in-ground post, position a temporary cut snugly, use a thick board for a base. Borax (Twenty Mule Team Borax laundry booster): pour into the hole around and under the base of the post. The boron in borax will prevent fungi.