In article , Frank Erskine
writes

Rather
than just plonking the posts into their holes and pouring concrete
around them, you should place a bit of slate or other impervious
material at the bottom of the hole, so that the post is reasonably
encapsulated from the soil.

How about putting the bottom of the post in a plastic bag? Something
like a rubble sack, and taping it up, before pouring the concrete around
it? In addition to engine oil/creosote?

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