Phil L wrote:

Lean mix concrete might be better, IE 10 gravel, 6 sand, 1 cement - just
something that won't wash away, but will be easily broken up when the need
arises - it will also allow water through, which is important with wood -
it's the cracks which appear in hard concrete around the base of the
shrunken timber which hold water and cause end rot - soft, gravelly concrete
eliminates this but still adds mass to the bottom of the post for stability,
also, mix it completely dry - no water at all.

Never tried this, but it sounds eminently sensible.