Protecting Fence Posts

I just built wood fence with a low 1.5' retaining wall at the base. The posts are pressure treated 4x4's spaced 7' apart and held in the ground with concrete. The wall is made of concrete retaining wall blocks that run between the posts up to the bottom of the fence. Before filling in soil behind the wall, I want to do something to keep the soil off of the posts. One thought I have is to mound up gravel as a barrier between the soil and the posts. What are the chances this will be effective? Any other ideas?
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