Protecting Fence Planks Against Soil

I have cedar plank fences in my back yard and want to add a shallow flower bed around the (inside) perimeter. The fence boards are treated with tar only on the bottom, and sit about at soil level in the grass. If I fill in with maybe 8 inches of dirt, what should I do to the bottom 8 inches of board (now with soil resting against it) to protect it. Is the same tar treating good enough or shoul I place some sort of physical barrier in there as well?
- Magnusfarce
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physical barrier in there

The tar isnt normally used on wood..Its generally used for concrete foundations.
"Leaning" garden soil against a fence isn't something I'd do but if you have to, I'd screw some green 1" X 8" PTL boards vertically along the fence, then I'd staple a 'row' of folded over 6 mil plastic to the PTL where the soil contact is going to occur. Some folks would put a double or triple row of PTL 4X4 s between the dirt and the fence.
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