There is no reason for soil to be a never-ending resource. It should run out, as with clay @10 to eventually nothing.
Might I also add that if the soil were to be "depleted" plants should not increase in quality at all upon harvesting. (It would still be difficult to even do that, because the soil would regenerate a point in 8 hours .)
I can see no cons and only pros to this mechanism being added to the game.