I have looked around for "Poplar Springs" in Dixon County (and immediately surrounding areas). I did not have any luck. Poplar Springs may have been re-named later, you never know. I thought about the community of Bon Aqua. In the late 1800s it became a place folks went to enjoy the waters. But Bon Aqua is in Hickman County (but not by much; as you know) at least based on present county boundaries. Also, Bon Aqua existed perhaps as early as 1842. So, I suspect the soldier would have called it by name. I'll keep looking.
Cal