Question
poor planning gave my cellar a concrete floor. i am looking at an area about 6'x8' that i would like to use as a root cellar. my question: while i am sure i can make it tight enough to be cool, what about the lack of moisture with a non-dirt floor?

Answer
Perfect humidity for a root cellar is between 70 and 95 degrees. Cement will just have a little lower humidity than a dirt floor. You can always set buckets of water on the floor or just sprinkle water on the floor if you find you don't have enough humidity.  If you get much temperature fluctuation in your cellar, or have too much humidity you will notice some condensation on the ceiling and walls, and this can cause spoilage.

You want it to be cold - between 34 and 40 degrees, and well vented, with good air curculation. If you can, have a vent you can open and close to the outside for air circulation, or you can use a fan.

Keep your shelves a couple of inches away from the walls, this will improve circulation.