Batteries and Electroplating Part One 

Check out the Facebook page. I'm making edits on there. But dirt is just the electrolyte. The real source of voltage comes from the reduction and oxidation potentials of the electrodes. There are concentration batteries though. In that case, the electrodes could be the same (inefficient as hell) and this will push voltage to make both cells have the same concentration. In this case different types and amounts of soils could push the reaction and create voltage. But it's incredibly inefficient. Might as well just use salt or ash. Easier to control concentration with a more pure substance.