Soil batteries?

In an attempt to demonstrate the basic principles of common batteries, my teacher assigned me to demonstrate a simple experiment to the kids at the elementary school. A tank filled with saltwater, copper ring used as cathode, cola tin as anode and a motor is powered etc. Surprisingly one little guy there asked if we could use unprocessed soil with natural salts and follow the same technique on a large scale. 


I am not keen on disappointing future (maybe) scientists so I answered: yes, but using the soil as an electrolyte is, as it seems inefficient. My teacher said I should search and come with a whole answer so here I am. What about on other worlds? What about on mars? Perhaps using supercritical $\ce{CO2}$ could be the thing. Can a soil battery be useful at all? I like the concept.




Unfortunately, the little guy would be a bit disappointed down the line. The energy of the cell comes from the oxidation of the aluminum (the soda pop can) and that Al anode is eventually consumed. Aluminum is produced at relatively high cost, using electrolysis, so wet soil (or seawater) is not the issue. Same deal with the standard Zn and Cu electrodes stuck into a lemon, potato, watermelon, or other fruit/vegetable: the oxidation of the anode metal is the ultimate energy source and the electrolyte medium just plays a necessary supporting role. (Of course, some hydrogen gas is produced at the cathode, but that does not help much.) Still, this makes a nice demo for the kids and the corrosion tie-in is great!


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