It looks like the idea has been around for quite a while ( the first battery made using soil was in 1830) so I figure that it must not be feasible for large scale. I like the idea of charging the cell phone though, and even more the idea of the houseplant generating enough electricity to power its' own grow light!
I wonder now, if is was not feasible as in not a big enough money spinner? In SA we had the electricity supplier (who is spectacularly failing to meet the demand btw) arguing that wind generated electricity is not worth the cost and won't work… Yeah, right.