Can electricity be harvested from the soil around plants?
Electricity may be indirectly generated from plants through the use of a microbial fuel cell, in which biologically-catalyzed chemical reactions are used to drive an electrochemical cell.

A non-technical description of the technology can be found here.  The basic idea is that plants produce organic compounds, which are broken down by soil microorganisms to produce carbon dioxide, electrons, and hydrogen ions.  Ordinarily the hydrogen ions and electrons combine with oxygen to form water, but there are various ways to divert the electrons to travel through an electrical circuit before the oxidation reaction takes place.  This flow of electrons (electricity) can be used as a power source.

This technology is still in its early stages but the process has been demonstrated in some laboratory proof-of-concepts:


Strik, David P. B. T. B.; Hamelers (Bert), H. V. M.; Snel, Jan F. H.; Buisman, Cees J. N. (2008). "Green electricity production with living plants and bacteria in a fuel cell". International Journal of Energy Research 32: 870–876. doi:10.1002/er.1397.
De Schamphelaire, Liesje; Van den Bossche, Leen; Dang, Hai Son; Höfte, Monica; Boon, Nico; Rabaey, Korneel; Verstraete, Willy (2008). "Microbial fuel cells generating electricity from rhizodeposits of rice plants". Environmental Science and Technology 42 (8): 3053–3058. doi:10.1021/es071938w.


The E-Kaia company has not released enough information to say whether their product is in fact a plant microbial fuel cell, a simple earth battery, or a mere scam.