Is it possible to power an arduino-like device from soil?
This isn't going to work.  Think about it.  If dirt to power worked, it would be done regularly.  There are some ways that soil can help to get some energy, but these will be difficult to extract or the amounts would not be useful for most purposes:

Sufficiently wet soil can be the electrolyte of a battery.  It wouldn't be a very good electrolyte, and the energy would come more from the plates that you'd insert into the soil than the soil itself.  Therefore saying you're getting power from soil is a bit of a stretch.

From thermal differences.  You can use soil as a big temperature averager, especially if you go a few feet or more down.  The instantaneous temperature difference between the ground and the air represent power that can in theory be extracted.

Note that the Carnot efficiency is not your friend here.  Carnot says that the maximum possible theoretical efficiency of a heat engine is Tdiff/Thot, when the temperatures are measured on some linear absolute scale, like Kelvin.  For example, Lets say the soil is 68 &deg;F and the air 90 &deg;F.  That's 293 &deg;K to 305 &deg;K, for a Carnot efficiency of only 4%.  That means you have to have a large thermal connection to both the soil and the air to extract meaningful power levels.



I would look into solar cells.  Perhaps these could keep a battery charged so that the device can run whenever it wants to, whether the sun is shining at that moment or not.