Electricity Generated From 2 Wires Stuck Into the Ground or Dirt

There simply must be a better way to generate electricity. Or, to produce energy in any form for that matter.

I stuck two wires into the ground and they generated 0.8 volts (1/2 a normal battery). Why can't this be used on a practical level?

Electricity was generated:
1. with poles
2. - 4. with an aluminum soda can and a copper wire
3. with two wires stuck into the ground
6. with two wires stuck into an earth box
7. with two wires stuck into a flower pot

Step 1: Details