After following some links around the net that peaked my curiosity I came upon something that all my schooling and instincts said "no way". So naturally I decided to try it and the results were simply amazing. You can get electricity from your yard, and almost anything in it.
so here's what I did, try it, you will be amazed too, and begin to wonder what else is not as we were taught.

Step 1: Tools

you will need a voltmeter, scraps of wire, some water and paper towels.
some stiffer wire, roll of wire, PVC tubing, brass and steel rods later if you decide to follow up with phase 2 and 3

Step 2: Phase 1

I have a patio with retaining wall around it, gravel sub base, and flag stones to walk on. All the ingredients are there.
What you need is gravel, or cement or sand or sand dirt mix in the lawn.
Hmm sound like most peoples yards?
Dampen 2 spots on the retaining wall block or cement block on opposite sides, using the wet paper towel as a contact point read the voltage that's available. Before you say 'yeah. right' JUST TRY IT!!
I had reading from .5 to 1.3v dc.
I now grabbed the garden hose and hosed the wall down, soaking it well. (excitement builds,as this isn't possible)
2 blocks read the same as the two added together, 3 blocks read the same as the three added together. hmm
Grab the scraps of wire! Wire 2 blocks in series, the voltage increases a small amount, same with three blocks.
Grab the left over blocks, I have 6. stack them up with wet paper towel between. My voltmeter reads 7.4v, unbelievable!
Now for the real test, Grab a small fan from a computer, hook it up. .....
Mine ran for about a minute before stopping, slower than normal, but I didnt have 12v to start with.
I disconnected one wire and pondered what just happened for a few minutes.
Then reconnected the wire just for kicks, and the fan started again and ran about a minute.
No Way, it recharged in a couple minutes? grab the stop watch!!
After several try's they apparently they recharge in 15 to 30 seconds!!
How about 2 stacks of 3? gave me a voltage of 3.6v
The fan runs about a min and a half, 20 second recharge, another 1.5 min.
ok now on to phase 2

Step 3: Phase 2

This time I need to try pushing brass welding rod and steel coat hangers into the ground.
the steel is the center of the new battery. the first brass rod is pushed in to the water soaked ground until spacing is found that gave me 1.5v. larger spacing gives higher voltage. the steel are separated by 6 inches and the same for the brass till I have 8 cells. wire the brass to the steel so the cells are in series, giving about 12v when done. my fan ran a total of 5 minutes. Again about 20 secs recharge time and it ran again.
I pulled every other wire out and re-spaced to 12 inches. the fan ran about 7 minutes. ( when I say about 7 minutes, that's using my cell phone clock, not a real stop watch.) So wider spacing gives more amperage, cool.
You have to be aware that apparently quartz needs to be present, hence the sand, or concrete. Black dirt or humus provides to small a voltage. But the quartz seems to help the transfer. crushed Granite or rock such as class 5 has is good too.
on to phase 3

Step 4: Phase 3

can I manufacture a battery from these components?
Out comes the PVC tubing in 4" diameter.
drill hole thru an end cap for a wire, bare the end about 2 ", flare it out and place it between 2 layers of paper towel in the cap.
place the cap on one end of the PVC. Also drill a couple water drain holes in the cap.
Fill with sand to about 3 inches, and test the voltage. This gave me 1.5v, yours can vary, by how wet the sand is, so soak it well.
Put a layer of wet paper towel, and another layer of sand.
repeat till you have 8 layers, and about 12v
last layer of paper towel and connection wire, I eventually cut down a cap ti fit inside the PVC pipe wit a press fit and drain holes, to hold pressure on the wire connections.
Oh no, the fan only runs just over a minute, and now wont recharge!!
Wait, grab a shovel, bury the tube either vertically or horizontally, soak the ground liberally.
Now the fan runs almost 2 minutes and recharges in 20 secs. Apparently the power is coming from the ground.
Not overly impressive for a 4" diameter battery 24" long, but the fact that it recharges every 20 secs is!
now on to the last step.

Step 5: Challenge

If a series of batteries buried will deliver power for 2 minutes, recharge for 20 secs, and repeat as long as the ground is wet.
How do you cycle 2 or more cells to deliver power continuously?

Can a series of cells deliver high enough power to convert to AC? thru an inverter?

How many cells in parallel give adequate amperage to convert to AC?

Can a backyard deliver enough usable power? for even minor appliances?

What could an abandoned gravel pit deliver?

What circuits would be needed to rotate the cells properly?

One thing I can tell you, LED lighting works great on the one cell, and lasts about 4 hrs, so a slow draw extends the discharge rate.

if you work out any usable circuits or battery styles please let me know, I think this could be a very good emergency backup, or even more.