Re: Zapping a potatoe field!!
Since a great many of the potato producers grow their crops under a center pivot, the solution might be easy. The ground is placed at the pivot base and the positive wire is run to the end tower and connected to an electrode that drags on the ground, centered between the tires.
The signal can be constant when the pivot is running.
As for the frequency, have a couple of slides prepared and send them to someone experienced with a synchrometer.
I do not think that any special voltage or current will be needed. Most automatic pivots have 12 volts available.
As for sound or ultrasonic, this may work, but I am not sure if it will penetrate as well as electricity. Light definitely will not.
I used to write software for radio remote control of center pivots for potato growers.