The dirty electricity that is on the surface of the Earth that creates a magnetic field which can be measured with a gaussmeter. The source is electricity - power lines, household electricity - as stated in my original question.
Would using an insulator like rubber tires partly buried in the ground + partly protruding above the land surface to create a continuous perimeter around a piece of land possibly shield the land inside of it from the accumulation of charge on the surface of the Earth which occurs in the vicinity...