It is common knowledge that a pair Earth stakes will pick up a small amount of current. Hobbyists have been using the idea for years to run small radios. Like

It is common knowledge that a pair Earth stakes will pick up a small amount of current. Hobbyists have been using the idea for years to run small radios. Likewise that same ground current will screw up Telegraphs (etc) which try to use an earth-return path. The problem for large scale energy harvesting is that the Earth is conductive, so any voltage differences are shorted out by the Earth itself and dissipated as heat. To put it another way, the Earth is a lossy medium, so any signals are heavily attenuated by distance.