Pure water does not have enough ions to conduct electricity. A mixture of metals such as iron, zinc and copper in the wet soil can trigger electrolysis that requires excess energy in the form of over potential to conduct electricity. The excess energy is needed due to limited self-ionization of water. The wet soil then can conduct current when positive and negative ions are present. The water ions begin to flow from anode (positive electrode) to cathode (negative electrode) to be oxidize and produce electricity.