If you want to measure the electric properties of soil, there's no reason to have 70V between the electrodes used. A few volts are absolutely sufficient. Perhaps you came to the conclusion that you need 70V because you have a large electrode distance in mind? Better have them only about 1 cm apart, because that way, you can keep the electrodes small, have a moderate, easy to measure impedance, and have full control over the soil sample you are measuring, plus some confidence that this sample is quite homogeneous.
I wrote impedance, not resistance, because it is preferred to measure with pure AC, no DC component, to avoid corrosion and a flow of ions that, on the long run, changes the soil sample. Doing so, you can conveniently model the soil as a parallel resistor and capacitor. By measuring amplitude and phase, both components can be identified. The electrodes can even be covered with an insulator if this is carried out properly.