First any electrode in soil is not going to be permanent as soil is an "active" matrix and anything used will eventually fail.
Graphite electrodes are used all the time in generating hydrogen in caustic (0.4m NaOH) solutions, with a voltage higher than 1.23V, (usually around 2 volts) and 0.5 amps per square inch of electrode surface. these graphite electrodes last months in this caustic solution. So the thought is that using these in soil they should last significantly longer. Also graphite rods are cheap if you purchase them on eBay
As it is the breakdown of the graphite rod binders on the anode side my thought was that these electrodes would last longer and not emit any toxic elements from the decomposition of the anode. For DC resistance measurement only the anode would have to be graphite the cathode could be iron or some other conductive material
If used in a a potted container (with drainage) any buildup of accumulated degradation products should wash away with each watering of the container. (pending completed wetting of the soil in the container)
As it is autumn here and growing season is coming to an end I wanted to see if anyone ever experimented with graphite rods as probes.
I have ordered a few and I plan on testing their longevity on some indoor plants when they arrive.