Soil moisture-sensing by hacking a solar light
For my probes, I choose to use a small perf board, with all of its holes routed together, on both sides. I chose it because it was what I had laying around and it had a large conducting surface area. I cut it in half to make the two probes. The two halves where then hot glued to the end of the pathway light. Last, wires where piped through the shaft to be soldered to the probes from the main circuit.
The probes could be made of many other things as well. Galvanized nails, a PCB board with copper tape on them, two steel rods, and other like things would also work fine as probes. Some things you should consider when choosing your probes is how moist your soil will be and if corrosion is going to become a large factor. If so, you might want to go with a metal that is less likely to corrode, such as galvanized steel. Also, you can encase your probes in a gypsum plaster to help prevent corrosion and keep your probes at a set distance.