lost_and_confused:
Yeah, the two nails are the way I am wanting to go.
I'm using a couple of stainless steel cooking skewers about 15cm long as moisture sensors, just pushed into the soil. They're mounted in a chop block electrical connector to keep them parallel and this also gives a convenient way to attach the wires. I'm using the technique of only powering the sensor when I need to take a reading, and reversing the pulse after the measurement to avoid any ionization effects. It took me a while to figure out the resistance and timing to get a consistent reading that reflects water content within the range I'm measuring, but it seems to be working OK now.