https://en.wikipedia.org/wiki/Time-domain_reflectometer#TDR_...
https://en.wikipedia.org/wiki/Measuring_moisture_content_usi...
Details above. Looks like you can get 3.5ps resolution with an off the shelf PIC after calibration which is pretty impressive!
So the price is..
3$ for the microcontroller, 3$ for the 4 ns comparator, and a few cents for the resistors, capacitors, and 7 GHz transistors (I couldn't find the exact model), and probably some dollars for the rest of the materials such as steel rods and tubes. All in all it looks doable for <50$ easily. That's pretty amazing!
On the PIC solution you posted, how do you get around the fact that the comparator is only good to 4ns (regardless of whether you can time down to 4ps)? Lots of measurements and look at the stdev?
We bought some pot plants to keep alive and some cheap soil sensors; they're just two electrodes and you pass a current through the soil, measure the resistance and get a proxy for soil moisture. We left the sensor continuously logging on Friday, came back on Monday to find the plant very unhappy and the probe contacts almost completely corroded. Within a day or two it was a stump, never recovered.
Turns out you're not supposed to leave these things powered on 24/7 and capacitative sensors are a much better solution. TIL you can electrocute plants.