This is a very cheap analogue board containing a comparator and and a pot to give both an analogue and digital (threshold) output.

Various sensor configurations can be found which include an LDR, NTC thermistor, two resistive water/moisture detectors and more besides.

The water/moisture detectors utilise a 2 pin header which can allow us to reuse the comparator board for any particular device we have available. For example, the top picture shows a flat style 10K thermistor along with the moisture detector kit.

Due to the circuit's simplicity, the digital output tends to oscillate when the two inputs to the comparator have a similar magnitude.

This is a reversed engineered schematic, the capacitor values are unknown.

After 3 months of use all I can say is this probe is mostly unusable, the resistance value appears to be make little sense until at least you notice that the probe is corroding. I have placed a second probe in the soil but this time I have insulated the probe header (the other rusted) and now have the probe against the inside edge of the pot rather than anywhere near the middle, maybe it will last longer…

Conclusion. The moisture probes are a waste of time.