Electronic – How much time to wait before sampling new data from soil moisture sensor

Brainstorming:

I've been reading about capacitive sensing recently. Capacitive-touch sensors have the great advantage that they can be completely hermetically sealed behind a thin layer of plastic, so there is no exposed metal to corrode. ("Electronics and Robotics: Sensing touch through a thick surface?")

Because you can't get DC current to flow through the plastic, such sensors must use AC, as Leon Heller suggested.

Alas, most of the discussion I've seen has been about how to respond only to intelligent finger-presses; what little discussion there is about water an moisture is about how to reduce the sensitivity of capacitive sensors to water splashes. ("Sparkfun forums: Capacitive sensors and waterproofing"). So clearly they are sensitive to water, so perhaps you could use this "flaw" and turn it into a feature.

A likely mechanism for the operation of the sensor is by creating a Galvanic Cell battery out of the probe electrodes, with the moisture in the soil acting as the electrolyte / "salt-bridge".

From the Wikipedia page referenced above:

The "sensor probe" would consist of two dissimilar metals such as Zinc and Copper, which @JimDearden correctly surmises in his answer.

However, the actual current flow due to Galvanic redox reactions is what would create the energy to deflect the needle of the sensor, rather than conductivity of the moist soil alone. The moisture here works merely as an electrolytic medium for the electrochemical reaction.

This sensor would fail if used in distilled or otherwise very pure water, or moisture containing no dissolved salts / electrolytes.