the moisture sensor will signal the pump that it's time to re-hydrate. There are sensors sensitive enough to pick up minute ...

Common “soil moisture meter module arduino raspberry compatible free shipping ... what it takes to build a soil-embedded sensor that can actually survive contact with a plant.

Although there are ways to get around this, a 2022 paper by Maja Škiljo and colleagues in Sensors covers an interesting way to make use of this signal attenuation property of changing moisture ...