Basically a Raspberry Pi with a bunch of Grove sensors and among them the Moisture sensor.
Today I found out that who designed this sensor did a very poor job, using a direct current to detect the level of humidity.
The result after months of use is depicted in the pictures below.
Yep. One side is totally corroded !!
No wonder the reading was erratic.
Update.Not sure if it was there when I bought it, but the sensor page now reports that the sensor should be used only for brief tests and not left in the soil because corrosion can happens.
Still I found totally silly to use something to design a system and then not be able to use it in a final design because it destroy itself.
You spend time experimenting with a sensor that can not be used ! Can I say is just stupid ?