Selecting the sensor & testing the sensor:

After some research I decided to go with cheap Capacitive sensors. They are sold for $2 in typical makers maketpaces such ebay or aliexpress. possibly they are not great but for the purpose of the projects accuracy is not a big deal. I expect to be able to control a simple scale of moisture such dry,almost dry, good, wet and very wet. This levels needs to be calibrated in the configuration. But for now this will not be relevant.

Al details of this kind of sensors can be found here: https://wiki.dfrobot.com/Capacitive_Soil_Moisture_Sensor_SKU_SEN0193

Schematic: https://github.com/Arduinolibrary/DFRobot_Capacitive_Soil_Moisture_Sensor/blob/master/SEN0193%20%20Capacitive%20Soil%20Moisture%20SensorV1.0.PDF

Key parameters for this project:

1. Works with 3.3V as input. The sensor has its own voltage regulator to 3V.
2. Consumes 5mA in operation. This is not Low power friendly. [power need to be gated]
3. Analog output is pulled down with a 1M resistor and will be less 3V. The output diode will drop the output voltage 2.8-2.4V.
4. High voltages  means dry, low voltages means wet. According to the documentation arround 1V range can be managed.

To test:

1. Verify power consumption stand alone.
2. Test response time on power up.
3. Test variablity of consecutive readings.
4. Dynamic test: Record one run of the sensor from wet soil to dry soil. To check linearity.