Either one of these sensor types can be used if you choose to flash the code from my github repository (with sensor output connected to ADC0/GP26, Pin 31):

Power supply capacitive sensor HW-390 is connected to VBUS Pin 40 (5V), but that doesn't matter actually because there is a 3,3V voltage regulator on the sensor board.

Power supply resistive sensor ME-110 is connected to Pin 36 (3,3V).

Schematic ME110 (ERDE = SOIL):