This is not a tricky, but a super simple and cheap moisture measurement device.
A resistive type moisture sensor (Iduino ME110) evaluates the soil moisture, a Raspberry Pi Pico microcontroller converts it to a digital value that gets shown on a single 7-Segment-Display in a 0-9 range.
Excluding the Pico, the material costs are around 2€ (I bought a whole bag of 7-Segment-Displays on discount for around 3€, for example).
This project is easy to modify, e.g. displaying two digits or using some other sensors respectively microcontrollers.