If it is only 20 trees, you could use off the shelf sensors.
Its much better and cheaper to design from scatch, use an $0.04 ultra-low power Arm microcontroller with ADC or a Padauk, add a single layer pcb with two well proportioned pcb traces in a plastic bag as a capacitive moisture censor (better than resistive sensor) and a $0.12 solar cell or rechargable battery.
Single sensor cost around $0.41 excluding labor, mass produced $0.08 including labor and sensor network
That's a great point. I have experience w/ Arduino and have made a carrier board one time [1], but am not familiar with full custom PCB + MCU designs. Do you have any suggestions on where to get started for someone with my medium level of experience here? And also, how difficult would you anticipate a custom MCU to be?
A custom MCU+pcb is at least 3 months work: downloading an design for an FPGA, testing it, using openlane to make an custom chip, taping it out, writting boot code and the main loop ( I would do that in assembly, much simpler than using C and some vendors library). The big issue is taping it out to a chip FAB, around $25K, see my comment below.
I would get started with an off the shelf MCU with good documentation, writing the software on a good development platform with the same assembly as the final microcontroller. For the $0,04 ARM microcontroller I mentioned above as final target with analog to digital controller (ADC) I would first write the program (the main loop) on a Mac on a high level language like Squeak or Python and use the $4 raspberry pi pico as ADC. I would then rewrite the working software in ARM assembly and test it directly on the pi pico. Then I would flash it on the $0.04 arm microcontroller wired on a breadboard to the moisture sensor.
Then I would debug it again, with simple flashing leds to see where your software goes wrong.
morphle at ziggo dot nl for more questions like how to find a 4 cent arm chip. I would start at lcsc.com , use their sister company and eda tool to design the pcb only with lcsc.com parts and order it fully assembled. Then I would open source it so others can directly order assembled boards
I would avoid Arduino like the plaque, in essence its just a precanned overpriced bloated C library on an overpriced microcontroller. You always get better results writing from scratch. See 'is it complex or did we make it complicated', a lecture by Alan Kay at Quallcom on Vimeo or Youtube, on designing systems from scratch or buy vendor stuff: https://vimeo.com/82301919
Its much better and cheaper to design from scatch, use an $0.04 ultra-low power Arm microcontroller with ADC or a Padauk, add a single layer pcb with two well proportioned pcb traces in a plastic bag as a capacitive moisture censor (better than resistive sensor) and a $0.12 solar cell or rechargable battery. Single sensor cost around $0.41 excluding labor, mass produced $0.08 including labor and sensor network