The hardware is consists of an Arduino microcontroller (here an Arduino Uno R3) and a pre-wired soil moisture sensor module. The soil moisture sensor module , built around the LM393 comparator, gives ...

My soil-moisture reading was in the low 700s in soil that had around 30 percent moisture (according to a comparison with a commercial soil-moisture sensor), and 661 in a drier pot.

I'm planning to have a "spider" of moisture sensors off one Arduino - 5 or 6 coming off one box. Again, I doubt you need continuous monitoring even with multiple sensors.

A competent Arduino implementation (SparkFun RedBoard) can get down to around 5mA from USB (if you remove the power LED) in deep sleep mode. The cheap Chinese ones do draw a lot more (20mA in deep ...