Using an Arduino module for moisture monitoring as an analog device
Well, you could contact the seller and ask for a datasheet, or you could take a look at this datasheet which appears to be the same (minor differences.)

Those modules appear to put out a voltage proportional to the moisture of the dirt.

So, you should be able to use them without an Arduino.



Note:

The datasheet says that the sensors are prone to corrosion and so not suited to being put in the soil and left for long periods.