The resistive version doesn't need a trigger IC, you simply put some voltage to one pin (might be a nail) you put into the soil and measure the voltage you get on the other pin (nail) a few millimeters away, which is connected to GND via a resistor to build a voltage divider.
This kind of sensor is known to have problems with corrosion so don't use it in a productive environment.
The other type of sensor uses some high frequency signal to measure the moisture in the soil but for that type I don't know the details.