Using a MOSFET as a switch
The gate of the MOSFET can be regarded as a pure voltage sensor which doesn't draw any current. (The only time this reasoning doesn't apply is in a high speed or high frequency application; whereas here, the signal from the sensor is changes slowly so that it is DC for all intents and purposes).

Note that the sensor is in series with another resistor.  The two form a voltage divider. The resistance of the plaster of Paris is expected to vary with moisture content and change the voltage at the midpoint of the voltage divider. If the sensor dries up completely, it basically does not conduct, and then the other resistor "pulls up" the voltage at the MOSFET's gate right to the positive supply voltage.

There is no diversion of current; the gate voltage alone controls the transistor's channel.