The Soil Moisture Sensor uses capacitance to measure dielectric permittivity of the
surrounding medium. In soil, dielectric permittivity is a function of the water
content. The sensor creates a voltage proportional to the dielectric permittivity, and
therefore the water content of the soil.
The sensor averages the water content over the entire length of the sensor. There is a
2 cm zone of influence with respect to the flat surface of the sensor, but it has little
or no sensitivity at the extreme edges. The figure above shows the electromagnetic
field lines along a cross-section of the sensor, illustrating the 2 cm zone of influence.