To your right you see a picture of a soil moisture probe. They are also called water content reflectometers and consists of two stainless steel probes connected to a circuit board. There is a shielded four conducter-cable that is attached to the circuit board and a power supply. This enables the probe to monitor unfrozen soil moisture content.

There are high-speed electronic components in the circuit board that are configured as bistable multivibrators. The probes act as a wave-guide to the output of the multivibrators, which is connected to the rods. The frequency of the multiviebrator is dependant on the dielectric constant of the media (soil) being measured. The dieletric constant is primarily dependant on the water content. The circuitry in the probe scales the output (increases the signal sinewave's amplitude) to an AC voltage that can be measured by the data logger. Proving once again that you don't have to understand technology to use it.