Gypsum Blocks
Gypsum is a naturally occurring porous mineral. When shaped into a block and buried in the soil, water from the surrounding soil moves into and out of the gypsum block as though it were another piece of soil.
A gypsum block sensor consists of two electrodes embedded in a block, 'tablet' or cylinder of gypsum. When water moves into the gypsum block some of that gypsum dissolves, allowing a current to move between the electrodes. As the amount of water in the block changes so does the resistance to current flow.
As the soil dries out, water leaves the gypsum block and the resistance between the electrodes increases. Conversely, as the soil wets, water is drawn back into the gypsum block and the resistance decreases. These resistance values are then translated into soil moisture tension readings which have the units of kiloPascals (kPa).
Ceramic (Tensiometers)
The hollow ceramic tips of tensiometers are porous, allowing water to move into and out of a sealed water storage 'reservoir' or tube inside the tensiometer shaft.
As the soil dries out, water is sucked out of the tensiometer through the porous ceramic tip. This creates a partial vacuum inside of the tube, wich is registered by a vacuum gauge.
Tensiometers usually operate over the range 0kPa to -80kPa.
Sensors utilising the Dielectric Constant or Dielectric Permittivity of soil
There are a number of soil moisture content sensors that use electric fields to monitor a property of soil called its 'dielectric constant'.
Water greatly changes a soil's dielectric constant. Dry soil has a dielectric constant of between 2 and 5. Pure water has a dielectric constant of 80. So as the moisture levels in the soil change, so does the dielectric constant.
The class of sensors that look at Dielectric Permittivity can be built to monitor soil moisture content when buried directly in the soil or when 'looking' at the soil through the walls of a buried access tube. A key advantage of these sensors is that mineral particles such as salt barely effect the dielectric constant of soil so the soil moisture readings are largely unaffected.
Capacitance or Frequency Domain Reflectometry (FDR) Probes
Capacitance probes utilise the fringing effect of two metal ring electrodes located one above the other on the probe to measure soil moisture. The fringing effect is the tendancy for the electric field to flow or jump from one electrode to another - similar to arcing.
So, these two metal rings form the plates of the capacitor with the soil acting as the dielectric or insulation in between. Capacitance measures the ability of this soil to hold an electrical charge when you apply a voltage to it. The ability to hold a charge is very dependent on the dielectric constant of the material between the electrodes or in this case the soil between the metal rings. As mentioned earlier dry soil has a constant of between 2 and 5 whereas water has a constant of 80.
The sensor applies a voltage and creates a circuit (flow of electrical current). This current which will oscillate or vibrate at a (resonant) frequency that is dependant on the amount of water in the soil. When you add water to the soil its ability to hold charge (capacitance) changes, which then changes the vibration (resonant frequency) of the circuit.
The probe measures this change in (resonant frequency), and uses it to determine the soil moisture content.
The best way to think of it, is like a row of glass bottles, each with a different level of water in it - the sound each one makes when you tap it tells you roughly how much water it contains.
Time Domain Reflectometry devices (TDR)
TDR devices also make use of the dielectric constant to measure the soil moisture content. They employ microwave technology and send a high-frequency electrical pulse down two parallel probes embedded in the soil. The signal reaches the end of the probe and is reflected back along the probes to a sensor.
The time it takes for the signal to travel to the end of the probe and back again varies with the soil dielectric constant and therefore can be related back to the water content of the soil surrounding the probe. The more water content the slower the electric field will travel.
ThetaProbes
ThetaProbes use an array or four rods pushed into the soil to enclose a well-defined cylinder of soil. ThetaProbes send radio waves down the middle one of the steel rods. The radio wave is deformed as it travels dependant on the soil moisture that is present.
The ThetaProbe actually measures the change in the amplitude of the radio wave rather than the change in frequency of the signal (as per a capacitance probe) or the time taken for the signal to travel (as per TDR probe). The amplitude change is highly dependent on the soil dielectric permittivity and therefore can be used to work out the soil moisture content.
These are the most accurate soil moisture content sensors, working in all soil types.
This technology is also employed by the WET Sensor and the Profile Probe.