Soil Sensor Types and Technologies

With all the different types of soil sensors and measurement technologies on the market and the different technologies that they employ, choosing the right one can be a confusing and time consuming process. Here we outline the different types of soil sensors available as well as their advantages and disadvantages.

Coaxial Impedance Dielectric Reflectometry

The "Coaxial Impedance Dielectric Reflectometry" method of soil moisture measurement employs an oscillator to generate an electromagnetic signal that is propagated through the sensor (usually by metal tines or other wave guide) and into the soil. This technique was originally developed by the physics department at Dartmouth College.

Part of this signal will be reflected back to the unit by the soil, and the sensor will measure the amplitude of this reflected signal and the incident signal in volts. The ratio of these raw voltages is used in a mathematical numerical solution to Maxwell’s equations to first calculate the impedance, then both real and imaginary dielectric permittivities which in turn is used to accurately estimate soil water content. This method fully characterizes the dielectric spectrum using a radio frequency at 50 MHz.

The HydraProbe by Portland, Oregon-based Stevens Water Monitoring Systems is the only commercially available sensor to use the Coaxial Impedance Dielectric Reflectometry method along with complex computations in soil measurement, resulting in very high measurement accuracy through the lifetime of the sensor. Another advantage is that sensors using this technology do not require calibration for most soils.

According to the International Soil Moisture Network, this sensor technology is used in more water supply forecast and climatological networks than any other soil sensor technology.

Complex mathematical computations performed by an onboard microprocessor process the reflected signal measurements to accurately determine the soil’s dielectric permittivities, the key parameters behind the soil moisture and bulk EC measurement. Because the soil measurement computations are performed by a microcontroller inside the sensor, the probe can output results in standard engineering units.

Coaxial Impedance Dielectric Reflectometry also provides low inter-sensor variability, so every sensor measures the same without the need to calibrate it.

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Frequency Domain Reflectometry (FDR)

There are several soil probes on the market today that use the Frequency Domain Reflectometry method of soil measurement. This method of measurement also uses an oscillator to propagate an electromagnetic signal through a metal tine or other wave guide, but with this method, the difference between the output wave and the return wave frequency is measured to determine soil moisture.

FDR probes are considered accurate but must be calibrated for the type of soil they will be buried in. They offer a faster response time compared to Time Domain Reflectometer (TDR) probes and can be connected to a standard data logger to collect readings. Examples of sensors in this category include the Decagon 5TM, and the Sentek EnviroSCAN Probe. While frequency capacitance type soil sensors are call “FDR” sensors, this is somewhat of a misnomer because many of these probes only use a single frequency and not a “domain” of many frequencies. The Delta-T Theta probe is a coaxial differential amplitude reflectometer.

Time Domain Reflectometry (TDR)

Sensors that use the Time Domain Reflectometry (TDR) function in a somewhat similar way to FDR probes, but the mechanics behind the measurement system are different. TDR sensors propagate a pulse down a line into the soil, which is terminated at the end by a probe with wave guides. TDR systems determine the water content of the soil by measuring how long it takes the pulse to come back. Examples of this sensor include the Campbell Scientific CS650 and the IMKO Trime-Piko 32.

TDR soil moisture measurement devices require a device to generate the electronic pulse and need to be carefully calibrated in order to precisely measure the amount of time it takes for the pulse to propagate down the line and back again. They are also sensitive to the saline content of salt and relatively expensive compared to some measurement methods. However, TRD devices do respond quickly to varying soil moisture.

Time Domain Transmissometry (TDT)

Time Domain Transmissometry (TDT) is similar to TDR, however it measures the transmission, rather than reflection, of a pulse along a looped, or closed circuit, rod. TDT measures the time taken for an electromagnetic wave to propagate (travel) along a given length of a transmission line in the soil. With TDT, a step pulse with a fast rise time is transmitted into a transmission line. The step pulse travels down the transmission line and a voltage threshold is detected at the other end of the transmission line. There is no complex waveform analysis as with TDR. The shape of the transmitted waveform is not relevant, but a measurement of the pulse travel time through the transmission line will allow an estimate of the dielectric constant of the medium.

Similar to TDR, a pulse measured via TDT will be slower in wetter soils than drier soils. The velocity of the pulse is related to the dielectric constant.

TDT sensors, being a more refined version of TDR, generally provide greater accuracy and lower power consumption than TDR sensors. However, they are usually more expensive.

Examples of this type of sensor include the GroPoint sensor family and the Acclima TDT Soil Moisture Sensor.

Gypsum Blocks

Gypsum blocks use two electrodes placed into a small block of gypsum to measure soil water tension. Wires connected to the electrodes are connected to either a portable hand-held reader or a data logger. The amount of water in the soil is determined by the electrical resistance between the two electrodes within the gypsum block. More water present in the soil will reduce the resistance, while less water will increase it.

While gypsum blocks can be relatively inexpensive and easy to install compared to other types of soil sensors, they have to be replaced periodically as the gypsum disintegrates. Gypsum blocks are also more sensitive to having readings throwing off by soil with high salinity (salt content).

Neutron Probes

Neutron probes are another way to measure soil moisture content. A probe inserted in the ground emits low-level radiation in the form of neutrons. These collide with the hydrogen atoms contained in water, which is detected by the probe. The more water content in the soil, the more neutrons are scattered back at the device.

Neutron probes are very accurate measurement devices when used properly but are expensive compared to most other measurement methods and generally have to be registered with the federal government due to radioactive elements used to emit the neutrons.

Gravimetric Probes

Gravimetric soil moisture measurement involves taking a sample of the soil from the site, weighing the sample, drying it in an oven for 24 hours and then re-weighing it to determine how much water was lost.

This soil measurement technique is inexpensive and easy to execute, but the long wait time from sample to result and the process cannot be automated. It is also difficult to obtain data from moderate soil depths as digging a new sample each time is necessary.