Soil Sensor Technologies

There are several different technologies used to measure the volumetric water content of soil in today’s commercial electromagnetic soil sensors. They differ primarily in the accuracy provided and cost to manufacture, but also affect the overall durability of the probe.

Technology	Principal of Operations	Physical Measurement	Basis for Soil Moisture	Typical Frequency
Time Domain Reflectometry (TDR)	Measures time for an electromagnetic wave to travel out and be reflected back from the end of the probe. Transmission line oscillators generate a voltage pulse inside the sensor head which propagates along the waveguide, with the arrival of the reflected pulse triggering the next pulse. The number of voltage pulse reflections over a certain time interval is recorded and a period (microseconds) that is inversely related to the number of reflections per second is output. The period is directly related to volumetric soil moisture via empirical calibration.	Time for a voltage pulse to travel along parallel rods and reflect back	Apparent permittivity	1000 MHz
Time Domain Transmissometry (TDT)	Measures electromagnetic wave time of travel in one direction over the length of the probe	Time for a voltage pulse to travel the length of looped or closed circuit rod	Apparent permittivity	150 to 2000 MHz
Capacitance (Frequency) / Frequency Domain Reflectometry (FDR)	Measures the change in frequency of an electromagnetic wave traveling out and being reflected back to the sensor head.	Difference between output wave and the return wave frequency	Apparent permittivity	150 MHz
Capacitance (charge)	Measures the charge time of a capacitor that uses the medium surrounding the probe as the dielectric material.	Capacitor charge time	Capacitance	n/a
Differential amplitude (Simplified Impedance)	Measures the difference between the incident signal and the reflected signal to calculate impedance and apparent permittivity.	Difference in reflected amplitudes	Apparent permittivity	75 MHz
Coaxial Impedance Dielectric Reflectometry	Measures the ratio of reflected voltage to an incident voltage of a 50 MHz signal, which is dependent on the impedance of medium between the probe rods. The tines serve as wave guides for a 50-MHz-signal that is generated in the probe body. When the tines are pushed into soil material, the probe measures the behavior of an electromagnetic wave in the soil between conductors.	Ratio of reflected amplitudes to measure the impedance	Real dielectric permittivity	50 MHz
Neutron Probe	Based on measuring fast-moving neutrons that are slowed (thermalised) by an elastic collision with existing hydrogen particles in the soil.	Collisions of emitted neutrons with hydrogen atoms	Apparent permittivity	n/a
Gravimetric Soil Analysis	Gravimetric water content is measured by weighing a soil sample, drying the sample to remove the water, then weighing the dried soil.	Difference in mass (weight) of soil before and after being dried to remove all water	Mass of water	n/a

Prevalence of Soil Measurement Technologies in Soil Moisture Networks Worldwide

Source: International Soil Moisture Network, 2016

Capacitance (Charge)

Capacitance determines the dielectric constant (Ka) by measuring the charge time of a capacitor, which uses soil as a dielectric medium. The charge time of the capacitor is a linear function of the dielectric permittivity of the soil. Capacitance sensors measure the dielectric constant of …

Coaxial Impedance Dielectric Reflectometry

The Coaxial Impedance Dielectric Reflectometry (also referred to as “Ratiometric Coaxial Impedance Dielectric Reflectometry”) method of soil moisture measurement employs an oscillator to generate an electromagnetic signal that is propagated through the sensor by metal tines and into the soil. It …

Frequency Domain Reflectometry (FDR) / Capacitance (Frequency)

Capacitance can be measured from the change in frequency of a reflected radio wave or resonance frequency (Kelleners 2004). This method of measurement uses an oscillator to propagate an electromagnetic signal through a metal tine or other wave guide. The …

Gypsum Block

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 …

Neutron Probe

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 …

Time Domain Reflectometry (TDR)

Sensors that use the Time Domain Reflectometry (TDR) are somewhat similar way to FDR probes, but the mechanics behind the measurement system are different. TDR sensors use parallel rods, acting as transmission lines. A voltage is applied to the rods …

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 …