Soil sensor types and technology
With all the different types of soil sensors and measurement techniques on the market and the different technologies that they employ, choosing the right one can be a confusing and time consuming process. This section outlines the different types of technologies available as well as their advantages and disadvantages.
Frequency Domain Reflectometry (FDR)
Stevens Hydra Probe soil sensor
Sometimes referred to as capacitance sensors, soil probes that use the Frequency Domain Reflectometry method of soil moisture measurement employ an oscillator to generate an electromagnetic signal that is propagated through the unit (usually by metal tines or other wave guide) and into the soil. Part of this signal will be reflected back to the unit by the soil. This reflected wave is what is measured by the FDR probe, telling the user what the water content of the soil is. Several popular brands of soil probes employee this technology, including the Stevens Hydra Probe.
Frequency Domain Reflectometer probes are considered highly 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. Other examples of sensors in this catagory include the Adcon C-probe, the Delta-T Theta Probe and the Decagon ECHO probe.
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 measure the 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 CR616 and the IMKO Trime.
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.
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
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.
