Soil Sensor Types
Volumetric Water Content Sensors
Sensors that measure the volumetric water content are typically referred to as soil moisture sensors. “Volumetric Water Content” or VWC is a measure of the amount of water held in a soil expressed as a percentage of the total mixture, and is often called simply “soil moisture”. The amount of water that can be stored by a soil and its availability to plants both depend on soil type.
Tensiometers (Soil Matric Potential Sensors)
Tensiometers measure the soil water potential or matric potential. Soil matric potential is the pressure it takes to pull water out of the soil and is an indicator of stress to plants and crops. It can be used to determine soil water fluxes and available water held in the soil.
Most soil sensors are single-point sensors, which means they take a measurement (or series of measurements if more than one soil attribute is being measured) at a single location.
Examples of single-point soil probes
Soil Profiling Probes
Profiling probes measure soil moisture (and often other attributes like temperature) across a vertical soil profile, typically spanning a range of 30cm to 120cm. Soil profiling probes typically consist of multiple single-point sensors housed within an elongated enclosure, although some, like the GroPoint Profile feature modular segments which form a single antenna for continuous measurement across the entire length. A soil profiling probe is installed in the soil vertically.
Measuring soil moisture at multiple depths is important for optimizing irrigation, as it helps characterize the penetration of water throughout the root zone.
The chief advantage of using a soil profiling probe is the elimination of the cost of multiple single-point sensors and the need to excavate a large hole in order to bury them at the appropriate depths.
Profiling probes are usually manufactured as parallel pairs of rings along a probe or rod, and are typically installed in plastic or PVC access tubes where the electric field between the sensor and the soil must pass through the plastic tube. This design imposes uncertainties as to whether the access tube, soil, or meniscus that can form on the outside of the access tube following rainfall or irrigation, is being measured. The volume of measurement and its geometry are also uncertain (Topp 2003). Profiling probes which don’t require an access tube will typically provide greater accuracy for this reason. The GroPoint Profile (which also uses the superior TDT method of moisture measurement) and Hydrascout HSTi are examples.
Examples of profiling probes
Permanent and Semi-Permanent Installations
Soil sensors are typically buried and left for continuous long-term monitoring (if connected to a data logger or wireless remote telemetry) or on-demand monitoring (using a handheld reader). They can remain buried indefinitely, subject to the durability of the sensor and especially the cable.
Portable Soil Sensors
Portable soil sensors are designed to provide the user an instantaneous reading of soil moisture in a battery-powered, self-contained unit that can be taken anywhere. Readings are displayed either on an integrated display, or on the user’s smartphone which communicates with the sensor unit wirelessly (Bluetooth or WiFi).
Examples of portable probes