Soil Sensor Calibration
With the exception of sensors based on the Coaxial Impedance Dielectric Reflectometry method of measurement, all sensors measuring the dielectric constant require calibration to obtain accurate volumetric soil moisture. Most manufacturers provide a universal “factory calibration”, others provide calibration(s) based on soil texture (normally sand, silt or clay) and nearly all recommend field calibration.
In almost every case, a sensor’s published accuracy specification can only be achieved with soil-specific field calibration.
Soil moisture sensors are typically delivered from a manufacturer with a percentage output. This value is often derived from a generic soil probably from the yard where the manufacturer is located. Needless to say, this calibration will be different from your particular soil.
Field calibration is not as simple as applying a correction factor. Soil moisture must be measured at different levels of soil moisture content to develop a relationship—an equation to correct the sensor measurements.
Manufacturers of soil water content sensors supply their product with an output registering volumetric water content (VWC), typically expressed as a percentage. But where does this number come from and what does it actually mean?
In all likelihood, the output value from the sensor will not be equal to the actual VWC of your particular soil. The output value is not entirely incorrect, per se, rather it is an estimation of the actual, or true, value of VWC of your soil. Where this number comes from, and how it is derived, can be a major source of error in soil moisture measurements.
The second biggest source of error is sensor to sensor variation. There will inevitably be slight variations in sensor output due to electronics and the manufacturing process. Even a variation of 1% to 2% VMC can cause serious miscalculations in scientific modelling, catchment water budgets, or irrigation scheduling. Note that when calibrating your soil moisture sensor, you will need to consider an acceptable level of accuracy. Many researchers calibrate a particular model of soil moisture sensor for each different type of soil or substrate they will encounter (for example, a sandy soil versus a loam soil). However, soil moisture sensors are never exactly the same even if they are the same model or produced from the same manufacturer. This leads to sensor to sensor variation in measurements. Therefore, many researchers calibrate every single soil moisture sensor for each type of soil or substrate they will be measuring.
Note that due to how the Coaxial Impedance Dielectric Reflectometry method measures both the real and imaginary dielectric permittivity, sensor-to-sensor variation is not a problem.