All sensors measuring the dielectric constant require calibration to obtain volumetric soil moisture. Most manufacturers provide a universal “factory calibration”, others provide a calibration(s) based on soil texture (normally sand, silt or clay) and nearly all recommend field calibration.
HydroServices measurements have shown that all sensors should be field calibrated to accurately measure volumetric soil moisture V%. At a trial site in Canterbury a number of different sensors are installed in a silt loam soil, the “best” soil texture for dielectric type sensors. Each sensor has had the “factory” calibration applied (universal or silt loam calibration). Aquaflex is the only sensor to approximate the gravimetric and the neutron probe soil moisture measurements (Figure 1). The sensors have very different responses. Soil moisture measured by the 5TM and Acclima sensors dry to V% close to wilting point for this soil type - likely under a well irrigated pasture. Aquaflex is less sensitive and does not dry to the same degree as the 5TM and Acclima sensors.
Field calibration is required to ensure accurate soil moisture readings.
Field calibration is not as simple as applying a correction factor. Soil moisture (V%) must be measured at different soil moisture content to develop a relationship – an equation to correct the sensor measurements. Figure 2 shows a “factory” calibrated 5TM sensor and the soil moisture following field calibration – it is not a simple offset or correction factor.
Field calibration has an important outcome for available soil moisture. Once field calibrated the readily available soil moisture increases from about 12mm (factory calibration) to 19mm (field calibration) in the 0-20cm depth for this soil. The accurate measure of available water has a significant effect on: