The second method is a two-step procedure as employed by Bogena et al. (2010) [13], [24]. In this method, the output signal is first related to apparent relative permittivity (e.g., based on the standardized sensor characterization method using standard solutions with known relative permittivity [25]), and is then related to volumetric water content . The – relationship in the second step is relatively well understood [24], [26], [27]. An advantage of the two-step calibration method is that, assuming that the – relationship in the second step is valid for the soils of interest, recalibration of all sensors is not required when the sensors are installed in different soils. On the other hand, in the direct approach, all sensors need to be recalibrated for each soil [24]. Therefore, it will cost a significant amount of time and efforts to obtain a number of OS- – data points, in order to determine a reasonable functional relationship, in different cultivated media. In fact, a calibration method that requires less OS- – data points while maintaining accuracy would be ideal to users.