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-θ/Ka–θ data points, in order to
determine a reasonable functional relationship, in different
cultivated media. In fact, a calibration method that requires less
OS-θ/Ka–θ data points while maintaining accuracy would be
ideal to users.