Why not make it easy with a non-capacitance based soil sensor like the HydraProbe?

This study published in American Meteorological Society's BAMS journal demonstrates the extent of variability in readings one can expect with a soil moisture sensor based on capacitance technology. 

To obtain the highest level of accuracy that is required for remote sensing applications, capacitance-based soil sensors often need to be recalibrated. The HydraProbe is an impedance-based sensor that that takes the measurements one step further and rarely needs to be recalibrated. If a custom calibration is necessary, it can be used with any HydraProbe (low inter-sensor variability). The HydraProbe will hold that calibration (custom or factory) for many years (5 years guaranteed) which will save operational costs in long term monitoring networks.