Calibration of an apparent electrical conductivity sensor using capacitance probes for determining soil water content

Abstract

Soil apparent electrical conductivity (ECa) measurements, although more suitable for soil salinity quantification, are often used to determine volumetric soil water content, θv, in site‐specific crop management. Generally, the approach is to develop a field‐specific calibration function between ECa and measured θv, θv(m). Such a calibration function is possible when soil water dominates ECa at a field. When this is not the case, θv(m) data taken over time may be helpful to develop a function. Data regarding θv(m) can be costly and time‐consuming as soil samples are needed to measure bulk density and gravimetric soil water. We postulate that capacitance probes, which measure scaled frequency (SF) as an indicator of the dielectric constant to estimate θv, can be used to establish ECa‐θv calibration functions. Capacitance probes that simultaneously provide temperature readings allow for ECa to be temperature‐corrected to 25°C, enabling development of ECa25‐θv calibration functions. A field experiment was set up to establish whether SF readings can be used to estimate θv, θv(SF), in structured high montmorillonite clay soils accurately and determine if ECa25‐θv(SF) calibration functions are possible. Our results revealed that a single SF‐θv(m) calibration function representing 12 probes, or a specific soil form, is impossible. Each capacitance probe should be calibrated separately. The parameters for ECa25‐θv(m) calibration functions were like ECa25‐θv(SF) functions. The R2 for ECa25‐θv(m) calibration functions was higher compared to the ECa25‐θv(SF) functions. Further investigation of using capacitance probes to estimate θv for developing ECa‐θv functions is needed before this approach can be applied confidently.