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

Abstract

Soil apparent electrical conductivity (EC_a) measurements, although more suitable for soil salinity quantification, are often used to determine volumetric soil water content, θ_v, in site-specific crop management. Generally, the approach is to develop a field-specific calibration function between EC_a and measured θ_v, θ_v(m). Such a calibration function is possible when soil water dominates EC_a at a field. When this is not the case, θ_v(m) data taken over time may be helpful to develop a function. Data regarding θ_v(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 θ_v, can be used to establish EC_a-θ_v calibration functions. Capacitance probes that simultaneously provide temperature readings allow for EC_a to be temperature-corrected to 25°C, enabling development of EC_a25-θ_v calibration functions. A field experiment was set up to establish whether SF readings can be used to estimate θ_v, θ_v(SF), in structured high montmorillonite clay soils accurately and determine if EC_a25-θ_v(SF) calibration functions are possible. Our results revealed that a single SF-θ_v(m) calibration function representing 12 probes, or a specific soil form, is impossible. Each capacitance probe should be calibrated separately. The parameters for EC_a25-θ_v(m) calibration functions were like EC_a25-θ_v(SF) functions. The R² for EC_a25-θ_v(m) calibration functions was higher compared to the EC_a25-θ_v(SF) functions. Further investigation of using capacitance probes to estimate θ_v for developing EC_a-θ_v functions is needed before this approach can be applied confidently.