Prediction of Pore Water Electrical Conductivity Using Real Dielectric and Bulk Electrical Conductivity in Soilless Substrates

Electrical conductivity (EC) is commonly used as an indicator of fertilizer levels in soilless substrates. The EC can be determined as bulk EC (bEC, the EC of the combined solid, water and air phases) and as pore water EC (pwEC, the EC of the solution in the substrate). Since pwEC represents the EC of the solution that roots are exposed to, this measurement is more relevant for crop production. In situ EC sensors can simplify EC measurements and allow for continuous monitoring of substrate fertility level over time. However, these sensors generally determine bEC. Hilhorst developed a model to estimate pwEC from bEC and dielectric permittivity (ɛ_b, directly related to substrate volumetric water content [VWC]). One of the parameters in the Hilhorst model is the permittivity of dry soil/substrate (e’_σb=0), which is assumed to be similar for different soils/substrates. However, e’_σb=0 may depend on the dielectric properties of the substrate and the measurement frequency of the dielectric sensor. Our objective was to determine e’_σb=0 using four different sensors to optimize pwEC measurements in two soilless substrates (peat:perlite and peat:vermiculite). We collected data in both substrates, using a wide range of substrate VWC (0.22 to 0.55 m³·m^-3) and three different fertilizer levels (0.5, 1.5, and 2.5 g·L^-1) to get a broad range of pwEC values. Substrate temperature, ɛ_b, and bEC were measured with four different sensors (GS-3, Decagon Devices; HydraProbe II, Stevens Water Monitoring Systems; SigmaProbe and WET-2, Delta-T). A small amount of substrate solution was subsequently sampled using a juice press and the EC of this solution was measured. The solution EC was assumed to represent pwEC. These data were used to back solve the Hilhorst equation to calculate e’_σb=0. We found that e’_σb=0 is not a constant and depends on ɛ_b, bEC, and their interaction. The value of e’_σb=0 also differed among sensors and substrates. More accurate estimates of e’_σb=0 can result in more accurate pwEC measurements. However, the accuracy of pwEC measurements still needs to be evaluated with an independent data set. Accurate pwEC measurements could be used to help guide fertilization decisions and potentially for automated fertigation control.