In Situ Measurements of the Electrical Conductivity of Substrates: The Relationship Between Bulk EC, Pore Water EC, and Substrate Water Content

Electrical conductivity (EC) is commonly used as an indicator of fertilizer levels in soilless substrates. EC is normally measured using laboratory tests or the pour through method. In situ sensors can simplify EC measurements, and allow for continuous monitoring. In situ EC sensors generally determine the bulk EC of the substrate, by measuring how well the substrate (the combination of substrate components, substrate solution, and air spaces) conducts electricity. Bulk substrate EC depends on both the amount of dissolved salts and substrate water content (θ). The confounding effect of θ makes interpretation of bulk EC difficult, and models have been developed to estimate pore water EC from bulk EC and θ (e.g., the Hilhorst model). Since pore water EC represents the EC of the solution that the roots are exposed to, it may be more relevant for plants than bulk EC. We determined the relationship between bulk EC, pore water EC, and θ.  Fixed amounts of fertilizer (0.5, 1.5, or 2.5 g) were mixed with different amounts of water.  These solutions were added to 1 L of soilless substrate (peat/perlite and peat/vermiculite) to produce substrates containing the same amount of fertilizer but different θ (0.22 to 0.55 m^-3∙m^-3). Bulk EC was measured with seven different EC sensors [W.E.T sensor and SigmaProbe (Delta T Devices), ECHO-TE, 5-TE, and GS-3 (Decagon Devices), SMEC 2000 (Spectrum Technologies), and HydraProbe II (Stevens Water Monitoring Systems)]. After these measurements were taken, pore water was sampled to determine the pore water EC with a standard solution EC meter. Both bulk EC and pore water EC increased with increasing amounts of fertilizer. The response to θ was more complicated: bulk EC increased and pore water EC decreased with increasing θ. The increase in bulk EC with increasing θ was likely due to a decrease in tortuosity, making it easier for current to flow between the electrodes of the EC sensors. Pore water EC decreased with increasing water content, because of dilution of the dissolved salts as θ increases. Bulk EC data can be misleading since low values can either indicate low levels of dissolved salts or low θ. Since pore water EC is more relevant for plants than bulk EC, the collected data were used to model pore water EC based on bulk EC and θ. The ability to monitor pore water EC will help growers fertilize more efficiently, and may allow for automation of fertilization.