Plant root growth affects FDR soil moisture sensor calibration

To acquire accurate volumetric water content (VWC) measurements from horticultural substrates using dielectric sensors, a substrate-specific calibration is critical. Calibrations typically are conducted with a substrate without plants, but water in the root system may affect soil moisture sensor readings. We investigated the effect of root growth on the measured VWC. Lettuce seedlings (Lactuca sativa L. ‘Joek Chi Ma’) were grown in 10 cm round containers (440 mL) filled with soilless substrate. Four EC-5 soil moisture sensors (Decagon Devices Inc., Pullman, WA, USA) were used to determine the effect of root system size on sensor calibration over an eight week period. Both calibration coefficients (slope and intercept) decreased (P < 0.001) with increasing root system size. The actual VWC at 8 weeks after transplanting (WAT) was 9% lower at 65% VWC compared to the estimated VWC from the substrate-only calibration. Multiple regression models indicated that various root size indicators (WAT, root fresh weight, root dry weight, and root water content) all had a negative effect on the estimation of VWC. The effect of root system size on estimated VWC may be tolerable in production, but should be considered in research applications. Careful interpretation is needed when using FDR soil moisture sensors to monitor substrate VWC in the presence of a growing root system.