Comparison of FDR Sensor Calibrations for Various Coir Dust and Perlite Mix Ratios

Recently developed FDR (frequency domain reflectrometry) soil moisture sensors are suitable for measuring volumetric water contents (VWC, v/v) of horticultural substrates in real time, which can provide a good tool for substrate moisture based-automated irrigation of horticultural crops. For FDR sensors, substrate-specific calibration is required to acquire accurate measurements of VWC, but sensor companies typically provide only a general calibration for all potting soils (and sometimes no specific calibration for horticultural substrates), which cannot represent the variety of horticultural substrates. To investigate and compare the substrate specific calibrations for various coir dust and perlite mix ratios (1:0, 8:2, 6:4, 4:6, 2:8, 0:1, v/v), we conducted individual calibration in a range of 0.1 m³·m^-3 to near the container capacity for each substrate mix. Sensor outputs (in mV and ADC) of various VWC levels of the specific substrates were collected with four EC-5 and GS3 sensors (Decagon Devices Inc., Pullman, WA, USA) connected to CR1000 data logger (Campbell Scientific Inc., Logan, UT, USA) with 2.5 V-DC and 12 V-DC excitation, respectively. Regression analyses were conducted to acquire VWC calibration equations for each substrate mix. The actual VWC level was calculated as (wet substrate weight - dry substrate weight) (g) / substrate volume (mL) after drying the substrate in a drying oven. All the tested substrate mixes had linear relationship between EC-5 sensor output and VWC with high R² (> 0.93) except 100% perlite, and their slopes were significantly different across the substrate mixes (P = 0.03). However, most of the substrate mixes had the similar calibrations with the estimated VWC differences within 0.06 m³·m^-3. GS3 data had the best fit with a logarithmic relationship between sensor output (ADC) and VWC, and the slopes among the substrate mixes were not significantly different (P = 0.13). Furthermore, the potting soil calibration equation provided by the company differed from the substrate-specific calibration equations, resulting in large errors (up to 0.5 m³·m^-3 difference) in measuring actual VWCs. Therefore, substrate-specific calibration is required to acquire accurate VWC measurements with FDR sensors.