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SOIL & WATER MANAGEMENT & CONSERVATION

Performance of a Capacitance-Type Soil Water Probe in a Well-Drained Sandy Soil

^a Univ. of Florida, IFAS, Citrus Research and Education Center, 700 Experiment Station Rd., Lake Alfred, FL 33850
^b Greenhouse and Processing Crop Res. Center, Agriculture and Agrifood Canada, Harrow, ON N0R 1G0, Canada

^* Corresponding author (Wijeb{at}ufl.edu).

Most soils in the Central Florida Ridge (CFR) area are Entisols that contain >95% sand, <3% clay, and <2% organic matter. Field capacity (_fc) is commonly 0.08 m³ m^–3. Therefore, accurate estimation of soil water content (_v) is important in these soils. The objective of this study was to evaluate the performance of ECH₂O probes when estimating _v for scheduling irrigation in CFR soils. Probes were tested for (i) probe-to-probe output variability, (ii) soil volume sampled, (iii) sensitivity to salinity, temperature, and air pockets close to the sensor surface, (iv) pockets of very dry soil close to the sensor surface, and (v) performance after installation in the field. According to the calibration, a 1% change in water content corresponds to a probe output of 17 mV. Laboratory testing suggested that output variability from probe to probe can be a problem in these soils. The sampling volume of the probe was within 1.5 cm from either side of the sensor surface. Salinity induced during fertigation increased the output by about 200 mV, and for each 1°C drop in temperature, the sensor output dropped by 2.3 mV. When the bulk density was changed from 1.56 to 0.94 Mg m^–3, the output decreased by 3.5 MV for each 1% drop in air-filled porosity. When very dry soil lenses with <0.01 m³ m^–3 _v were associated with the probe surface, the probe failed to sense the wet soil even 1 cm away from the sensor surface. Sensor failure was common due to water leaking into the circuit when sealing material deteriorated or casing material was damaged by insects. These issues need to be addressed before the probes can be considered reliable to estimate _v or used in automated irrigation.

Abbreviations: CFR, Central Florida Ridge