Calibration method to address influences of temperature and electrical conductivity for a low-cost soil water content sensor in the agricultural field

The performance of a low-cost soil water content sensor (5TE, Decagon Devices, Inc.) for monitoring volumetric soil water content in an agricultural field was investigated. Prior to the in-field measurements, calibration methods for the sensor to correct the influences on soil temperature and electrical conductivity (EC) of the soil water were established. Dried soil collected from the agricultural field, pure water, and an in-laboratory thermostatic reservoir were used. First, calibration formulas for the soil temperature and EC were obtained using a dataset of laboratory measurements. Second, the soil water content in the field was measured using the 5TE sensor with three different calibration curves, applying: (A) no correction of EC for curves at 20 °C, (B) temperature correction only, and (C) temperature and EC corrections. The differences between the sensor and actual measurements were (1) 2.5%, (2) 2.5%, and (3) 2.2% expressed as volumetric soil water content (m3 m3). Higher accuracy was confirmed in the 5TE-sensor measurements under condition (3) at higher EC values. Finally, we demonstrated that the 5TE sensor with the proposed calibration method is useful for monitoring soil water content in agricultural fields.