Calibration of capacitance sensor for Andosol under field and laboratory conditions in the temperate monsoon climate

Abstract

Capacitance sensors (CS) were developed to continuously measure soil water content using the dielectric properties of soils. Although CS are calibrated in soils of various types, the default calibrated functions may not work for all conditions as in the case of this soil that has high organic matter content, hence, soil-specific calibrations are sometimes required to achieve reasonable accuracy. This study (1) evaluated the suitability of the default calibrated functions of a Decagon 5 T M CS for Andosol (Kuroboku), and (2) derived soil-specific calibration functions to Andosol. A field monitoring experiment was conducted at Sakaecho (western suburb of Tokyo) from August 2016 to July 2017, where volumetric soil water content (θv) was measured in bare field and repacked soils using both gravimetric method with oven drying and CS installed at 5 cm depth. The values of θv monitored using soil cores sampled from the field under natural condition and CS revealed large errors under both field and laboratory conditions when the default factory supplied calibration function (FSC) was used. The deviations depicted significant (P < 0.001) underestimations of the observed θv measured in soil cores by 0.117 - 0.199 and -0.004 - 0.131 cm3 cm−3 for field and laboratory conditions, respectively. Accordingly, soil-specific calibration functions were developed by correlating the dielectric permittivity of the soil with θv measured from the soil core samples. The θv recalculated based on the site-specific calibration function under field condition best fitted to the observed θv. Calibration of CS improved the θv measurement error from 15% with FSC to ≤ ±2%. Whereas the improvement with laboratory calibration was from ±15% with FSC to ±4% when the function was implemented to the field measured data, hence, it still underestimated the observed field θv. The deviation between the field and laboratory procedures was attributed to the deformation of the well-aggregated soil structure and its consequent changes in hydraulic properties due to crushing when a 2-mm sieve was used for sample preparation. Quasi-field calibration of the 5 T M CS under natural condition is highly recommended for real-time monitoring of θv in Andosol. In cases when field calibration is impractical, laboratory calibration further verified with field data could also offer a reliable method for calibration of the 5 T M CS for Andosols.