Soil-specific calibration of capacitance sensors considering clay content and bulk density

Abstract

Soil hydrology research requires the accurate measurement of soil water content. Recently, less expensive capacitance sensors (CS) have become popular for the measurement of soil moisture across soil profiles, but these sensors need to be calibrated for precise results. The purpose of the present study was to determine the effect of clay content and bulk density (ρb) on the calibration of CS. Two different CS (10HS and 5TM) were considered for the study. Clay content and ρb of the soils were determined from two different sites and from three different depths (0–5, 25–30 and 50–60 cm) of an experimental field in Gembloux, Belgium. Custom calibration (CC) equations were developed using packed soil columns following the same ρb at sequential volumetric water content (θ) levels. The factory-supplied calibration (FSC) showed an overestimation of θ (0.04–0.07 m3 m–3) with the 10HS sensor, and an underestimation of θ (0.06–0.077 m3 m–3) with the 5TM sensor for the entire calibration range. The variance in raw sensor outputs for different ρb and clay content of soil depths was not highly significant because the soil had limited range of variability in ρb and clay content. However, the CC is recommended in parallel with FSC for the precise measurement of soil moisture with CS.