Calibration of time domain reflectometry for forest soil humus layers

Time domain reflectometry (TDR) has become a widely used method for determining the volumetric water content of soils. However, due to the differences in bulk density and surface area, the relationship between the dielectric constant and water content in organic soils is very different from that in mineral soil. It is therefore impossible to have a universal calibration suitable for all soil types. In this article we describe the relationship between the apparent dielectric constant (Ka) and volumetric water content ([theta]v) for homogenized and undisturbed humus layers (Of + Oh) from forested soil using three empirical models. There was a clear relationship between the Ka and [theta]v and this relationship was best described (R2 = 0.968) with a logarithmic equation of the form [theta]v = aln(Ka) – b. Accurate determination of sample volume was the main source of variation in the calibration, having a greater effect on the calibration results than differences in bulk density.