Calibration and study of soil physical-hydric relations using a frequency domain reflectometry probe

Abstract

The FDR Diviner 2000® probe is widely used to monitor soil moisture and to study physical-hidric relationships.
The calibration equation provided by the manufacturer may not correctly reproduce the actual moisture value for the same type of soil with different bulk densities.
In addition, it was postulated in this experiment that the capacitance probe can be used to determine the soil water tension and the initial soil water retention curve.
Thus, the objectives were: to assess whether the calibration equation parameters of the FDR probe changes with the bulk density variation; estimate the soil water tension using the FDR probe;
and verify the effect of the probe calibration in determining the initial soil water retention curve for the Red Oxisol at three bulk density levels (1.2, 1.4 and 1.6 Mg m^-3).
The results showed that for the Red Oxisol the calibration equations of the Diviner 2000® probe changes according to bulk density variation,
with a greater difference occurring in the 1.4 Mg m^-3 bulk density, whose measurement error using the default equation can be up to 0.17 m³ m^-3.
In addition, the FDR probe can be used to estimate the soil water tension, if the soil is less than 1.4 Mg m^-3 bulk density.
Finally, the use of the capacitance probe to determine the initial water retention curve can only be performed by prior calibration of the equipment specifically for the desired bulk density.