SUMMARYThe paper is addressed to soil scientists who use Time‐Domain Reflectometry (TDR) technology to measure soil moisture. The practical aspects of the measurement calibration are discussed, and an empirical approach to establishing the existence of a universal calibration function is presented.Samples of 11 mineral soil horizons and seven organic soil horizons with different chemical and physical properties (including magnetic properties) were selected with the aim of determining their dielectric constant‐volumetric water content relationship as calibration functions for TDR soil moisture measurements. These samples were supplemented by other, soil‐like, capillary‐porous reference materials (montmorillonite, glass beads, washed sand and a sand from a C horizon).The study showed that a unique calibration function for mineral soils and another distinct calibration function for organic soils can be esta