Soil moisture using the time domain reflectometry (TDR) has been evaluated using probes or waveguides (length > 0.10 m), with little information in the literature for lower lengths. The purpose of the study was to evaluate the estimation of soil moisture from the dielectric constant using TDR probes with different rod lengths. TDR waveguides were fabricated with different rod lengths (0.30, 0.15, 0.12, 0.10, 0.08, 0.06, and 0.04 m). In the laboratory, the soil was placed for drying in air, crumbled, passed through a sieve of 2 mm and filled in PVC tubes (diameter 0.196 m). Immediately after saturation, the readings were performed with a TDR reflectometer until the moisture values approached the lower limit of soil water availability. Analyses of moisture values using different rod lengths showed that the length had an effect on the soil moisture readings for values above 0.25 m³ m-3 and below 0.14 m³ m-3. The readings of the probes with the different rod lengths were similar for constant soil moisture values in the range of 0.14 - 0.25 m³ m-3. The performance of the TDR probe with a length of 0.04 m was satisfactory for volumetric water contents above 0.20 m³ m-3.
apparent dielectric constant; soil water content; TDR calibration
