Evaluation of Time Domain Reflectrometry Device in Saline Clay and Sandy Soils

Measuring the precision quantity of soil moisture in agricultural science is very important. Therefore, it is important to have accurate and fast measurement of soil water content, especially to achieve soil characteristics by a non-invasiveness way. In recent years, application of time domain reflectometry (TDR) as a well-known for measuring soil water and electrical conductivity in short time has been common. The objective of this study was to evaluate the accuracy of the TDR probe under laboratory conditions in clay and sandy soils at four soil salinity levels of 2, 6, 12 and 18 dS m-1 which were packed in a PVC cylinder with 27 cm in diameter and 300 cm in length. The TDR underestimated dielectric coefficient and volumetric moisture in heavy soil due to the effect of bound water in heavy soil which reduced the time delay. Also soil dielectric coefficient increased with increasing soil salinity, resulting in overestimation of the soil moisture. The result of soil salinity and texture interaction showed that under high soil salinity levels (12 and 18 dS m-1) the estimated moisture by the device was less than the volumetric method, whereas under low soil salinity it was vice versa. This indicated that in high salinity level, the effect of clay caused the low estimation of soil water content but in the low soil salinity level, electric conductivity would lead to over estimating moisture content.