The effects of amount and type of clay minerals on soil dielectric constant for measuring water content with Time Domain Reflectometry

Time domain reflectometry (TDR) is used for determining soil water content nowadays. Amount and type of clay minerals in heavy soil can affect soil dielectric constant and cause the error in soil water content. The effect of the amount of clay minerals on soil water content was the subject of many researchers, but they did not work on the effect of the type of clay minerals. The aim of this research was the effect of type and amount of clay minerals on dielectric constant in heavy soils. For this purpose soil samples with different textures including clay, clay loam, loam, silty clay and sandy clay were taken. The results showed that with increasing the clay content, due to bound water and delay time, the soil dielectric constant decreased and consequently soil water content has been shown less than the real amount. The results also showed that Smectite group increased soil dielectric constant. Our results showed that the presented model with four phases including solid, air, bound water and free water in non saline soils presents acceptable results for volumetric water content. But in saline soil this model did not show accurate results. Consequently, soil salinity and clay type can affect on soil dielectric constant and water content.