Effect of Soil Organic Matter on TDR Calibration for Measurement of Moisture

There are different methods for measuring soil moisture. The TDR method (Time Domain Reflectometry) is a relatively modern method in which soil water content is estimated based on the velocity of electromagnetic waves. The effect of soil compositions on calibration curve necessitates further calibration of the instrument. The aim of the current study was to present a calibration equation for soils with five different amounts of organic matter. This study was carried out in laboratory on three soil textures i.e. light, medium, and heavy. The results showed that the moisture measured in windows of 10 NS was more accurate than that of other windows. Also, in low moisture contents, the amount of moisture measured by TDR was higher than the gravity method and the difference between the two methods increased with increase in the clay and organic matter content. With higher organic matter in soil, RMSE increased. It was shown that soil organic matter content influenced the accuracy of TDR. Analyses of variance showed significant difference between TDR and gravity method for moisture content of soils with different amounts of organic matter (P<0.05). Finally, calibration curves with high coefficients of regression were obtained for the studied soil textures.