Possibility of Measuring the Water Content of Saline Coarse-Textured Soils Using the Time Domain Reflectometery (TDR) Method with a Coated Contact Censor

Document Type : Research Article

Author

Research and Education Center for Agricultural and Natural Resources of East Azarbaijan, Agricultural Research, Education and Extension Organization (AREEO), Tabriz, Iran.

10.22034/sps.2026.71015.1030

Abstract

Background and Objectives
Accurate and continuous monitoring of soil water content is critical for sustainable agricultural practices, as soil moisture plays a crucial role in biological processes such as seed germination, plant growth, and nutrient uptake. Among the various methods available for soil moisture measurement, the Time Domain Reflectometry (TDR) technique has gained widespread acceptance due to its speed, non-destructive nature, and the high accuracy in determining volumetric water content based on the soil’s dielectric constant. While TDR performs reliably in normal soils, particularly those with medium textures, its accuracy is severely compromised under anomalous soil conditions, including high bulk density, extreme temperatures, high organic matter content, and notably, high salinity. Salinity poses a particular challenge because the presence of highly concentrated dissolved ions significantly increase the electrical conductivity of the soil solution. This high electrical conductivity leads to excessive signal attenuation and scattering of the electromagnetic pulse emitted by the TDR device. Consequently, the measured dielectric constant is overestimated, resulting in inaccurate or impossible determination of the actual soil moisture content. Previous research has indicated that standard TDR sensors with conventional burial probes fail to measure soil moisture when the electrical conductivity of the saturation extract exceeds approximately 15 dS/m, especially in fine-textured soils. The main purpose of this research was to design, manufacture, and calibrate a new coated contact sensor for the TDR device that overcomes the technical limitations imposed by high soil salinity and allows the TDR device to measure the moisture content of coarse-textured saline soils with high accuracy based on the regression models obtained.
 
Material and Methods
For this purpose, coarse-textured soil (sandy loam) was prepared from the lands around Khajeh district, 20 kilometers east of Tabriz metropolis, Iran. For the aforementioned soil, an experimental salinity model was first prepared, which calculated the amount of salt for the desired electrical conductivity. Based on the salinity model, soils with electrical conductivity of 20, 35, and 50 dS/m were prepared and, after being poured into cylindrical containers, were brought to the bulk density of 1.57 g/cm3. Then, through trial and error, the type and amount of coating installed on the sensor waveguide rods for the aforementioned soils at the specific humidity of 30% was determined. The behavior of the coated waveguide rods for other soil moistures (air-dry to saturated) was unknown. For this purpose, soils with the aforementioned electrical conductivity were also prepared and poured into drained cubic containers and compacted until reaching the bulk density of 1.57 g/cm3. After the samples were saturated, the coated waveguide rods were installed vertically in the soil and the soil moisture measurement inside the containers was started by gravimetric and TDR methods. It took 130 days for the soil moisture to reach air-dry state, during which 29 measurements were made in three replicates. The data obtained after averaging the replicates were plotted in a graph and the most appropriate regression equations were presented. These equations were able to calculate the actual moisture content of saline and very saline soils with high accuracy by using the TDR method.
 
Results
The implementation of the specialized polymer coating on the TDR sensor rods successfully resolved the fundamental problem of signal failure in highly saline soils (up to 50 dS/m). The coated sensor reacted proportionally to changes in soil moisture content, much like a conventional sensor operates in non-saline soil. This outcome represents a significant achievement, enabling TDR measurements in soil conditions previously considered intractable. However, a systematic error was observed, particularly as the soil moisture content decreased from the saturation point towards air-dry conditions. Although the coating reduced the electrical interference, a noticeable difference persisted between the TDR-measured values and the true volumetric moisture content. This difference necessitated the development of specific calibration models for each salinity level to ensure high-accuracy estimation of the true soil moisture. Statistical analysis, utilizing indicators such as the Root Mean Square Error (RMSE), Relative Error (RE), p-value, and the coefficient of determination (r2), were performed to evaluate the efficiency of various regression models. The optimal regression equations derived from the calibration data varied depending on the level of salinity:
1-For ECe = 20 dS/m: A linear regression model provided the most accurate fit. 2-For ECe = 35 dS/m: A quadratic (second-degree polynomial) regression model was required. 3-For ECe = 50 dS/m: A cubic (third-degree polynomial) regression model demonstrated the best performance. These findings indicated that the non-linear relationship between the TDR reading and the actual moisture content was more complex (requiring higher-degree polynomials) as the soil salinity increased. Applying these specific calibration equations allows researchers and farm managers to calculate the actual soil moisture content with high precision, overcoming the inaccuracies associated with direct, uncalibrated TDR readings in saline environments.
 
Conclusion
This research enables the measurement of moisture in coarse-textured saline soils, even at very high salinity levels (over 50 dS/m), using contact sensors coated with polymeric materials using the time-domain reflectometry (TDR) method. The newly developed sensor, calibrated with empirical regression equations (linear, quadratic, and cubic), enables the measurement of soil moisture content in saline soils.
Data Availability Statement
Data is available on reasonable request from the author.
 
Acknowledgements
The author is thankful to the Research and Education Center for Agricultural and Natural Resources of East Azarbaijan for financial supports.
 
Ethical considerations
The author avoided data fabrication, falsification, plagiarism, and misconduct.

Keywords

Main Subjects

References

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Journal of Soil and Plant Science
Volume 36, Issue 1
December 2026
Pages 43-60

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