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Monitoring Salt Dynamics in Three Saturated Soils by Using Time-domain Reflectometry (tdr)

A. B. M. Zahid Hossain
Scientific Officer
Bangladesh Rice Research Institute (BRRI) Regional Station, Kushtia.

M. A. Mojid
Professor
Department of Irrigation and Water Management, Bangladesh Agricultural University, Mymensingh 2202.

Abstract/ Executive Summary:

Continuous, nondestructive and real time monitoring of salt dynamics in soil is important for salt management in the field. Time- domain reflectometry (TDR) was applied to monitor salt dynamics in saturated soil columns in laboratory at the Department of Irrigation and Water Management, Bangladesh Agricultural University (BAU), Mymensingh during 2004 - 2005. Three different textured soils were salinized with solutions of sodium chloride (NaCl) of electrical conductivity 2.71 and 5.84 dS/ m, and subsequently desalinized by applying tap water. Several TDR sensors were constructed to measure water content and electrical conductivity of the soil columns at two depths. TDR- measured water content is fairly comparable to that of gravimetric method within ±2% accuracy. Second-degree polynomial functions of TDR- measured impedance ratio with sensor- dependent coefficients provide an accurate estimate of electrical conductivity over a wide range of measurement. The bulk- soil electrical conductivity versus time curve of the salinization process is always steeper than that of the desalinization process for the 3 soils. The time of salinization and desalinization processes is the same for a 15 cm column of coarse sand when it was salinized with a salt solution of 2.71 dS/ m. But, it differs considerably for sandy loam and silt loam soils for both low and high salinities. TDR is found a prospective non-invasive technique to monitor salt dynamics in soils.

Keywords: Monitoring, Soil salinity, Salt dynamics, TDR sensors

Location: Bangladesh Agricultural University farm, Mymensingh

Start Date: 01-01-2004

End Date: 30-06-2005

Program Area: Crop-Soil-Water Management

Commodity: Soil salinity

Objectives:

(i) To evaluate the performance of TDR sensors that were constructed locally,

(ii) To monitor salt movements during salinization and desalinization processes in three different textured soils under saturated condition

Methodology:

Time- domain reflectometry (TDR) was applied to monitor salt dynamics in saturated soil columns in laboratory at the Department of Irrigation and Water Management, Bangladesh Agricultural University (BAU), Mymensingh during 2004 - 2005. Three different textured soils were used in different experiments of this study. One soil sample was collected from the bank of the river Brahmmaputra and the other two were collected from two different locations of the Bangladesh Agricultural University farm, Mymensingh in 2004. TDR actually measures the dielectric constant (s) of the soil. TDR sensors in measuring soil-water content, air-dry silt loam soil was packed uniformly in separate polyvinyl chloride (PYC) cylinder, 8.7 cm in diameter and 14 cm high with the base closed with polyethylene sheet. The soil was then spread on a clean tray to spray some water and mixed properly. This soil was packed uniformly in PYC cylinder in 3 to 4 steps. In this way, 31 soil samples, ranging water content from air dry to water saturated, were prepared. TDR automatically recorded the dielectric constant and an impedance ratio of the soil at the specific time interval set in the TDR program. TDR readings became constant when the soil became equilibrium with the added solution and addition of salt solution was stopped to complete the salinization process of the soil column. To start desalinization, tap water was applied with Marriotte bottle just before the removal of the salt solution from the surface of the soil. The salt in the soil column started leaching out with the flowing water. As before, the dielectric constant and impedance ratio of the soil was recorded at interval. TDR reading reached at constant value when the salt leached out completely from the soil thus completing the desalinization process. The salinization and desalinization processes of the soil column were also done for the 0.05 N solution. Similar experiments were conducted with all three types of soil.

Source of Information: JOURNAL OF AGRICULTURAL ENGINEERING, THE INSTITUTIONAL OF ENGINEERS, BANGLADESH, Vol. 32 (A), December 2006, pp. 45-54, ISSN 0379–4318

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

The parallel 3- rod TDR sensors work well in measuring soil- water content and electrical conductivity. A linear 1: 1 relationship obtained between the water content measured by TDR and that measured by gravimetric method shows the accuracy of TDR method. At the dry range of water content <0.10), TDR overestimates soil- water content. TDR measures a wide range of electrical conductivity of solutions and bulk soils by the parallel 3-rod sensors. The relationship between the impedance ratio measured by TDR and electrical conductivity of salt solution is governed by second- degree polynomial function. The coefficients of this function differ among different sensors, and individual calibration of the sensor is needed. The bulk-soil EC versus time curves for coarse sand are steeper during salinization as compared to the desalinization process. Equal time is required both for the salinization and desalinization processes when the sand is salinized with a salt solution of low concentration. But, more time is required during the salinization process as compared to the desalinization process for solution of high concentration. In the loamy sand soil, the salinization curves are steeper than the desalinization ones. Time required for desalinization is higher than that for salinization. Like loamy sand soil, the salinization curves are also steeper than the desalinization curves in the silt loam soil. More time is required in the salinization process over the process of desalinization. The results of this study have importance in several practical contexts. The information regarding the movement of dissolved substances in the subsoil is a vital requirement for successful subsurface management. For the estimation of leaching requirements for reclamation of salt- affected soils, it is necessary to know the nature of the time span of desalinization process of different textured soils.

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