Determination of soil pore water salinity using an FDR sensor working at various frequencies up to 500 MHz

Abstract

This paper presents the application of a frequency-domain reflectometry (FDR) sensor designed for soil salinity assessment of sandy mineral soils in a wide range of soil moisture and bulk electrical conductivity, through the determination of soil complex dielectric permittivity spectra in the frequency range 10-500 MHz. The real part of dielectric permittivity was assessed from the 380-440 MHz, while the bulk electrical conductivity was calculated from the 165-325 MHz range. The FDR technique allows determination of bulk electrical conductivity from the imaginary part of the complex dielectric permittivity, without disregarding the dielectric losses. The soil salinity status was determined using the salinity index, defined as a partial derivative of the soil bulk electrical conductivity with respect to the real part of the soil complex dielectric permittivity. The salinity index method enables determining the soil water electrical conductivity value. For the five sandy mineral soils that have been tested, the relationship between bulk electrical conductivity and the real part of dielectric permittivity is essentially linear. As a result, the salinity index method applied for FDR measurements may be adapted to field use after examination of loam and clayey soils.

Keywords: complex dielectric permittivity; dielectric spectroscopy; frequency-domain reflectometry; salinity index approach; soil salinity; soil water content.

Figure 1.

Real and imaginary parts of the complex dielectric permittivity of a sample of soil no. 601 wetted with distilled water to approximately 50% of saturation water content.

Figure 2.

Determination of bulk electrical conductivity of a sample of soil no. 601 wetted with distilled water to 50% of saturation water content; C_b is directly proportional to the intercept, according to Equation (5). The regression equation, R² and standard error of regression are given on the plot. The standard error of determination of C_b is about 2%.

Figure 3.

On the left: bulk electrical conductivities of all measured soil samples vs. the real part of the dielectric permittivity. On the right: salinity index X_S calculated as a slope of a linear C_b vs. ε′ relation, with respect to the conductivity of the moistening solutions C_s. Regression equations, R² and standard errors of regressions are given on the plots.

Figure 4.

Soil water conductivities C_w versus ε′ = Re(ε*) for linear and quadratic salinity index models for all moistening solutions. Solid lines represent C_w values calculated with the use of X_S obtained from the slopes of C_b vs. ε′ linear relations.

Figure 5.

Conductivities of the moistening solutions calculated from the linear salinity index model vs. assumed conductivities for all tested soils. The straight line has a slope equal to 1 and corresponds to the perfect agreement.