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. 2023 Jun 5;23(11):5357.
doi: 10.3390/s23115357.

Broadband Measurements of Soil Complex Permittivity

Justin Stellini  1 , Lourdes Farrugia  1 , Iman Farhat  1 , Julian Bonello  1 , Raffaele Persico  2 , Anthony Sacco  3 , Kyle Spiteri  3 , Charles V Sammut  1
Affiliations

Broadband Measurements of Soil Complex Permittivity

Justin Stellini et al. Sensors (Basel). .

Abstract

Agriculture is a major consumer of freshwater and is often associated with low water productivity. To prevent drought, farmers tend to over-irrigate, putting a strain on the ever-depleting groundwater resources. To improve modern agricultural techniques and conserve water, quick and accurate estimates of soil water content (SWC) should be made, and irrigation timed correctly in order to optimize crop yield and water use. In this study, soil samples common to the Maltese Islands having different clay, sand, and silt contents were, primarily, investigated to: (a) deduce whether the dielectric constant can be considered as a viable indicator of the SWC for the soils of Malta; (b) determine how soil compaction affects the dielectric constant measurements; and (c) to create calibration curves to directly relate the dielectric constant and the SWC for two different soil types of low and high density. The measurements, which were carried out in the X-band, were facilitated by an experimental setup comprising a two-port Vector Network Analyzer (VNA) connected to a rectangular waveguide system. From data analysis, it was found that for each soil investigated, the dielectric constant increases notably with an increase in both density and SWC. Our findings are expected to aid in future numerical analysis and simulations aimed at developing low-cost, minimally invasive Microwave (MW) systems for localized SWC sensing, and hence, in agricultural water conservation. However, it should be noted that a statistically significant relationship between soil texture and the dielectric constant could not be determined at this stage.

Keywords: dielectric constant; soil water content; water conservation.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
A simplified three-dimensional model of the two waveguide sections after being connected together, with the sample holder in between and the bolt screws inserted accordingly.
Figure 2
Figure 2
Schematic diagram of the two rectangular waveguide sections employed in our study (left and right) and an empty sample holder (center). It should be noted that these are not drawn to scale and are only meant to give a general idea of the respective dimensions. All dimensions are in centimeters.
Figure 3
Figure 3
Layer of Kapton tape attached to the end of a waveguide section. The layer thickness shown in the figure is not to scale.
Figure 4
Figure 4
Different conversion algorithm dielectric constant profiles for (a) FR4; (b) PTFE; and (c) PE-30. The results shown are for ‘uncovered’ samples.
Figure 5
Figure 5
Dielectric constant vs frequency profiles comparing the ‘uncovered’ and ‘covered’ results for (a) FR4; (b) PTFE; and (c) PE-30.
Figure 6
Figure 6
ε′ frequency distribution for (a) B3; (b) G1; and (c) Ħ9. The three profiles in each graph correspond with different soil densities achieved by manual compaction.
Figure 7
Figure 7
Dielectric constant as a function of frequency for all given soil samples at (a) low, and (b) high density, respectively.
Figure 8
Figure 8
Moisture results at low and high density, respectively for (a) Bajjad 3, and (b) Garigue 1.
Figure 9
Figure 9
Calibration curves for B3 (left) and G1 (right) obtained from data taken at a 9.04 GHz transect. While the scatter points correspond with the measured data, the trendlines represent the fitted cubic models. The error bars are associated with the percentage difference between the SWC values measured by the moisture analyzer and those obtained after substituting ε′ values into the relevant models.
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Grants and funding

This research was funded by the Energy and Water Agency Malta within the framework of the National Strategy for Research in the Energy and Water Sectors.

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