Dielectric Characterization of Vertisol and Inceptisol Soils at Multiple Microwave Frequencies

Evaluated the dielectric and electromagnetic characteristics of five agricultural soils obtained from the Jalna district, Maharashtra, at five microwave frequencies: L-band (~2.0 GHz), S-band (~3.0 GHz), C-band (4.785 GHz), J-band (7.6 GHz), and X-band (9.685 GHz). For each soil–frequency combination, twelve parameters were determined on a rectangular waveguide bench operating in the TE10 mode: real permittivity (ε′), dielectric loss (ε″), loss tangent (tan δ), AC conductivity (σ_AC), penetration depth (d_pen), skin depth (δ_skin), complex magnitude (|ε*|), phase angle (φ), quality factor (Q), reflection coefficient (Γ), refractive index (n), and extinction coefficient (κ). The soil set comprises four Vertisols (black cotton; ~58% montmorillonite clay) and one Inceptisol (alluvial clay loam; 42% clay). Across all samples, ε′ fell steadily from L-band (7.39 ± 0.26) to X-band (2.87 ± 0.13), a trend consistent with Maxwell–Wagner interfacial polarisation relaxing above roughly 3 GHz. Dielectric loss reached its highest values at S-band (2.60 ± 1.50), driven by ionic conduction losses in clay double layers; this elevated loss created an unexpected depth-of-penetration reversal in which C-band (0.046 m) probed deeper than S-band (0.037 m) despite operating at a higher frequency. Soil emissivity, calculated from the Fresnel equations, peaked for V-polarisation between 60° and 70° incidence at every band, while L-band nadir emissivity (~0.777) was noticeably lower than X-band (~0.924). Organic carbon (r = 0.82 at C-band), electrical conductivity (r = 0.74 at S-band), and clay fraction (r = 0.70 at C-band) emerged as the principal chemical controls on dielectric variability.