The effect of clay and organic matter content on the relaxation time distribution in mineral-organic soils

The spectra of complex dielectric permittivity of three soils with different granulometric composition and different organic matter content were measured in the frequency range from 15 MHz to 15 GHz and in the moisture range from dry state to the minimum water-holding capacity at a temperature of 25°C. From the complex dielectric permittivity spectra obtained the relaxation time distribution functions were retrieved. Analysis of the relaxation time distribution functions revealed up to nine relaxation processes for all specimens, grouped by characteristic times: 6–50 ps, 20–120 ps, 800–20·103 ps and from 104 to greater than 105 ps. Comparison with literature data allowed attributing the slowest group to the Maxwell- Wagner relaxation, the second and third groups to bound water relaxations, while the first group includes contributions from both unbound and bound water. It was found that an increase in moisture leads to an increase in relaxation times and a broadening of their distribution, most noticeable in the third group. A qualitative similarity in the effect of clay and organic matter content on relaxation parameters was demonstrated, while the times of the extreme high-frequency and low-frequency processes remain practically independent of soil composition. Non-monotonic changes in the times of intermediate relaxations with varying organo-mineral composition were noted.