Dielectric properties of a tilled sandy volcanic-vesuvian soil with moderate andic features

The apparent dielectric constant, ɛ, of many agricultural soils, measurable with the Time Domain Reflectometry (TDR) method, may be used to estimate soil water content, θ, with the Toppʹs empirical model. However, organic soils and those of volcanic origin do not obey this model, which has been termed “universal”. In particular, volcanic soils have singular physical properties in terms of bulk density, porosity, surface area, and variable charge essentially arising from the state of structural aggregation of particles of the solid mineral phase. After drying and/or intensive long-term tillage in arid and semi-arid climate, properties of these soils tend to change irreversibly. These modifications affect pore-sizes distribution and more stable larger size aggregates are formed. This in turn modifies porosity, surface area, variable charge, thus changing the dielectric characteristics and the relationship θ(ɛ) of the porous system. In this paper we compare the dielectric properties of two horizons of a volcanic-vesuvian soil profile. θ(ɛ) experimental relationships were measured in the laboratory on 16 undisturbed soil samples taken from Ap and Bw horizons of the soil profile. The subsoil Bw has kept its physical characteristics, typical of a volcanic soil having better expressed andic features. By contrast, topsoil Ap has been intensively cultivated and has altered its properties, partially losing its andic features. Physically based dielectric approaches explicitly taking into account the contribution of the changed properties are necessary for comparing the dielectric properties of the two soil horizons. The experimental θ(ɛ) were thus interpreted by using Maxwell–De Loorʹs approach and the so-called α-model which both consider soil as a multiphase system. The Toppʹs model was also used for evaluating its interpretative capability for the two soil horizons. By using the multiphase models in their three- and four-phases (including the effect of the bound-water) configurations on both the unaltered subsoil and the altered topsoil, it was possible separating the contribution of the different physical factors mainly involved in determining the singular dielectric properties of volcanic soils. In terms of model comparisons, we found no major differences between the three- and four-phase versions of the Maxwell–De Loorʹs and α-models, thus demonstrating the substantial irrelevance of the bound-water on the dielectric properties of the investigated volcanic soils. Besides, this confirms the strength of the completely physically based Maxwell–De Loorʹs model for interpreting soil dielectric properties without using any fitting parameters.