The apparent dielectric constant () of many agricultural soils, measurable with the time-domain reflectometry (TDR) method, may be used to estimate soil water content () with the empirical model of Topp et al. (1980). 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 and porosity essentially arising from the state of structural aggregation of particles of the solid mineral phase, and from the presence of ferrous hydroxides and allophanic clays with a high specific surface and high affinity with water. This led us to suppose that the retention characteristics of volcanic soils might show atypical dielectric behaviour. In this work we demonstrate that some physically-based alternative models, which include bulk density and porosity as parameters, are able to interpret the anomalous dielectric behaviour that may be observed in a volcanic-Vesuvian soil in the Campania plain (near Naples, Italy). The experimental relations -, measured in the laboratory on 16 undisturbed soil samples taken from horizons Ap/BW and 2Ap/BW of the soil profile in question, were interpreted by using both Topp’s model as well as Maxwell’s (De Loor, 1964, 1990) and the model of Dobson et al. (1985) which consider soil as a multi-phase system. An empirical model developed by Regalado et al. (2003) for a volcanic soil, pedologically classified as an andosol, was also taken into consideration. We found that porosity plays an important role in the dielectric response of the Vesuvian soil. Further, the andic properties of the soil examined may provide a key diagnostic criterion to estimate a priori the deviation of - relations from Topp’s model.
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