Mapping field-scale physical properties of soil with electrical resistivity

The spatial variability of physical properties that significantly influence the fate of water and solute in soils needs a large number of measurements to be quantified. Surface electrical resistivity techniques could be used as a simple and practical method to determine this spatial variability. Electrical sounding and profiling measurements were taken on a small agricultural field (30 by 60 m) under two different soil conditions (dry and wet conditions). The soil profile is composed of three layers: a highly permeable sandy loam (alluvial terrace) overlying a gravelly sandy till that covers a friable sandy to silty shale. The soil physical properties (grain size distribution, porosity, hydraulic conductivity, bulk density, and organic matter content) of the uppermost layer were measured in the laboratory on undisturbed soil cores taken at three different depths on a 6 by 15 m grid in the field. Correlations were established between these parameters and the electrical conductivity. The best correlations were between the electrical conductivity and the sand, silt, clay, and organic matter contents. Their correlation coefficients, r, were, respectively; 0.64, 0.53, 0.64, and 0.65 for the dry conditions and 0.54, 0.45, 0.53, and 0.52 for the wet conditions. No relation was established between the electrical conductivity and the porosity, the bulk density, or the hydraulic conductivity. The correlation coefficients were, respectively, 0.04, 0.16, and 0.10 for the dry conditions and 0.14, 0.12, and 0.14 for the wet conditions. Electrical conductivity seems to be more influenced by the soil texture, i.e., by the electrical properties of the soil constituents, than by the structure, i.e., the water-related properties. Besides, the two sets of electrical resistivities obtained in dry and wet conditions are not significantly different, as shown by the regression between them (slope = 0.92, r = 0.71) and by the isoresistivity maps. This study seems to indicate that the electrical method could be used to evaluate the spatial variability of some soil properties when their variability is sufficiently large, i.e., when the investigation scale or the level of contrast is large enough.