Soil properties influencing apparent electrical conductivity: a review

The most common method for in situ assessment of soil salinity, namely the electrical conductivity (EC) of the soil solution (ECw), is to measure the apparent electrical conductivity (ECa) and volumetric water content (θ) of the soil and apply measured or predicted ECa(ECw, θ) calibration curves. The water content and electrical conductivity of a soil solution are indeed the major factors affecting its apparent electrical conductivity, which justifies the assessment of salinity from apparent EC measurements. However, the ECa(ECw, θ) relationship depends on some additional soil and environmental attributes affecting the soil ECa. Non-spherical particle shapes and a broad particle-size distribution tend to decrease ECa, and when non-spherical particles have some preferential alignment in space, the soil becomes anisotropic, i.e., its ECa depends on the direction in which it is measured. The electrical conductance of adsorbed counterions constitutes a major contribution to the ECa of medium- and fine-textured soils, especially under conditions of low solution conductivity. In such soils and with such salinity levels, the temperature response of the soil ECa should be stronger than that of its free solution, and care should be taken when extrapolating from field-measured ECa values to obtain the ECa at a given temperature. The above-mentioned and other secondary findings should, on one hand, indicate some limitations for the application of existing ECa-ECw models, and, on the other hand, can serve as guidelines for further development of such essential models.