Effects of Liquid‐phase Electrical Conductivity, Water Content, and Surface Conductivity on Bulk Soil Electrical Conductivity

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Abstract
Recent research has demonstrated that field soil salinity can be inferred from four‐electrode soil electrical conductivity (ECa), if the soil profile is near “field capacity” and calibration curves, based on saturation extract salinity (ECe), are available. To extend the use of this field method to arbitrary water contents, we studied EC in the laboratory as a function of water content (θ) and in situ soil water conductivity (ECw). We collected undisturbed cores of four soil types (fsl, vfsl, l, cl) using Lucite column inserts, which were tapped for later insertion of electrodes. The cells were equilibrated with waters of a desired ECw and, using a pressure membrane apparatus, adjusted to a desired θ. Values of ECa were calculated for each ECw − θ equilibration from measured four‐electrode resistances and an appropriate cell constant. Our results indicated that over the θ‐range of practical concern, ECa = (transmission coefficient) × θ × ECw + surface conductivity. This relationship is derived using a simple capillary model, which assumes that liquid phase and surface conductivities (via exchangeable cations) behave as resistors in parallel.

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