Simulation of a potential error in determining soil salinity from measured apparent electrical conductivity

A common method for estimating soil salinity is by in situ measurements of the apparent electrical conductivity, ECa, usually by a four-electrode probe. It is assumed that the ECa can be regarded as taking place in parallel mode by two conductors on a bulk scale: the dissolved (ECb) and the adsorbed (ECs) ions. Therefore, the contribution to the ECa by the soil solution electrolytes, ECb, which serves to assess its salinity, can be deduced by subtracting the estimated ECs from the measured ECa. This assumption is wrong and leads to an evaluation of ECb higher than its real value. This study was conducted to characterize the error in estimating soil solution electrical conductivity, ECa. A simplified model of a randomly diluted and pore-size-distribution-decorated simple cubic lattice serves to describe the pore network of a saturated soil. It is assumed that only within each pore can the electrical conductance be represented by a sum of two conductors: the dissolved and the adsorbed ions, acting in parallel. Using Monte Carlo lattice simulations, it was shown that the error due to the assumption of parallel mode on a bulk scale increases with increasing broadness of the pore-size distribution, decreasing connectivities, and increasing cation-exchange capacity. An illustrative example of real soils, typical of irrigated soils, indicated errors of up to 25% for electrolyte concentrations.