Evaluating soil salinity status from bulk electrical conductivity and permittivity

Abstract

SummaryIn the range of volumetric water content, θ, from about 0.12 cm3 cm–3 to saturation the relation between bulk electrical conductivity, Cb, and bulk electrical permittivity, ε, of mineral soils was observed to be linear. The partial derivative ∂Cb/∂ε appeared independent of the moisture content and directly proportional to soil salinity. We found that the variable Xs = ∂Cb/∂ε determined from in situ measurements of Cb(θ > 0.2) and ε(θ > 0.2) can be considered as an index of soil salinity, and we call it the ‘salinity index’. Knowing the index and sand content for a given soil we could calculate the electrical conductivity of the soil water, Cw, which is a widely accepted measure of soil salinity. The two variables from which the salinity index can be calculated, i.e. Cb and ε, can be read simultaneously from the same sensor by time‐domain reflectometry.Quantities and symbolsa constant /dS m–1b constantc constant /dS m–1C b electrical conductivity of bulk soil /dS m–1C b′ constant equal to 0.08 dS m–1C s electrical conductivity of a solution used to moisten soil samples /dS m–1C w electrical conductivity of soil water defined as the soil salinity /dS m–1C wref reference salinity (that truly existing) resulting from the procedure of moistening samples, expressed as Cs + Cr/dS m–1C r baseline value of Cs due to residual soluble salts present in the soil /dS m–1d constantD dry soil bulk density /g cm–3l sloper ratioS sand content /% by weightt time /sX s salinity index /dS m–1X si initial salinity index when distilled water is used to moisten soil samples /dS m–1Y a moisture‐independent salinity‐dependent variable /dS m–1z coordinate along direction of flow of the soil solutionε′ constant equal to 6.2ε relative bulk electrical permittivity (dielectric constant) of the soilθ volumetric water content determined thermogravimetrically using oven‐drying /cm3 cm–3