Evaluating soil salinity status from bulk electrical conductivity and permittivity

Summary In the range of volumetric water content, θ , from about 0.12 cm 3 cm –3 to saturation the relation between bulk electrical conductivity, C b , and bulk electrical permittivity, ε , of mineral soils was observed to be linear. The partial derivative ∂ C b /∂ ε appeared independent of the moisture content and directly proportional to soil salinity. We found that the variable X s = ∂ C b /∂ ε determined from in situ measurements of C b ( 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, C w , which is a widely accepted measure of soil salinity. The two variables from which the salinity index can be calculated, i.e. C b and ε , can be read simultaneously from the same sensor by time‐domain reflectometry. Quantities and symbols a constant /dS m –1 b constant c constant /dS m –1 C b electrical conductivity of bulk soil /dS m –1 C b ′ constant equal to 0.08 dS m –1 C s electrical conductivity of a solution used to moisten soil samples /dS m –1 C w electrical conductivity of soil water defined as the soil salinity /dS m –1 C wref reference salinity (that truly existing) resulting from the procedure of moistening samples, expressed as C s + C r /dS m –1 C r baseline value of C s due to residual soluble salts present in the soil /dS m –1 d constant D dry soil bulk density /g cm –3 l slope r ratio S sand content /% by weight t time /s X s salinity index /dS m –1 X si initial salinity index when distilled water is used to moisten soil samples /dS m –1 Y a moisture‐independent salinity‐dependent variable /dS m –1 z coordinate along direction of flow of the soil solution ε′ constant equal to 6.2 ε relative bulk electrical permittivity (dielectric constant) of the soil θ volumetric water content determined thermogravimetrically using oven‐drying /cm 3 cm –3