Soil electrical conductivity

The quality of soil water measurements which apply electromagnetic fields to wave guides is affected by soil electrical conductivity. The propagation of electromagnetic fields in the configuration of the CS650 is predominantly affected by changing dielectric permittivity due to changing water content, but it is also affected by electrical conductivity. Free ions in soil solution provide electrical conduction paths which result in attenuation of the signal applied to the waveguides. This attenuation both reduces the amplitude of the high-frequency signal on the sensor rods and reduces the bandwidth. The attenuation reduces oscillation frequency at a given water content because it takes a longer time to reach the oscillator trip threshold.

It is important to distinguish between soil bulk electrical conductivity and soil solution electrical conductivity. Soil solution electrical conductivity refers to the conductivity of the solution phase of soil. In the laboratory, extraction methods can determine soil solution electrical conductivity, σ_solution. These extraction methods separate the solution from the solid then measures the electrical conductivity of the extracted solution.

The following describes the relationship between solution and bulk electrical conductivity (Rhoades et al., 1976)

where σ_bulk is the electrical conductivity of the bulk soil; σ_solution, the soil solution; σ_solid, the solid constituents; θ_v, the volumetric water content; and T, a soil-specific transmission coefficient intended to account for the tortuosity of the flow path as water content changes. See Rhoades et al., 1989 for a form of this equation which accounts for mobile and immobile water. This publication also discusses soil properties related to CS650 operation such as clay content and compaction. The above equation is presented here to show the relationship between soil solution electrical conductivity and soil bulk electrical conductivity.

Most expressions of soil electrical conductivity are given in terms of solution conductivity or electrical conductivity from extract since it is constant for a soil. Bulk electrical conductivity increases with water content so comparison of the electrical conductivity of different soils must be at the same water content.

The calibration equation in the CS650 operating system corrects the oscillation frequency for the effects of σ_solution up to 3 dS/m for the CS650 and up to 10 dS/m for the CS655. This is equivalent to σ_bulk values of approximately 0.8 dS/m and 2.7 dS/m respectively. If σ_bulk exceeds these limits, the CS650 sensor returns 99999 for dielectric permittivity and volumetric water content. The measured period average and voltage ratio values continue to report even if the bulk EC is outside the operational range of the sensor.