Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 25, 2005
Publication Date: January 3, 2006
Citation: Hunt, A., Logsdon, S.D., Laird, D.A. 2006. Percolation treatment of charge transfer in humidified smectite clays. Soil Science Society of America Journal. 70:14-23.
Interpretive Summary: Variation of electrical charges in the soil are often related to soil properties such as amount of water, clay, or chemicals in the soil. These electrical properties vary for soils at different positions in the landscape. Knowledge of the amount of water and chemicals in the soil are needed to make sure the crop has enough water and the proper amount of soil chemicals for growth. This study examined how the charges move through the water associated with the clay and shows that the charges generated came from broken water molecules within the clay layers. This information is primarily useful for scientists to understand how charges develop in soils high in clay, and how this affects measurement of soil chemical levels and measurement of soil water content.
Bulk electrical conductivity of soil is generally assumed to be dominated by the electrical conductivity of the soil solution, with perhaps a small contribution from surface charges associated with soil solids. Soils high in smectites often exhibit high electrical conductivity due to water associated with the clays. These charges are either associated with the exchangeable cations or with proton migration in the clay-associated water. The purpose of this study was to use percolation principles to elucidate mechanisms of charge transfer in humidified smectites. The theory predictions were compared with electrical conductivity spectra measured for mono-ionic humidified smectites. The measured spectra were consistent with a percolation model of proton hopping in the maximum interlayer spacing beyond a threshold water content, plus an offset term for some samples. The threshold water content for percolation in the maximum interlayer-spacing was 0.07 m**3/m**3, which was assumed the same for all basal spacings. The spectra should converge at the phonon frequency, but for some samples at the higher humidity levels, the spectra were offset. The samples with this offset were determined by examining the spectra for the same mono-ionic smectite at different humidity levels. The mean offset level was 0.015 m**3/m**3 and was likely related to charge transfer on external surfaces. The phonon frequency fitted across all samples was 3.54x 10**8 Hz, lower than the 10**12 Hz often assumed. These results support the idea that water associated with smectite clays generated the observed electrical conductivity.