Submitted to: Soil Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/16/2006
Publication Date: 10/10/2006
Citation: Logsdon, S.D. 2006. Uncertainty effects on electrical conductivity and permittivity spectra. Soil Science. 171:737-746. Interpretive Summary: Two electrical measurements are often related to soil properties. One is called "dielectric", which measures the rotation movement of water molecules when an electrical field is applied. Soil dielectric properties are often used to determine soil water content. The other is "electrical conductivity" due to charges that develop in the soil water and carry an electrical current. Soil electrical conductivity can be related to salts and clays in the soil. This study was conducted to determine if the uncertainty in the measured and derived values for soil electric properties could be caused by the instrument, fitting the data to an equation, or sample-to-sample variation. The results showed that the uncertainty to sample-to-sample variation was the greatest. This was caused by differences in how densely the soil was packed into the holder, how the soil clumps (aggregates) fit in the holder, and variations between different soils. The uncertainty due to fitting the data to mathematical equations was very small. This information is important for scientists who use soil electrical properties to determine soil water content, salt content, or clay content.
Technical Abstract: Soil permittivity and electrical conductivity values are usually reported without error bars, which limits comparisons among different studies using different equipment. Uncertainty in measured permittivity spectra could be due to the measurement system (hardware) or sample-to-sample variation. There is also uncertainty in parameters fitted from the spectra. The objective of this study is to determine the measurement uncertainty on permittivity and electrical conductivity spectra. Uncertainty related to fitting parameters, instrument, aggregate size, density, and soil are considered. Different aggregate sizes of three soils were packed into a truncated coaxial cell. Electrical conductivity and permittivity spectra were calculated from reflection scattering parameter measured by a vector network analyzer. Measurement uncertainty was around ten times greater than fitting uncertainty, and 1/3 to 3/4 that of sample-to-sample uncertainty. The square root of the apparent permittivity significantly related most strongly with soil water content but also with soil differences (bound water) and bulk density (partial R2 of 0.71-0.74, 0.09, and 0.03-0.04). Derived direct current (d.c.) electrical conductivity also significantly related to soil water content, bound water content, and bulk density, but also to aggregate size (partial R2 of 0.53-0.54, 0.05, 0.03, and 0.02). Derived d.c. electrical conductivity leveled off at low water contents thus providing a "cutoff" water content. The cutoff water content was significantly related to bound water and aggregate size. The effect of aggregate size on electrical conductivity spectra and derived factors, while significant, was small compared with water content effects. The greatest variation was sample-to-sample within a soil, and among soils.