Vibration-induced Conductivity Fluctuation Measurement for Soil Bulk Density Analysis

Soil bulk density is a physical property of soils that affects water storage, water and nutrient movement, and plant root activity in the soil profile. The ability to quantify soil bulk density using vibration-induced conductivity fluctuation was investigated with possible field applications in the future. The AC electrical conductance of soil was measured using a pair of blade-like electrodes while exposing to periodic vibration. The blades were positioned longitudinally and transversally to the direction of the induced vibration to enable the calculation of a normalized index. This normalized index was expected to provide data independent from the vibration strength, and to reduce the effect of soil salinity and water content. Simulations on a simplified resistor lattice indicate that the ratio of transversal and longitudinal fluctuation decreases as soil bulk density increases. The experiment was conducted on natural and salinized fine sand at two moisture conditions and four compaction levels. The blade-shaped electrodes improved electrode-soil contact compared to cylindrical electrodes, and thereby, reduced measurement noise. Dry sand measurements showed an inverse correlation between the normalized conductivity fluctuation and soil bulk density for both longitudinal and transversal fluctuation. The wet natural and salinized soils performed very similarly as hypothesized, but their normalized VICOF response was not associated with bulk density changes. This lack of sensitivity might be attributed to the heavy electrodes and/or the specific vibration method used. The effects of electrode material, vibration method and soil properties on the experiment need further study.