Contribution of water content and bulk density to field soil penetration resistance as measured by a combined cone penetrometer–TDR probe

Soil strength as measured by cone penetrometers depends on several parameters, but it is mostly affected by the soil water content (θ) and bulk density (ρ). In order to better understand the effect of the water content and bulk density on soil strength we developed a combined penetrometer–coiled TDR probe to determine simultaneously the depth distribution of penetration resistance and water content in a soil profile. Field experiments carried out for a Yolo soil allowed the fitting of the effect of θ and ρ using a combined power–exponential equation. Using the combined cone penetrometer–TDR probe data, the fitted equation may be used to estimate soil bulk density.