Relating mobile sensor soil strength to penetrometer cone index

A horizontally operating on-the-go soil strength profile sensor (SSPS) was previously developed so that the within-field spatial variability in soil strength could be measured at five evenly spaced depths down to 50 cm. Force divided by the base area of the sensing tip of the SSPS was defined as a prismatic soil strength index (PSSI, MPa), similar to the cone index (CI, MPa) of a vertically operating cone penetrometer. This study was conducted to establish theoretical and empirical relationships between PSSI and CI data to aid interpretation of PSSI data with respect to the many previous CI studies. Comparison of mathematical soil failure models for the SSPS and a conventional cone penetrometer, along with a sensitivity analysis of model parameters documented patterns of CI and PSSI in different soil and operating conditions. Patterns for both the soil strength indices were: (1) linear as a function of unit weight of soil, cohesion, adhesion, and operating depth, (2) exponential as a function of internal friction and soil-tool friction angles, and (3) quadratic as a function of operating speed. When a single model parameter was varied, the simulated CI and PSSI data showed highly significant linear relationships. Field data showed that, in general, both CI and PSSI were greater with higher bulk density, lower apparent soil electrical conductivity (ECa, where lower values indicate coarser texture), and lower gravimetric soil water content. Relationships between soil strength indices and the other measured soil variables were different when the data were divided into sub-groups by operating depth and ECa range. In models estimating CI, the effects of PSSI and its interaction with other variables were relatively clear at 30- and 40-cm depths. CI prediction models with the highest coefficients of determination were also found at these depths. Across entire fields, including operating depth in the regression model decreased RMSE values slightly from 0.35 to 0.32 MPa and from 0.53 to 0.51 MPa for the two test sites. These results would be useful to understand different approaches to soil strength measurement and to relate data obtained with the SSPS to the extensive research literature where CI has been used to quantify soil strength and manage the soil for better plant growth.

► CI and PSSI were linear as a function of weight of soil, cohesion, adhesion, and depth.
► The patterns were exponential as a function of internal and soil-tool friction angles.
► Soil strength was greater with lower electrical conductivity and water content.
► Including depth in the CI regression model increased coefficients of determination.