Design and Validation of an On-the-go Soil Strength Profile Sensor

Soil strength has traditionally been determined using the cone penetrometer, an instrument that provides highly variable discrete point measurements, making it difficult to detect statistically significant differences in the soil strength profile among treatments or locations. Generally, this problem has been addressed by obtaining a large number of measurements, a process that is time-consuming and labor-intensive. Our objective was to develop a soil strength profile sensor (SSPS) that could take measurements continuously and more efficiently while traveling across the field. The on-the-go SSPS was designed and fabricated using an array of load cells, each of which was interfaced with a soil-cutting tip. These multiple prismatic tips were extended forward from the leading edge of a vertical blade and spaced apart to minimize interference from the main blade and adjacent sensing tips. Prismatic soil strength index (PSSI, MPa) was defined as the force divided by the base area of the sensing tip. The sensing tip had a 60° cutting or apex angle and a base area of 361 mm2. The design maximum operating depth was 0.5 m, and the upper limit and resolution of soil strength were 19.4 and 0.14 MPa, respectively. Field tests determined that the optimum extension and spacing of the cutting tips were 5.1 and 10 cm, respectively. A significant ( = 0.01) linear relationship between PSSI and penetrometer cone index (CI), with a slope of approximately 0.6, was found for field data collected at a 30 cm depth. The ability to develop such relationships comparing penetrometer and SSPS data will allow SSPS data to be interpreted with respect to the available body of penetrometer literature.