Improvement and field testing of a combined horizontal penetrometer for on-the-go measurement of soil water content and mechanical resistance

Soil water content has a significant effect on penetrometer resistance, so simultaneous on-the-go measurement of both these parameters can provide useful information on within-field spatial variations in soil compaction. In this study we improved and refined the electronic circuitry and mechanical construction of an existing sensor and evaluated it as a combined horizontal penetrometer in the field. This paper concentrates on its ability to determine soil water content (volumetric basis) in the field. Stationary calibration was first performed in the laboratory on remoulded loam soil (gleyic Cambisol) at gravimetric water content 0–0.28 g g−1 and wet bulk density 1.4–1.8 g cm−3. A quadratic relationship was found between sensor output and volumetric water content. Simulations of the sensor fringe field using a finite element model showed that at a dielectric constant of 5 (dry soil) the zone of influence of the sensor (i.e. the distance from the centre of the sensor) was approximately 70 mm, whilst at a dielectric constant of 30 (an estimate of wet soil) it was 40 mm. The dielectric sensor was then implemented together with a load cell on a tine for field measurement of water content and penetrometer resistance, and a series of transect measurements were conducted to assess its accuracy over a range of natural soil water contents in the field. Within the volumetric water content range 0.30–0.40 cm3 cm−3, a linear relationship fitted the sensor output versus water content data, which was in close agreement with the laboratory calibration. Additional studies on the effect of travelling speed on dielectric sensor performance showed no clear trends, but further investigations are needed. A significant negative correlation was observed between penetrometer resistance and water content. The developed combined horizontal penetrometer can be a promising useful instrument for future studies in soil compaction and precision agriculture.