Development and field testing of an integrated sensor for on-the-go measurement of soil mechanical resistance

• Instrumented disk coulter and penetrometer integrated to measure soil compaction.
• The coulter measures the penetrating depth of the coulter into the topsoil layer.
• The soil mechanical resistance at two discrete depths measured with two tips.
• The integrated sensor is used to map soil compaction profile to a depth of 30 cm.
• Disk coulter should be installed in offset position.

Within-field variability of soil physical and chemical properties is generally regarded as the main causes contributing to variability in crop yield. High soil strength due to soil compaction has been recognized as an important negative factor in crop growth and yield. Measurement of soil strength is a common indirect measure of soil compaction. In this research, an integrated sensor – consisted of vertical and horizontal sensors – was developed and evaluated. The vertical sensor was a 60-cm diameter instrumented disk coulter with an ultrasonic distance sensor for measuring the penetrating depth of the coulter into the topsoil layer in a depth range of 0–20 cm. For measuring the soil mechanical resistance at two discrete depths (20 and 30 cm), a horizontal penetrometer with two 30° prismatic tips was integrated with the disk coulter. The integrated sensor was tested in a field with silty clay loam soil and at two gravimetric water contents (WCs) of 6 (low) and 13.5% (medium) in three replications. At the time of testing, standard soil cone penetrometer measurements along the same transects were also taken and the cone index (CI) was calculated. The results showed that the penetrating depth of the disk coulter was sensitive to changes in surface soil strength. At low WC, the measured horizontal soil resistance index (HRI) at each depth with/without vertical sensor was the same and therefore its value was independent of soil failure induced by the disk coulter ahead of the prismatic tips. However, at medium WC, due to deep penetration of the disk coulter and thus, probably reduction of geostatic pressure, the measured HRI at the depth of 20 cm with vertical sensor was decreased. To minimize the interaction of the coulter-induced failure zone with the upper tip, it is suggested that to install the disk coulter in offset position relative to the shank of the horizontal penetrometer. A significant negative linear relation between the operating depth of the disk coulter and the CI was found. Also, the relationships between the HRI and CI at both depths were significantly linear. Therefore, with offset positioning the sensors on the frame, the developed integrated sensor could be used to map the mechanical resistance of a soil profile to a depth of 30 cm.

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