Impacts of load and repeated wheeling from a lightweight autonomous field robot on the physical properties of a loamy sand soil

The increasing size of agricultural machinery has intensified the risk of soil compaction. Fleets of lightweight autonomous robots have been proposed as a solution. However, because of their lower capacity and narrower working widths, they are likely to increase the area affected by traffic and repeated wheeling in the fields. The aim of this study was to evaluate the effects of load and repeated wheeling from a lightweight autonomous robot on soil physical properties. We conducted ten passes on the same wheel track with a 55 kW 4WD robot (model ROBOTTI 150 D with 320/65 R16 tyres) in two configurations: unloaded (total weight 33.6 kN) and carrying a filled sprayer (total weight 37.2 kN). After the first, fifth and tenth passes, we measured rut depth, apparent cohesion and penetration resistance. We took minimally disturbed 100-cm3 soil cores at 10 cm depth to measure air permeability and effective air-filled porosity in the laboratory. The results show that load configuration was significant for rut depth and apparent cohesion, whereas the number of robot passes was significant for all parameters. We found significant differences in penetration resistance for the different numbers of passes down to 0.26 m depth. Penetration resistance maximum values were 1.5 and 2 MPa for five and ten passes. Effective air-filled porosity and air permeability were negatively affected between, respectively, the second to fifth and sixth to tenth pass, reaching 0.15 cm3 cm−3 and 8 µm2, which are close to but above what is considered critical for crop growth. The pore organisation index showed blocking of the large conductive pores, especially after ten passes. Our results show that even though critical limits were not exceeded, topsoil functionality was affected by repeated wheeling, especially after six to ten passes. Thus, despite the relatively light wheel loads (< 10 kN), the recommendation is to limit the number of wheel passes to reduce the risk of compaction.