Effects of load and repeated wheeling from lightweight autonomous field robots on soil structure

Lightweight agricultural robots are expected to be widely used in the future and will use the same tracks for many operations within a season (i.e. repeated wheeling) and perhaps across seasons. The objectives of this study were to characterize the effects of repeated wheeling and wheel load from the traffic of a lightweight autonomous field robot on soil structural properties and the potential risk of soil compaction.

The experiment was conducted on sandy loam soil at water content close to field capacity. The field had not been tilled for approx. a year and stubbles were remaining from the previous crop. The robot used for the experiment was the AGROINTELLI ROBOTTI 150D. In total, three wheeling scenarios with one, five and ten passes were conducted both with the robot alone (3.3 Mg, inflation pressures 60-80 kPa) and at its maximum load with an implement (3.8 Mg, inflation pressures 70-90 kPa). For each treatment, rut depth and apparent cohesion were measured in the field and soil cores were taken at 10 cm depth for measuring air permeability (k_a) and effective air-filled porosity (ε_pyc) in the laboratory.

The results show that both wheel load and repeated wheeling had a significant effect on rut depth and apparent cohesion. Rut depth seemed to increase linearly with the number of wheel passes. However, apparent cohesion decreased after one pass, then increased linearly with the consecutive passes. Thus, a single pass weakened the soil structure and made it more sensitive to compaction for the following passes. Both k_a and ε_pyc, decreased significantly with repeated wheeling but not with wheel load. The average value of k_a at the tenth wheel pass was 7 µm², being five times lower than the first pass for both loads. The values of ε_pyc for the fifth and tenth passes were similar for both loads (approx. 0.15 cm³∙cm^-3). This was not the case for the value of the first pass, which was higher for the robot alone compared to loaded (0.21 and 0.17 cm³∙cm^-3 respectively), although not significantly. 

Repeated wheeling from lightweight autonomous field robots can cause significant compaction even for a soil that has not been tilled recently. Even though soil properties were not critical for crop growth, the compacted wheel tracks may serve as hotspot areas e.g. water erosion. Thus, attention should be drawn towards avoiding traffic and limiting the number of wheel passes.