The soil structural cost of traffic from heavy machinery in vertisols

The soil structural cost of traffic from heavy machinery in vertisols

The agricultural industry has a strong and continuing trend for the incorporation of heavy machinery into the farming system, in order to create operational efficiencies. It is therefore important to understand the soil structural cost of such machinery, which was the objective of this work. Using the John Deere 7760 (JD7760) cotton picker (soil surface stress at the rear wheel ≈0.5 MPa), as a case study, seven randomly allocated experimental sites within the Australian cotton industry were investigated for changes in soil bulk density after traffic with the JD7760. The modified Proctor test optimum moisture content (OMC) for compaction was measured, based upon the JD7760 imposed surface stress, and compared to the field results for compaction. Soil water deficits, calculated for the modified Proctor test OMC, were determined and used to discuss the soil structural implications of heavy machinery, as well as threshold soil water content for safe traffic. All sites underwent significant soil compaction within the 0.3 m depth. More than 50% of sites exhibited compaction to the limit of investigation (0.8 m depth), with the remaining sites having significant reduction in spatial heterogeneity of Vertisol cracks and macropores for the same depth. General equations for OMC and plastic limit, based on clay content and OMC, respectively, were developed. These were used to facilitate extrapolation of experimental data to an open-database of 116 Vertisol sites. For these data, it was determined that safe traffic thresholds did not exist above to the lower limit (soil matric potential −1.5 MPa). Implications for soil structural relations and soil-water movement are discussed.

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