Contents lists available at ScienceDirect Soil & Tillage Research journal homepage: www.elsevier.com/locate/still The soil structural cost of traffic from heavy machinery in Vertisols J. McL. Bennett a, ⁎ , S.D. Roberton a , S. Marchuk a , N.P. Woodhouse a , D.L. Antille a , T.A. Jensen a , T. Keller a,b,c a University of Southern Queensland, Centre for Sustainable Agricultural Systems Toowoomba, Qld, 4350, Australia b Agroscope, Department of Natural Resources and Agriculture, CH-8046, Zürich, Switzerland c Swedish University of Agricultural Sciences, Department of Soil and Environment, Uppsala, Sweden ARTICLE INFO Keywords: Compaction Controlled traffic farming Conservation agriculture ABSTRACT 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 ex- perimental 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 het- erogeneity 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. 1. Introduction There has been a recent and clear trend toward the use and devel- opment of larger and more powerful agricultural machinery to increase the effective capacity, or in field efficiency. This trend will likely con- tinue (Kutzbach, 2000; Bennett et al., 2015; Antille et al., 2016) at the risk of significant soil compaction, particularly in the subsoil. Such a trend means increased axle loads, in most cases, leading to continued increase in subsoil stresses (Keller and Arvidsson, 2004), with nu- merous studies (e.g., Chamen, 2015) suggesting stresses can be as great as 0.3 MPa at 0.4 m soil depth (e.g., from combine harvesters with an overall load ≥30 t).The effects of subsoil compaction are not easily remediated, resulting in often persistent impact, unless energy de- manding tillage is undertaken. However, the result of such tillage is variable and potentially short-lived, due to subsequent traffic(Logsdon et al., 1992; Alakukku, 1999; Tullberg, 2000; Chamen, 2015). Conse- quently, the tendency towards adoption of more efficient machines to reduce costs and increase work rates has brought about concern, due to the potentially negative effects of increased soil compaction and the associated need for tillage repair. Increased machinery size has the drawback of increased axle loads, and subsoil stresses (Keller and Arvidsson, 2004). In grain cropping, Chamen (2015) estimated an average 14-fold increase in subsoil stresses (from about 0.02–0.28 MPa at 0.4 m depth) between 1930 (horse-ploughing) and 2010 (30 Mg combine harvesters), respectively. What constitutes ‘heavy machinery’ should not simply be a function of the machine mass, but determined by mass at the wheel, the associated stress state distribution, and the characteristics of the soil that it will traffic. Håkansson (1990) states that the maximum load at the soil in- terface should be much less than 0.2 MPa. Whilst this has well been exceeded in modern agriculture, such a limit would minimise com- paction of Vertisols—the major soil type in Australian cotton produc- tion (McKenzie, 2001) — due to overcoming the average precompres- sion stress (≈0.1 MPa, and a volumetric soil moisture ratio average of 0.319, n = 170) with depth (Kirby 1990). Therefore, we define ‘heavy machinery’ as that applying stress > 200 kPa at the soil surface. https://doi.org/10.1016/j.still.2018.09.007 Received 10 December 2017; Received in revised form 26 July 2018; Accepted 8 September 2018 ⁎ Corresponding author. E-mail address: john.bennett@usq.edu.au (J.M. Bennett). Soil & Tillage Research 185 (2019) 85–93 0167-1987/ © 2018 Elsevier B.V. All rights reserved. T