Thomas Keller and Dani Or

PNAS May 16, 2022; 119(21)e2117699119

Significance

Mechanization has transformed agriculture over the past century, greatly improving crop production efficiency. However, the higher capacity has resulted in increased farm vehicle weights. We show that while machinery design maintains constant surface contact stresses, an insidious and largely overlooked threat of subsoil compaction has developed. We demonstrate that modern vehicles induce high soil stresses that now exceed critical mechanical thresholds for many arable soils, inducing chronic soil compaction in root zones below tillage depths and adversely affecting soil functioning. We draw parallels between modern farm vehicles and the heaviest animals that walked Earth: sauropods. We hypothesize that these prehistoric giants may have induced subsoil compaction, thus presenting a paradox for productivity of the land that supported them.

Abstract

Mechanization has greatly contributed to the success of modern agriculture, with vastly expanded food production capabilities achieved by the higher capacity of farm machinery. However, the increase in capacity has been accompanied by higher vehicle weights that increase risks of subsoil compaction. We show here that while surface contact stresses remained nearly constant over the course of modern mechanization, subsoil stresses have propagated into deeper soil layers and now exceed safe mechanical limits for soil ecological functioning. We developed a global map for delineating subsoil compaction susceptibility based on estimates of mechanization level, mean tractor size, soil texture, and climatic conditions. The alarming trend of chronic subsoil compaction risk over 20% of arable land, with potential loss of productivity, calls for a more stringent design of farm machinery that considers intrinsic subsoil mechanical limits. As the total weight of modern harvesters is now approaching that of the largest animals that walked Earth, the sauropods, a paradox emerges of potential prehistoric subsoil compaction. We hypothesize that unconstrained roaming of sauropods would have had similar adverse effects on land productivity as modern farm vehicles, suggesting that ecological strategies for reducing subsoil compaction, including fixed foraging trails, must have guided these prehistoric giants.

See https://www.pnas.org/doi/10.1073/pnas.2117699119

Figure 1: Scaling relationship between footprint size (contact area) and body or vehicle mass. Blue circles represent sauropods (data in SI Appendix, Table S3), and red squares represent agricultural machinery, with tire characteristics given in SI Appendix, Table S2. The orange circles are from Cumming and Cumming (17), and the light blue line is the scaling law of Michilisens et al. (18), also presented in the inset taken from Clemente et al. (72). Note the relatively narrow range of surface contact stress that spans the entire range of animals (extant and extinct) and agricultural machinery.