Which is worse for the soil: combine harvesters or dinosaurs?

Enlarge / Having this guy stomping around could mean things would have a hard time growing there afterward.

Words you didn’t expect to read in a scientific article this week: “The similarity in mass and contact area between modern agricultural vehicles and sauropods raises the question: What was the mechanical impact of these prehistoric animals on the productivity of the land? ” The paper, by Thomas Keller and Dani Or, raises what may be a major concern: Agricultural vehicles have grown in recent decades, to the point that they may be compacting the underground soil where crop roots extend. This represents a risk for agricultural productivity.

The paper then compares that compaction risk to that posed by the largest animals that have ever roamed our land: sauropods.

the big crunch

We think of soil as solid, but the spaces and channels within the soil are critical to plant life, allowing air and water to reach the roots. Soil compaction, in its extreme form, eliminates all of these spaces, making the soil much less hospitable to plants. And compaction is hard to reverse; it can take decades of plant and animal activity to break down compacted soil again and reestablish a healthy ecosystem.

There are a large number of agricultural tools dedicated to tilling the soil, breaking it up and making it more permeable to air and water. But this hardware doesn’t go that deep. A “subsurface” area remains below the tilled soil, but the subsoil is still within the area used by the roots of agricultural plants.

Obviously, having something heavy rolling on the ground is an easy recipe for compaction. And farm hardware is getting heavier as bigger equipment becomes more efficient. In 1958, a typical combine harvester weighed about 4,000 kg. Today, that average weight has grown to more than 35,000 kg. To avoid crushing the ground beneath this mass, the tires have become larger and are operated at lower pressures, allowing the tire to be spread over a larger area to limit compaction of the ground surface.

But the surface forces are translated to the subsurface in a way that is less dependent on the area over which the compressive force is distributed. Instead, at some depths, the stresses are highly dependent on the mass per wheel. As such, the increasing mass of combine harvesters (and other farm equipment) increases the risk of subsoil compaction. That effect will be difficult to detect and correct, but it may result in a loss of agricultural productivity.

Global risks. Besides the dinosaurs!

Compaction hazards are not evenly distributed. Drier regions have less groundwater to squeeze out and are therefore less at risk of compaction. Some countries also farm through smaller lots that don’t benefit from the efficiencies provided by large hardware. Still, many areas are at risk of compaction, including the eastern US, eastern Australia, Argentina, and much of Europe. In general, these areas align well with those that a separate study suggested suffer from reduced productivity due to soil compaction. Overall, about 20 percent of important agricultural areas are at risk.

Which brings us back to the topic of dinosaurs. Sauropods became considerably more massive than even the largest harvesters: they may have approached 80,000 kg. Its weight was spread over just four limbs, with footprints roughly comparable to modern tires (harvesters, by contrast, typically have six tires). Finally, to move, sauropods had to lift at least one limb at a time. All of these factors combined mean that sauropods should have exerted substantially more pressure on the ground.

If farm equipment now poses a compaction risk, the dinosaurs were almost certainly causing problems. At the same time, however, animals of that size required a thriving ecosystem to sustain them. “The potential for significant soil compaction by foraging sauropods appears inconsistent with productive land that supported renewable vegetation to feed these prehistoric herbivores,” as the researchers put it.

How to explain this apparent contradiction? Keller and Or, who specialize in agricultural studies, fall back on an idea that has been popular in paleontology circles for a while: Anything that big was almost certainly semi-aquatic, as it would have a hard time supporting its bulk. But rather than speak to a paleontologist for more information, the authors simply state that “solving this paradox is beyond the scope of this study.”

PNAS2022. DOI: 10.1073/pnas.2117699119pnas.org (About DOIs).

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