Increasingly heavy farm equipment is crushing fertile soils

Fat tires may keep giant combines from sinking into the field, but their tremendous weight still squeezes the life out of the soil deep down—potentially reducing crop yields across a lot of the planet.

If you start talking with Thomas Keller or Dani Or about farm machinery, sooner or later the conversation will turn to dinosaurs. Why would two experts in the biology and structure of soils segue from tractors and combines to extinct behemoths? Because today’s farm vehicles, they explain in a recent paper, have become nearly as heavy as the largest animals that ever stomped the Earth—and the sheer weight is crushing one of the world’s most precious resources: fertile soil.

“It's not rocket science,” says Or, who splits his time between the Desert Research Institute in Reno, Nevada, and the Swiss Federal Institute of Technology, in Zurich. “For over a century, we have had a persistent increase in the size of [farm] machinery. Something has to give.”

The trend toward the gargantuan has been especially pronounced over the last 60 years. Keller and Or found that from 1958 to 2020, the typical weight of a fully loaded combine harvester for corn or wheat increased nearly tenfold, going from 8,800 pounds 60 years ago to about 79,000 pounds today. A fully loaded sugar beet harvester can weigh more than 130,000 pounds, which is in the range of the heaviest sauropods—the super heavyweights of the dinosaur world. (The weight of the largest living land animal, the African elephant? A pixieish 20,000 pounds.)

“In a way it’s fantastic,” says Keller, a professor at the Swedish University of Agricultural Sciences in Uppsala. “If you see how much you can harvest in a certain amount of time today and how long it would have taken you 60 years ago...”  Some wheat harvesters, for example, can clear 30 acres in an hour.

That incredible efficiency enables about 5 percent of the world’s population to feed the other 95 percent. But it is taking a heavy toll on the world’s soils, Or and Keller argue. They estimate that oversize agricultural machines threaten about 20 percent of all arable land, mostly in North America, Europe, Brazil, and Australia, where the huge vehicles are most widely used. Once damaged, heavily compacted soil can take decades to recover, if it recovers at all.

“There's some evidence that we have been losing yields,” says Or. “Despite breeding for better wheat and other cereals, we have been consistently losing yields. You can blame it on many things, but this seems to be happening mostly in the mechanized world.” 

One recent study reported that soil compaction from heavy machines has reduced yields in some fields by as much as 50 percent. If current trends continue, the combination of compaction and erosion may eventually reduce global crop yields by as much as 20 percent.

Unseen damage

Soil compaction is insidious because it’s mostly invisible, even to farmers. Manufacturers have tried to limit the impact on the land by equipping heavy vehicles with giant tires, which distribute the load more widely and keep the vehicles from sinking too deeply into the soil surface.

But deeper soil layers still absorb the full weight of the machines. The compressive stress fans out under each point of the tire, and under the center the stress lines intersect and add up. Picture that tire as the base of an inverted, buried pyramid of stress: The more massive the vehicle, the deeper the apex pushes into the soil.

“If you have a [heavier] load, the stress decreases less quickly with depth,” says Keller. A motorcycle, for example, would leave a deeper track on the surface of a field than a tractor would, but the impact of the tractor’s greater weight would extend much more deeply into the soil. The fat tires are just spreading the impact to a place where we don’t notice it as much—a bit like the way tall smokestacks on a power plant spread air pollution far downwind.

But healthy soils are porous and alive, and we depend on the deep layers too. A teaspoon of garden soil might hold a billion bacteria, networks of air pockets, many yards of fungal filaments, and thousands of protozoa. “Soils are the most biodiverse habitats on the planet,” says Paul Hallett, a soil physicist at the University of Aberdeen, in Scotland. “That's because of this pore structure, which is really, really complex.”

Heavy machinery squeezes the life out of that rich habitat. Tilling repairs some of the damage by aerating the soil, but only in the upper few inches, and it exacerbates erosion. Today’s mammoth farm machines, say Or and Keller, are compressing soils at depths of a foot or more, crushing pores, lowering oxygen levels, and destroying the life that creates the basis for healthy soils, roots, and crops.

“Once you have compaction in the subsoil, it’s irreversible, it’s permanent,” says Markus Berli, an environmental physicist at the Desert Research Institute. “The natural processes that might alleviate compaction are much more active near the surface than they are in the subsurface.” Even the world’s most fecund biomes are not immune to the effects of compaction. Researchers in Brazil have found that the subsoil in a heavily logged region of the Amazon rainforest still hasn’t recovered 30 years after the heavy logging trucks and other machines departed.

Although the effect is hard to quantify, compaction also reduces the soil’s capacity to store carbon. Dense, compressed soils limit the growth of roots, which are one of the main pathways by which carbon enters the soil. All told, soils hold more than two trillion tons of carbon, about three times the amount found in the atmosphere. “Soil is the biggest terrestrial store of carbon,” says Hallett. “There's more carbon beneath the ground than there is above it.”

A case for small robots?

The obvious remedy to these many ills would be to use smaller machines. “Rather than having one giant tractor, I'd rather see a series of 10 small tractors [operated] remotely doing far less damage and getting the same efficiency,” says Or. “We can do that now. We should consider restricting the size of the tractors.”

How much smaller should they be?

“We started to lose the battle somewhere in the middle of the 1980s,” says Or. That’s when the average load per wheel on heavy farm vehicles rose above 11,000 pounds. Returning to the previous limits would help protect the soil’s deep root zone, reducing the stress to recoverable levels. “It’s a compromise,” says Or,  “between efficiency and damage.”

The sort of future Or imagines can already be glimpsed at Hands Free Farm, an experimental 86-acre project in Shropshire, England. Researchers there have shown that it’s possible to farm with a very light touch indeed—and with no human touch at all. The researchers use small conventional vehicles retrofitted to operate autonomously. One of the farm’s workhorses is a 3,000-pound, 38-horsepower robotic tractor.

Since 2016, researchers at Harper Adams University in Shropshire have been cultivating, growing, and harvesting crops with robotic vehicles. “We've been doing the whole cropping cycle, from soil preparation all the way through to harvest, entirely with autonomous equipment,” says James Lowenberg-DeBoer, a Harper Adams agricultural researcher.

The key to reducing the size of farm machinery, he argues, is to take humans out of the drivers’ seats. Farm machines have grown ever larger, Lowenberg-DeBoer says, for one simple reason: to maximize the amount of work done by a single individual. “Once you take the driver off the equipment,” he says, “then the size of the equipment matters less.”

Farmers work long hours, but they can’t match the work ethic of tireless automatons. “It’s possible during parts of the year to run farm equipment 24 hours a day,” Lowenberg-DeBoer says. “Very few farmers do that, but robots don’t care.”

The shift to robotics could help farmers lower their expenses. Lowenberg-DeBoer and his colleagues estimate that equipment costs on a 500-hectare farm—that’s 1,235 acres—could be cut by nearly two-thirds by adopting the sorts of machines used at Hands Free Farm.

What will it take to make that happen? “All the big companies have come to Harper Adams,” says Lowenberg-DeBoer. “And they’ve looked at this farm and said nice things. But they're not very interested in starting to sell [this equipment]. What we need is an outside influence, someone that would come in and start selling autonomous equipment, just like Elon Musk shook up the electric vehicle market.”

How the dinos did it

It’s tempting to write that maybe one day the Brobdingnagian combines, tractors, and other vehicles now squashing the Earth beneath them will eventually go the way of the dinosaurs. But that would be a disservice to those incredible animals. Sauropods thrived for more than a hundred million years. How did they manage to avoid destroying their own habitats by compacting soil beyond repair? Keller and Or call this “the sauropod paradox.”

The giant dinos had giant feet—the largest known sauropod footprints measure nearly six feet across—that kept them from sinking into the soil, as fat tires support a combine. But Keller and Or argue that it’s unlikely the imposing reptiles moved in large herds. 

“The picture you see in Jurassic Park of free-roaming sauropods is not very likely due to the damage they would have done to the landscape,” they write.

Instead, Or and Keller suggest that the sauropods restricted themselves to moving along narrow trails. In this scenario their long necks evolved to let them browse vegetation off their well-beaten paths without damaging the surrounding landscape. Alternatively, Keller and Or say, they may have spent much of their time suspended in water, again using their long necks to forage along shorelines.

Fast-forward a hundred million years to 21st-century Australia, where farmers are experimenting with “controlled traffic farming,” a strategy that restricts the wheels of heavy vehicles to fixed traffic lanes, leaving about 80 percent of any given field free from the risk of compacted soil. In that case, at least, maybe the big machines are going the way of the dinosaurs after all.

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