Thousands of years ago, beasts of burden helped make humanity what it is today. When farmers first started putting down roots, they’d plant and tend their crops by hand. With the power of oxen, they could drag plows across their fields before sowing, which boosted soil fertility and eliminated weeds. Today, that job has been made even easier by giant machines that rake the landscape.

Millennia of tilling, though, has come at a cost. While plowing releases nutrients in the short term, it degrades soil fertility in the long term, requiring farmers to load their fields with synthetic fertilizers. (The burst of microbial activity after roiling the ground also chews through accumulated carbon, returning it to the atmosphere as planet-warming greenhouse gas.) In addition, all this cultivation destroys the natural subterranean structures that hold onto water, meaning less is delivered to crops.

Fiber optic cables, of all things, have now exposed just how badly tilling messes with a farm’s ability to retain moisture. Using a technology known as distributed acoustic sensing, or DAS, scientists analyzed how seismic waves disturbed the cable as they rippled through harrowed fields compared to adjacent undisturbed plots. This created subtly distinct signals, showing that plowing obliterates the “capillaries” that carry water like tiny interconnected reservoirs. 

The findings point to a serious problem with modern agriculture, to be sure, but also to solutions. “Regenerative farming practices based on principles of no-till — combined with cover crops and a diversity of crops — can basically lead to less agrochemical reliance, better soil organic matter contents, comparable yields, [and] lower diesel use,” said David Montgomery, a geomorphologist at the University of Washington and coauthor of a new paper describing the research.

DAS exploits the extreme sensitivity of fiber optic cables, which transmit information as pulses of light. If there’s a disturbance along the path — an earthquake or even someone walking overhead — a tiny bit of light bounces back to the source. With a device called an interrogator, researchers can send pulses along a length of cable and analyze what returns. Because they know the speed of light, they can differentiate a disturbance a mile down the line from one just a few hundred feet away, as the former will take just a tiny bit longer to return. Whereas a traditional seismometer takes readings at a single point, a DAS system turns miles upon miles of fiber optic cable into one continuous sensor. 

Luckily enough for these researchers, Harper Adams University in the United Kingdom has run a 20-year outdoor lab where researchers have treated adjacent fields with different levels of tilling. Whereas using DAS to monitor for earthquakes relies on the planet’s deep seismic rumblings, in these fields the researchers laid cable at the surface and listened for what was happening above ground: human activities like cars, but also rain and wind hitting the cable. Basically, it was messier, seismically speaking, than Earth’s more consistent vibrations, but still informative. “A lot of noise for somebody is a signal for others,” said Marine Denolle, an earth scientist at the University of Washington and senior author of the new paper.

In the end, it was all about speed, aka seismic velocity. If a car drove by, it sent waves across the road, then into the fields. “If the soil has water, the wave will take longer to come to us than if the soil is dry,” Denolle said. 

Let’s leave the farm for a second and head to the beach to explain that. Where the ocean laps at the shore, the wet sand is so hard that you can run along it without sinking in and breaking your ankle. The nearby dry sand, on the other hand, is so loose that you might have a hard time trudging through it. “The only difference is the way these capillary forces glue the material together when there’s a sufficient amount of water,” Denolle said. “We do notice that action of stiffening and loosening of the soil, just due to that change.” 

Or think of undisturbed soils as a sponge, loaded with lots of pores for water to fill — leave one out on the counter and it hardens and contracts, only to soften and expand when you soak it once more. So on the experimental farm, the tilled soils might have looked like they’d better absorb water, what with their looseness, but the opposite is true. “It’s kind of counterintuitive, right?” Montgomery said. “You’d think that breaking up the ground surface would allow more water to get down into it. But if you plow it often enough, hard enough, you kind of pulverize it. And it’s all those little worm holes and the bug holes and the root holes that allow water to get down into the soil.” 

Why, then, would farmers keep plowing their fields for thousands of years? “Well, farmers don’t like weeds, and so a really good way to get weeds off your field is to plow it,” Montgomery said. “That can provide nutrients to a crop, so you get a little burst of fertility with tillage. But if you do it too often for too long, you wear out the batteries of the soil, in effect.” 

That’s why modern farmers add heaps of expensive synthetic fertilizers. Not only do these inputs take a whole lot of energy to produce, contributing to global warming, but they also run off of the land, poisoning waterways. (Nitrogen fertilizer is geopolitically perilous as well: Almost a third of it passes through the Strait of Hormuz, which Iran effectively closed after being attacked by Israel and the United States.) And because water doesn’t soak into tilled foils as well, much of it evaporates before reaching roots. 

Together, these trends drive up costs and will only accelerate as climate change exacerbates droughts. Researchers elsewhere in the world might also use DAS to better understand soil conditions on local farms. “I thought it was a neat application on the very small scale, but just showing how DAS can be used to solve these types of problems,” said Jonathan Ajo-Franklin, an applied geophysicist at Rice University, who studies the technology. (Ajo-Franklin wasn’t involved in the paper, but its lead author is a postdoc in his department.)

That solution, Montgomery said, is embracing regenerative agriculture to prioritize soil health, which means reducing physical and chemical disturbances. If a farmer is struggling with weeds, for instance, they can let loose livestock after a harvest, clearing the field for the next crop to get a head start. Or they might add cover crops, which further choke out weeds. Even if, say, an urban farm has to do some tilling, it can return carbon to the ground with compost.

Increasing crop diversity, too, will both recharge the soil (legumes, for example, “fix” their own nitrogen and add it to the earth for other plants to use as fertilizer) and make a field friendlier to beneficial microbes, which help lock carbon in the ground and keep it out of the atmosphere. “There’s all kinds of other reasons we would want to adopt those same suite of practices,” Montgomery said, “from reducing our reliance on agrochemical inputs, increasing on-farm biodiversity, reducing off-farm pollution, building soil organic matter, creating more profitable farms.”

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