Thousands of years ago, the Beast of Burden helped make humanity what it is today. When farmers first started planting their roots, they planted and tended their crops by hand. They could use the power of their oxen to drag plows across the fields before sowing, increasing soil fertility and eliminating weeds. Today, that task is made even easier by huge machines that scrape the landscape.
But thousands of years of cultivation came at a cost. Although tillage releases nutrients in the short term, it reduces soil fertility in the long term, requiring farmers to load their fields with synthetic fertilizers. (The burst of microbial activity after shaking the ground also chews up the stored carbon and sends it back into the atmosphere as a global-warming greenhouse gas.) Plus, all this cultivation destroys the natural underground structures that hold water, so less is available to crops.
Above all, fiber optic cables have revealed how much tillage is negatively impacting a farm’s ability to retain water. Scientists used a technique known as distributed acoustic sensing (DAS) to analyze how seismic waves rippling across the desolate landscape disturb the cables compared to adjacent, peaceful areas. This produces a subtly different signal, indicating that tillage eliminates the “capillaries” that transport water like small interconnected reservoirs.
The findings certainly point to serious problems in modern agriculture, but they also point to solutions. “Regenerative farming practices based on no-till principles, combined with cover crops and a variety of crops, can essentially reduce reliance on pesticides, improve soil organic matter content, provide comparable yields, (and) reduce diesel use,” said David Montgomery, a geomorphologist at the University of Washington and co-author of a new paper describing the study.
Marine Denor/University of Washington
DAS takes advantage of the extreme sensitivity of fiber optic cables, which transmit information as pulses of light. If there is an obstacle along the way (even an earthquake or someone walking overhead), a small amount of light will bounce back to the light source. A device called an interrogator allows researchers to send pulses along a cable and analyze what is returned. Because they know the speed of light, they can distinguish between a disturbance a mile away and a disturbance just a few hundred feet away. The former is because it takes just a little while to come back. While traditional seismometers take measurements at a single point, DAS systems convert miles of fiber optic cable into one continuous sensor.
Fortunately for these researchers, Harper Adams University in the UK has been running an outdoor laboratory for 20 years, and researchers have been treating adjacent fields with different tillage levels. Whereas earthquake monitoring using DAS relies on seismic rumbles deep within the Earth, researchers in these fields have laid cables on the surface and listened to what’s happening above ground: not only human activity such as cars, but also rain and wind hitting the cables. Basically, seismologically speaking, it was messier than the Earth’s more stable vibrations, but still beneficial. “A lot of noise to one person is a signal to another,” said Marine Denour, a geoscientist at the University of Washington and lead author of the new paper.
At the end of the day, it’s the velocity that matters: the seismic velocity. As cars passed by, the waves traveled over the road and into the fields. “If there’s water in the soil, the waves will take longer to arrive than if the soil is dry,” Denor says.
To explain that, let’s leave the farm for a moment and go to the beach. Where the ocean washes onto the shore, the wet sand is so hard that you can run on it without sinking and breaking your ankle. On the other hand, the dry sand nearby is very loose, so it can be difficult to trudge along. “The only difference is how capillary forces bond the materials together when there is enough water,” Denor says. “We realized that these changes had the effect of hardening or loosening the soil.”
Alternatively, think of undisturbed soil as a sponge with many pores that allow it to fill with water. If you leave one on the counter, it will harden and shrink, but if you soak it again, it will soften and expand. That is, on the experimental farm, the tilled soil may have appeared to absorb water better due to its loosening, but in fact, the opposite is true. “That’s counterintuitive, isn’t it?” Montgomery said. “You might think that if you break up the surface, more water can get down into the soil. But if you plow often enough and hard enough, the ground becomes kind of pulverized. And these little worm holes, worm holes, root holes allow water to get down into the soil.”
So why do farmers continue to plow their fields for thousands of years? “Well, farmers don’t like weeds, so a really good way to remove weeds from a field is to till the field,” Montgomery said. “Tillage gradually increases fertility by providing nutrients for crops. But if you till it too often and for too long, you effectively drain the soil’s battery.”
This is why modern farmers add large amounts of expensive synthetic fertilizers. These inputs require large amounts of energy to produce and not only contribute to global warming, but also run off from land and pollute waterways. (Nitrogen fertilizers are also geopolitically dangerous; nearly a third of them pass through the Strait of Hormuz, which Iran effectively closed after attacks by Israel and the United States.) Water also doesn’t penetrate the tilled foil, so much of it evaporates before it reaches the roots.
These trends combine to drive up costs, which will further accelerate as climate change worsens droughts. Researchers in other parts of the world may also use DAS to better understand soil conditions on local farms. “I thought we just showed how DAS can be used to solve these kinds of problems, even though it’s a very small-scale application,” said Jonathan Ajo Franklin, an applied geophysicist who studies the technology at Rice University. (Ajo-Franklin was not involved in this paper, but its lead author is a postdoctoral fellow in his department.)
The solution, Montgomery said, is to embrace regenerative agriculture to prioritize soil health, which means reducing physical and chemical disturbances. For example, if a farmer is suffering from weeds, he can get a head start by letting livestock out after harvest and clearing the field for the next crop. Or you might add cover crops to further suffocate weeds. For example, even though urban farms require some tillage, composting can put carbon back into the ground.
Increasing crop diversity will also replenish the soil (legumes, for example, “fix” their own nitrogen and add it to the soil for other plants to use as fertilizer) and make fields more friendly to beneficial microorganisms that lock up carbon in the soil and keep it out of the atmosphere. “There are other reasons why we want to adopt the same set of practices, including reducing dependence on pesticide inputs, increasing on-farm biodiversity, reducing off-farm pollution, building soil organic matter, and creating more profitable farms,” Montgomery said.
