For thousands of years, farmers have prepared their fields for planting by overturning the soil. It helps them the manage the moisture, nutrients and flow of air underfoot. But modern farming’s deep plowing and heavy machinery often do more harm than good, researchers report.
All that soil turnover disrupts the natural paths that water takes to percolate into the root zone. And this is not a problem just for growing crops. Breaking down the network of pores that water wants to move through also makes the soil less resilient to flooding and drought. So concludes a team of scientists in the April 16 Science.
A fiber-optic cable, here laid along the relatively bare soil, was able to pick up tiny seismic signals. They’d been triggered by the movement of rainwater in the soil below.Marine Denolle
Qibin Shi is a geophysicist with the Chinese Academy of Sciences in Beijing. He was part of a team that fashioned a dense array of sensors out of fiber-optic cables. Such cables find use in more than just high-speed internet. They also can become powerful tools for seismology — and not just for earthquakes. They’re proving sensitive enough to detect even the tiny vibrations triggered as water moves through soil.
Shi’s group installed these cables along the edges of 27 plots of land at a university test farm in Newport, England. The plots were treated one of three ways. Nine went unplowed. Another nine were tilled to a depth of 10 centimeters (4 inches). The last nine were turned over to a depth of 25 centimeters (almost 10 inches). Within each group, plowing machinery of different weights were used. Why? The heavier they are, they more they can compact the soil.
For three days in March 2023, the team used its seismic-sensing system to track how the plots responded to rain. Rainwater didn’t seep down much on plots with the most compacted and deeply plowed soil. Instead, it tended to pool near the surface. Here, it could evaporate swiftly once the sun came out.
Water moved down and around more easily in the plots that were less compacted and less deeply plowed.
To understand what was happening, Shi’s team created a computer program that modeled the water’s movements. Rain, they found, moves through porous soil in a way that sort of mimics what happens to blood in capillaries — the body’s tiniest blood vessels. This process is known as capillary action.
Scientists used fiber-optic cables to study how turning over the soil in spring will affect rainwater’s ability to infiltrate the soil at this research site in Newport, England.Marine Denolle/University of Washington
Like our capillaries, the tiny, tunnel-like airways between specks of soil are very thin. Gravity does not drive water’s flow through these holes. Instead, as water tries to alternately adhere to soil particles and bind to other water molecules, it gets caught in a push-pull action. And that moves it. When those narrow pathways get disturbed or crushed, however, the suction forces strengthen, impairing the water’s flow.
Fiber-optic sensing could offer farmers a quick and inexpensive way to monitor soil moisture on a large scale, Shi’s team says. Such monitoring also could provide real-time warnings of natural hazards, such as flooding and soil liquefaction during an earthquake (when saturated ground can suddenly turn unstable due to shaking).
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