Plowing, or tilling, is an age-old agricultural practice that readies the soil for planting by turning over the top layer to expose fresh earth. The method - intended to improve water and nutrient circulation - remains popular today, but concerns about soil degradation have prompted some to return to regenerative methods that disturb the soil less.
In a new study, a team led by University of Washington researchers examined the impact of tilling on soil moisture and water retention using methods originally designed for monitoring earthquakes. Researchers placed fiber optic cables alongside fields at an experimental farm in the United Kingdom and recorded ground motion from plots receiving different amounts of tillage and compaction from tractor tires pulling farm equipment.
The study, published March 19 in Science, shows that tilling and compaction disrupt intricate capillary networks within the soil that give it a natural sponge-like quality.
"This study offers a clear explanation for why the process of tillage, one of humanity's oldest agricultural activities, changes the structure of soil in ways that affect how it soaks up water," said co-author David Montgomery, a UW professor of Earth and space sciences.
The link between tilling and soil degradation has been established for quite some time, but the rationale is less robust.
"It's counterintuitive," Montgomery said.
Tilling is supposed to create holes for water to reach the roots of plants, but it breaks these small channels in the soil instead, causing rain to pool on the surface and form a muddy crust. Over time, this can increase erosion and flood risk. The researchers observed this phenomenon in detail using seismological methods.
For the past decade or so, physical scientists have been exploring ways to harness the fiber optic cable network to make remote observations. They use a technique called distributed acoustic sensing, or DAS, that records ground motion based on cable strain. Because the technology is so sensitive, it can also capture the speed at which sound waves pass through a substance, which is called seismic velocity.
When soil gets wet, seismic velocity changes. Sound moves slower through mud than dry dirt.
"We wanted to find out whether seismic tools could be used to understand how soil - under different treatment regimens - would respond to environmental variability," said senior author Marine Denolle, a UW associate professor of Earth and space sciences.
An experimental farm near Newport in the United Kingdom, affiliated with Harper Adams University, turned out to be an ideal testing ground for their experiment.
The farm is split into rows that have received consistent cultivation for more than two decades.
There are no-till rows, rows tilled 10 centimeters deep and rows tilled 25 centimeters. Compaction is a byproduct of tilling caused by tractors. Different levels of compaction were tested by modulating tractor tire pressure.
"We took advantage of a natural experiment that had already been done, but just not yet measured," Montgomery said.
The researchers lined their experimental plots with a fiber optic cable. They collected continuous ground motion data for 40 hours and combined it with weather data over the same period, which featured light to moderate rainfall and mild temperatures.
"We observed the natural vibration of the ground and found that it is really sensitive to environmental factors, including precipitation," said Qibin Shi, lead author and former UW postdoctoral researcher of Earth and space sciences, now at the Chinese Academy of Sciences.
They determined how each cultivation strategy impacted the soil's response to rainfall by comparing trends in seismic velocity across study sites. Shi developed various models to process the data and help the researchers understand seismic velocity in terms of soil moisture.
The method is straightforward, inexpensive and offers far better spatial and temporal resolution than previous monitoring tools.
The researchers believe it could help farmers understand how to manage their land, provide real time flooding alerts, improve earth systems models by refining estimates of atmospheric water content and better inform seismic hazard maps with data on liquefaction risk.
Additional co-authors include Abigail Swann, a UW professor of atmospheric and climate science, Nicoleta C. Cristea, a UW research assistant professor of civil and environmental engineering, Ethan Williams from the University of California Santa Cruz, Nan You formerly at Purdue University, Simon Jeffery, Joe Collins, Ana Prada Barrio and Paula A. Misiewicz from Harper Adams University, Tarje Nissen-Meyer from the University of Exeter
This study was funded by The Pan Family Fund, the Murdock Charitable Trust, the UW College of the Environment Seed Fund, the David and Lucile Packard Foundation, and a National Environmental Research Council cross-disciplinary research capability grant.
