Layer of Idaho dirt you can’t penetrate with a shovel? Here’s how water gets through

·5 min read

In much of Southern Idaho, a cement-like layer of soil lies just below the surface. Though people have trouble getting through it, water somehow finds a way.

By swinging sledgehammers and burying electrodes, Boise State University scientists discovered that water flows through on paths made by sagebrush roots — paths that can prevent flooding and help mitigate wildfires by allowing sagebrush to survive.

Though they long suspected these soil openings, researchers recently published their discovery in the soil-science journal Catena. Their work maps the area’s hard soil layer, composed of calcium carbonate.

“Anyone who has tried to dig a fence post, I guarantee you, in Southern Idaho has encountered this (layer) and said, ‘Gosh darn it,’ because it’s hard to dig through,” said Jen Pierce, a geoscience professor at Boise State and an author of the study, in a phone interview.

Calcium carbonate in dry soil is the same whitish stuff that appears in coffee pots or bathtub rings.

When “hard water,” which is just water with calcium or magnesium in it, evaporates, it leaves behind minerals. Rain water picks up calcium on its way into or through the ground and leaves behind the calcium carbonate layer, also called caliche, when it evaporates. In areas with less than about 20 inches of rain per year, the soil is fairly dry, and calcium carbonate can form, Pierce said.

If the soil is too moist, the minerals wash out with the water. Just as only the water’s edge gets a chalky layer in the bathtub.

Electrodes and sledgehammers

When researchers noticed that some, but not all, of their underground soil moisture probes at a monitored site in Southwest Idaho didn’t change much after a rainstorm, they suspected something unusual was happening.

So they chose a low-elevation area of about 600 square feet and buried electrodes in the ground. When they ran a current through some of these probes, they could see the electrical response of the others.

Based on this signal, researchers mapped what’s called the soil resistivity. This value is “very sensitive” to the amount of water in the soil, Sebastian Uhlemann, a geophysicist at Lawrence Berkeley National Lab not involved in the study, told the Idaho Statesman.

“It’s a little bit like medical imaging, just on the ground,” he added.

This method is also used to monitor areas for landslides. When soil on hills or mountains becomes too moist, the risk of a landslide increases, Uhlemann said. By monitoring the soil resistivity, an area may have advance warning of a disaster.

After a rainstorm, researchers saw the soil resistivity change only in certain channels, which showed them the corridors water took through the hard calcium carbonate layer.

To further investigate the soil structure, scientists also placed “little boxes on spikes” in a line, said Travis Nielson, another paper author, by phone. Researchers took turns swinging an 8-pound sledgehammer and letting the boxes vibrate up and down in response, he said.

Though this part of the project was exhausting, “you get used to it,” Nielson said. “It’s like CrossFit, practically.”

Originally, researchers planned to simultaneously map soil structure at a higher elevation. After rodents chewed through their cables, though, they focused just on the caliche-relevant site, Nielson said.

Capturing carbon in the soil

Understanding soil structure helps scientists figure out how much carbon is trapped in the soil. Since carbon dioxide in the air causes climate change, “we want more carbon in our soils and less carbon in our atmosphere,” Pierce said.

When calcium carbonate forms, it takes carbon dioxide from water that otherwise might end up in the air. This doesn’t happen only in barren, dry soil. Farmers also trap carbon in irrigated systems, Pierce said.

Since soil can’t suck up all the carbon dioxide in the air, as Idaho experiences more droughts, pathways through the hard calcium carbonate layer might help sagebrush survive, Uhlemann said.

Grasses such as cheatgrass compete with sagebrush for water in the top layer of soil. “(Cheatgrass) is happy to take (the upper-soil water), green up, dry out and burn over and over again,” Pierce said.

If sagebrush survives or helps channel water to a depth grasses can’t reach, wildfires may lose some of their fuel. Since cheatgrass thrives after a burning, fires breed more fires in an unfortunate cycle.

When it does rain, pathways through the rocky deposit can prevent flooding by providing water a route through the soil, Uhlemann said.

Soil flooding is a phenomenon that Matt Germino, a research ecologist at the U.S. Geological Survey, has seen in other areas where there aren’t pathways through the hard layer. He cautioned that there are some regions where the calcium carbonate layer is too tough for even plant roots to penetrate. So this research is not applicable to all sagebrush land.

Still, evidence of soil channels at the site mapped by researchers is “exciting,” Germino said. Especially because it shows how sagebrush can access deeper water in some important locations. The sagebrush may need the soil layer to beat grasses to water, but the soil also needs sagebrush to prevent flooding.

“We always supposed that these root pathways were important, but (the study) proved that,” Pierce said.

Sophia Charan writes for the Idaho Statesman on a fellowship through the American Association for the Advancement of Science. She has a Ph.D. in chemical engineering from the California Institute of Technology, where she focused on atmospheric chemistry, and a bachelor’s degree from Yale University.

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