A video by a University of Reading meteorologist that claims to show how soil absorbs rainwater went viral on multiple social media platforms in early August. In the video, the meteorologist put a cup full of water on wet grass, grass with a normal summer’s precipitation and dry grass fresh off of a drought and heat wave.
The cup emptied the fastest in the wet grass, which was meant to show that moist soil absorbs rainwater best. While that cup emptied within 15 seconds, it took nearly a minute for the cup to empty on grass that had a normal summer’s precipitation. The cup on top of the dry soil was still nearly full when the video ended after a full minute.
The video was viewed 4.1 million times on Reddit, 3.8 million times on Twitter and 86,000 times on YouTube. The point of the demonstration, explained the video’s social media captions, was to illustrate why heavy rainfall after a drought can be dangerous and might lead to flash floods.
But that conclusion runs counter to what most people might expect. If there’s a drought, and the soil, grass and plants are dry, then what could possibly be a better fix than dumping a lot of water on it? Many people cast doubt on the demonstration, or asked how the science behind it would even work.
Can drought make it harder for soil to absorb water?
- National Geographic Society
- Noble Research Institute, an agricultural research organization
- Yates, an Australian gardening company
- Government of Western Australia Department of Primary Industries and Regional Development's Agriculture and Food division
- Mr. Fothergill's Seeds, an Australian seed company
- United States Department of Agriculture (USDA)
- Ray Weil, Ph.D, a soil science expert at the University of Maryland
Yes, drought can make it harder for soil to absorb water.
WHAT WE FOUND
A drought can leave soil unable to absorb water, even though the soil is parched and in need of moisture. This, in turn, can make a drought-stricken place more prone to flash flooding when it finally rains. But soil won’t always behave this way in droughts, as its specific characteristics can change exactly how it will respond to dryness, heat and rainfall.
Droughts can make soil less absorbent because there’s usually something called organic matter at the top-most layer of the soil, which can become waxy and water-repellant, or “hydrophobic,” when dried out or when exposed to extreme heat.
Organic matter within soil is created when material from living things like plants, animals and fungi decompose, the National Geographic Society and the Noble Research Institute, an agricultural research organization, say. It’s the dark brown or black layer of soil you’d likely find directly beneath a grass-covered lawn.
“Turf grass and pastures usually look kind of like this, they tend to develop an organic layer on the surface,” said Ray Weil, a soil science expert at the University of Maryland. “This can become hydrophobic very easily when it's dry.”
Normally, microbes within the soil break down the waxy materials present in some organic matter, says Yates, a gardening company based in Australia, where droughts frequently make soil hydrophobic. But these microbes work best when soil is sufficiently moist and cool. So in a drought, or right after a wildfire, these microbes will often slow down. The waxy material can build up as a result.
When this happens, Weil said a thin, waterproof film will begin to form over the soil particles, particularly if the drought is accompanied by hot temperatures. When a drop of water lands on this film, it beads up into little round balls and runs off in whichever direction gravity will take it rather than sinking into the soil. Weil compared it to what happens to rainwater when it falls on a waxed car.
“So this means if you get a heavy rain, it all runs off the hill instead of soaking in, and that can cause serious floods,” Weil said, also mentioning mudslides as a possible outcome.
But ironically, organic matter present in normal conditions actually helps soil absorb water. The Noble Research Institute says organic matter that’s not dried out can hold up to 90% of its weight in water, acting as a kind of sponge for the soil. Which means organic matter can actually be part of the solution to preventing flooding in some cases.
“A long-term way to improve your soil is by adding well rotted organic matter, then mulching over the top to help prevent the soil from drying out,” says Mr. Fothergill's Seeds, an Australian seed company. “This will introduce microorganisms to your soil which will break down the waxy residue and also improve your soil biology.”
Another long-term fix is to simply water the hydrophobic soil — but slowly, and only a little at a time.
“If you then wait a while for some of that water to soak in and add a little more, it'll start soaking in,” Weil suggested. “So if you add a little at a time, it will overcome that [water-repellency] once you remoisten it. It'll dissolve those hydrophobic components and it'll become hydrophilic again, it'll absorb.”
Some companies sell quick, temporary fixes called wetting agents. Mr Fothergill’s Seeds says these wetting agents work by breaking the surface tension in the water so it can penetrate the soil more easily, and can even help break down the waxy coatings. Wetting agents aren’t a long-term solution to hydrophobic soil.
The United States Department of Agriculture (USDA) has recommended raking or hoeing the first few inches of dried-out forest soil after a fire to break up the water-repellent layer at the top of the toil.
And while solutions differ slightly for farms, the concepts behind those fixes are generally the same: Protect soil from drying out, remoisten the soil slowly if it does, give water paths into the soil, give the soil moist organic matter and look to break down the waxy soil coatings.
Soil isn’t the same everywhere, which is part of the reason why solutions differ between your home garden, a forest and a farm. Soils with more coarse textures are more prone to becoming hydrophobic, the Government of Western Australia says. And both Weil and the Noble Research Institute said prairies and pastures have much more organic matter than forests, which means they’re more susceptible to becoming water-repellent after a drought.
Weil noted that even healthy soil is unlikely to absorb rain that falls heavily enough, as the water will have nowhere to go once the soil has absorbed all the water it can. He said people should take good care of the land because it will reduce the likelihood of flooding, but there’s nothing that will eliminate the risk entirely in the event of extreme weather.