Drought, heat can make soil absorb water slower

A viral video claims that soils affected by drought and heat absorb water much more slowly than wet soils. VERIFY explains the science behind why this can happen.

A video allegedly showing the ground absorbing rainwater by a University of Reading meteorologist went viral on multiple social media platforms in early August. In the video, the meteorologist placed a cup of water on wet grass, grass with normal summer rainfall, and dry grass fresh from a drought and heatwave.

The cup emptied fastest in wet grass, which should show that moist soil absorbs rainwater best. While this cup emptied within 15 seconds, it took almost a minute for the cup to empty on grass with normal summer rainfall. The cup on the dry earth was still almost full when the video ended after a full minute.

The video has been viewed 4.1 million times on Reddit. 3.8 million times on Twitter and 86,000 times on YouTube. The point of the demonstration, the video’s social media captions explained, was to illustrate why heavy rains after a drought can be dangerous and lead to flash floods.

But this conclusion contradicts what most people might expect. When there’s a drought and the soil, grass and plants are dry, what could be a better solution than dumping lots of water on it? Many people doubt the demonstration or asked how the science behind it would even work.

THE QUESTION

Can drought make it harder for the soil to absorb water?

THE SOURCES

THE ANSWER

This is true.

Yes, drought can make it harder for the soil to absorb water.

WHAT WE FOUND

A drought can result in the soil not being able to absorb water even though the soil is parched and in need of moisture. This, in turn, can make a drought-stricken location more vulnerable to flash floods when it finally rains. But soil doesn’t always behave this way during droughts, as its specific properties can change exactly how it responds to drought, heat and rain.

Droughts can make soil less absorbent because the top layer of soil usually has something called organic matter that can become waxy and water-resistant or “hydrophobic” when dried out or exposed to extreme heat.

Soil organic matter is formed when material from living things like plants, animals and fungi breaks down, say the National Geographic Society and the Noble Research Institute, an agricultural research organization. It’s the dark brown or black layer of soil you’d likely find just below a grass-covered lawn.

“Lawns and pastures usually look like this, they tend to develop an organic layer on the surface,” said Ray Weil, a soil science expert at the University of Maryland. “That can become hydrophobic very easily when it’s dry.”

Normally, microbes in the soil break down the waxy materials present in some organic matter, says Yates, a horticulture company based in Australia, where droughts often render soil hydrophobic. But these microbes work best when the soil is sufficiently moist and cool. So in a drought or right after a forest fire, these microbes often slow down. This allows the waxy material to build up.

When this happens, Weil says, a thin, waterproof film begins to form over the soil particles, especially when the drought is accompanied by hot temperatures. When a drop of water lands on this foil, it bounces up into small round beads and runs off in the direction that gravity is taking it, instead of sinking into the ground. Weil compared it to what happens to rainwater when it falls on a waxed car.

“That means that when it rains heavily, everything will run off the hill instead of seeping in, and that can cause severe flooding,” Weil said, also noting mudslides as a possible result.

But ironically, organic matter that is present under normal conditions actually helps the soil absorb water. The Noble Research Institute says that organic matter that hasn’t dried out can absorb up to 90% of its weight in water, acting as a kind of sponge for the soil. This means that in some cases, organic matter can actually be part of the solution to avoiding flooding.

“A long-term way to improve your soil is to add well-rotted organic matter and then mulch over it to keep the soil from drying out,” says Mr. Fothergill’s Seeds, an Australian seed company. “This introduces microorganisms into your soil that break down the waxy residue and also improve your soil biology.”

Another long-term solution is to simply water the hydrophobic soil—but slowly and only a little at a time.

“Then if you wait a while for some water to soak in and add a little more, it starts seeping in,” Weil suggested. “So adding a little bit at a time will overcome that [water-repellency] as soon as you wet it again. It dissolves these hydrophobic components and becomes hydrophilic again, it absorbs.”

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 are not a long-term solution to hydrophobic dirt.

The United States Department of Agriculture (USDA) has recommended raking or hacking the first few inches of dry forest floor after a fire to break up the waterproofing layer on top of the work.

And while the solutions for farms are slightly different, the concepts behind these fixes are generally the same: protect the soil from drying out, moisten the soil slowly when it does, introduce water paths into the soil, enter soil moist organic matter and try to decompose the waxy layers of soil.

Soil is not the same everywhere, which is one of the reasons why solutions differ between your home garden, a forest and a farm. Soils with coarser textures are more likely to become hydrophobic, says the Western Australian government. And both Weil and the Noble Research Institute said prairies and pastures contain far more organic matter than forests, meaning they’re more prone to becoming water repellent after a drought.

Weil found that even healthy soil is unlikely to be able to absorb rain that falls hard enough, because once the soil has absorbed as much water as possible, the water has nowhere to go. He said people should take good care of the land as it would reduce the chance of flooding, but there is nothing that can completely eliminate the risk of extreme weather.

More from VERIFY: Yes, air conditioners can help filter wildfire smoke out of your home

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Alley Einstein

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