The Dirt on Dirt

The world’s soil helps us grow food and store carbon. So why are we letting it waste away?

May 13, 2015

Photo: Ron Nichols/ USDA, NRCS North Carolina

When I was eight years old, I saw a public-service announcement about the importance of soil. Rowlf, the piano-playing dog of Muppets fame, sang a lovely tribute to soil. “Be it ever so humble, there's no dirt like soil,” he warbled earnestly. Rowlf had interrupted my afternoon cartoons to warn me about soil loss, and I assumed it was the greatest threat facing mankind.

That was probably the last warning I heard about soil issues, but it stuck with me. For more than 30 years, I’ve wondered whatever happened to this problem. Did we solve it?

No, we did not. A commentary in Thursday’s issue of the journal Science explains how our soil management is abysmal, and the nutrient balance of soil worldwide is badly out of balance. And when people can no longer grow food, the authors warn, geopolitical instability can result. Rowlf would be very disappointed.

What Is Soil, Anyway?

It seems like a dumb question, but most people use what Supreme Court Justice Potter Stewart might have called the pornography approach to describing soil: They can’t define it, but they know it when they see it.

Here’s a much more specific definition, courtesy of soil scientist Ronald Admundson and his coauthors on the Science article. “Soil is commonly thought of as the ~1-m-thick [approximately one meter] layer of biogeochemically altered rock or sediment at the earth’s surface…Soil-forming chemical reactions create micrometer-sized electrically negative clay minerals that impart soil with plant-nutrient retention capabilities.”

The ability to attract and retain nutrients is what makes soil special. This is also part of the reason why our stewardship of fertile lands is so problematic. Approximately 38 percent of the world’s land is dedicated to agriculture—which is essentially a systematic depletion of nutrients from the soil. Monocultures, in particular, draw large stores of nutrients from the soil as they provide us with staple crops like corn and wheat.

The Phosphorous Cartel

When humans were just a ragged band of a few million people spread across the vastness of the planet, farming presented little problem to global soil health. The chemical and biological processes that turn surface-level rocks and sediments into productive soil could easily keep up with our growing habits. As with so many other earth systems, however, we eventually overwhelmed the planet’s ability to recover from our activities, forcing us to employ industrial techniques to replenish the soil with nutrients.

We’re pretty good at manufacturing nitrogen, a nutrient that helps plants build tissues and perform photosynthesis. Beginning in the early 1900s, the industrial process allowed humans to squeeze more food from an acre of land than previously thought possible. But we still have to mine other nutrients, like phosphorous, to replace what our crops take from the soil. Mining phosphorous is not only energy-intensive, it presents the potential for international strife. Morocco and China have the world’s largest stores of phosphorous, while the United States is running dangerously low. Since natural processes can’t even come close to replacing the phosphorous we extract from U.S. soil, American farmers may soon have to pay extremely high prices to import it. Amundson sounds the warning about future “phosphorous cartels.”

Photo: USDA NRCS South DakotaSoil erosion during a wind storm in South Dakota in October 2012

Soil Erosion

Perhaps more problematic than the loss of soil nutrients is the loss of the soil itself. Wind and rain can sweep loose soil into the air or waterways. Research shows that before Europeans arrived in North America, the rate of soil erosion was approximately 21 meters per million years. The removal of native plants, which help to hold dirt in place, accelerated that process. Today, the continent loses 2,000 meters per million years. In some parts of the world, such as upland China, the rate is as high as 10,000 meters per million years—nearly 500 times the background rate of soil erosion.

When soil erodes, natural processes can convert the underlying rock and sediment into new soil—that is, after all, how the original soil got there—but the process is slow. Most researchers think the replacement rate is at most 200 meters per million years, and many think it’s as low as 50. In the parts of the world with the fastest human-induced erosion rates, it might take 200 years to replace the soil lost in one year.

Climate Change

I know. You thought, for once, you were going to read an environmental science article that doesn’t mention climate change. No way. (We environmental journalists have all taken a secret blood pact to always mention it.)

Don’t worry, I’ll make it worth your time with this single datum: There are 2,300 gigatons of carbon stored in the top three meters of the earth’s soil. That’s more than the total amount in the atmosphere plus all of the world’s plants.

This is a dangerous situation. Disrupting the soil—either through farming or developing a piece of ground—releases carbon into the atmosphere. Farmers are already responsible for releasing around 60 gigatons of that carbon by disturbing the soil. If we don’t reform our agricultural practices, we could make things a lot worse.

But there’s also a bright side.

“This is a big opportunity,” says Claire O’Connor, a staff attorney in NRDC’s food and agriculture program (disclosure). “Because the soil carbon levels have been depleted, they are ready to be rejuvenated. We can mitigate climate change by regenerating soil and using it as a carbon sink.”

O’Connor recommends that farmers plant cover crops when giving the land a break from their primary crop. This offers two benefits. First, plants suck carbon from the atmosphere. When they decompose, that carbon returns to the soil. Second, cover crops help take nitrogen from the atmosphere and convert it into a form used by the plants and released back into the soil after they die. This lessens the need for synthetic fertilizers, which are often made from large amounts of fossil fuels. Farmers can also help replenish their land by forsaking tilling (which can exacerbate erosion), diversifying crops, and bringing nutrient-recycling livestock back onto farms that have long focused on monocultures.

O’Connor calls these techniques the “next generation” of land management strategies, though they bear a strong resemblance to the last generation.

“We’re finding that going back to our roots and taking a more holistic view leads to a more balanced and successful system,” she says.

That’s worthy of a Rowlf encore.  


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