One of the most significant carbon sinks on the planet is right below your feet. Soil, that layer of organic material and crushed-up rock that covers much of the terrestrial earth like a chocolate coating, contains about 2,500 billion metric tons of carbon. It’s the second-biggest carbon sink on the planet after the ocean, currently holding about three times as much carbon as the atmosphere. Some scientists and activists think it could do even more.
And increasingly, companies and governments agree. From Ben & Jerry’s to Unilever, companies are calling for more environmentally friendly farming practices as a way to meet net-zero goals. Meanwhile, the U.S. Department of Agriculture last year announced it would be investing $10 million to better monitor and measure soil’s carbon sequestration under its Conservation Reserve Program.
“Soil is the foundation of human civilization,” says Jeff Creque, director of rangeland and agroecosystem management at the Carbon Cycle Institute, an environmental organization based in California working to boost the carbon-sequestering power of soil and other natural carbon sinks. “We don’t have agriculture without fertile soils, and we don’t have fertile soils without carbon rich-soils.”
The Role of Soil
Carbon in soil takes two forms: organic (derived from living things) and inorganic. Inorganic carbon comes from carbon-containing rocks like limestone, marble, and chalk, which are most common in desert soils, as well as reactions between atmospheric carbon dioxide and minerals in the soil. But the majority of the carbon contained in soil is organic, and it’s this organic material that sets it apart from lifeless dirt.
Plants are the main source of organic carbon in soil and the main bridge carbon takes between the atmosphere and the earth. They absorb carbon from the atmosphere through photosynthesis, the process by which plants convert carbon dioxide into the carbohydrates they use for energy and to build their bodies. When plants die or shed leaves, petals, or other debris, the decomposers that live in the soil below consume them; they also eat the carbon-containing mucilage (a thick, gluey secretion) that roots exude while they’re alive.
The decomposers will re-release some of the carbon back into the atmosphere as they respirate; this carbon spends only a short amount of time in the soil. But several mechanisms can draw the carbon deeper into the soil, where it can be sequestered for years, decades, or longer. Rain, for example, can dissolve some carbon compounds and carry them deep into the groundwater. Mycorrhizal fungi, which form a symbiotic relationship with plants, carry carbon along their deep, rootlike hyphae and secrete compounds that help glue it in place.
And some carbon compounds can bind with the minerals in clay, a form of carbon sequestration that can last hundreds or even thousands of years. This chemically bonded carbon is part of soil’s stable carbon pool, together with carbon that has traveled deep enough in the soil (about 1 meter) to avoid being consumed and respirated into the atmosphere. (Carbon can also be sequestered long-term in frozen soil, as in the permafrost.)
How Farming Impacts Soil Health
All of these mechanisms are most effective in healthy, minimally disturbed soil with plenty of organic material from a thriving community of living things. Unfortunately, there’s less and less of this kind of soil left on the planet. Some of the most significant remaining swaths of soil are controlled by agriculture, which covers about 38% of the global land surface. But standard agricultural practices like tilling disrupt the downward path of carbon, exposing once-sequestered organic compounds to the air and allowing carbon to escape into the atmosphere.
That’s where regenerative agriculture, sometimes called carbon farming, comes in. This approach to agriculture focuses on restoring and maintaining soil health through a holistic set of practices, including reducing tilling, composting farm waste, and planting plots with cover crops such as clover so they continue receiving carbon when they aren’t being used for other things. In addition to absorbing more carbon, proponents say this approach can help recharge groundwater, prevent pests, and increase crop yields.
Regenerative agriculture is based on practices far older than modern industrial farming, championed in recent years by activists like Robert Rodale of the Rodale Institute and Allan Savory of The Savory Institute. Their initial acolytes tended to be small, experimental farmers and organic producers. But in the past decade, several multinational corporations have announced goals to adopt regenerative agriculture practices, including Unilever, PepsiCo, and General Mills. These commitments help corporations toward their net-zero goals, in addition to protecting their supply chains against the effects of global warming, drought, and desertification.
Using Soil as the Solution
Some corporate advocates of regenerative farming, including Ben & Jerry’s and Timberland, have formed a coalition with farmers to lobby Congress to include funds to support regenerative agriculture in the 2023 Farm Bill. This coalition, Regenerate America, argues in its policy recommendations that regenerating the soil can impact not only the climate but also rural economies, communities, and health outcomes.
Some farmers and scientists are experimenting with soil additives, called amendments, to further boost soil’s carbon-sequestering potential in conjunction with regenerative agriculture. One of the most promising amendments is rock dust. While most of the more familiar soil amendments, like compost and manure, boost the organic pathways for carbon to enter the soil, rock dust also jumpstarts the inorganic pathways.
The soil amendment currently garnering the most buzz may be rock dust, though it’s far from a new technology. “Rock dust has been applied to lands at a large scale for many years because farmers knew that ground-up rock holds important mineral nutrients for plants,” says Whendee Silver, a professor of ecosystem ecology and biogeochemistry at University of California, Berkeley. It’s been used in Europe since at least the late 19th century, when the German doctor Julius Hensel published the book “Bread From Stones” advocating for what he called “stonemeal manure” made from igneous rocks, which form through the cooling and solidification of magma or lava.
Today, researchers are experimenting primarily with crushed basalt, an igneous rock rich in minerals including iron, magnesium, and calcium—similar in composition to the rock found in the famously fertile soils that surround volcanoes. Basalt is one of the most common rocks in the upper layers of the earth’s crust, and mining operations bring up huge amounts of it as they search for more profitable things underneath. “Putting that material out onto soils is a win-win as long as the material is safe,” Silver says—that is, not contaminated with heavy metals or other toxic substances.
In the presence of water, the magnesium and calcium in the basalt react with the carbon in the atmosphere and soil to form bicarbonates, which can remain dissolved in the groundwater or eventually precipitate out as a solid. This makes the carbon unavailable for decomposers, so it won’t be respirated back into the atmosphere. Basalt also contains minerals like potassium and phosphorus that are essential for plants, which can help increase crop yields—and healthy plants absorb more carbon.
Another soil amending technology is biochar, a black substance made by applying heat to plant matter in a low-oxygen environment. Creating biochar releases less carbon dioxide than burning plants or allowing them to decay, two of the usual routes to get rid of the inedible parts of crops, grass, or trees that farmers clear to plant new fields.
About 50% of the carbon in the plants remains trapped in the biochar, which can then be added to soil to boost water retention and fertility. This method has been promoted as a more technologically feasible and localized alternative to carbon capture and sequestration technology; consumers can already buy cookstoves to make their own biochar at home.
“At this point in our history, we’re looking at every possible strategy,” says Creque. “The beauty of terrestrial sequestration … is that we see this enormous raft of co-benefits that emerges with those strategies.”
—With reporting by Jennifer Junghans
This article is part of a series on key topics in the climate crisis for time.com and CO2.com, a division of TIME that helps companies reduce their impact on the planet. For more information, go to co2.com
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