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Agriculture is responsible for 24 percent of our global greenhouse gas emissions. Despite this, UC Davis researchers say agriculture holds huge potential to be one of the biggest solutions to climate change. Carbon farming may hold the key. In this episode of Unfold, we examine how scientists are adding rock dust to crops to see if it can sequester carbon while also increasing yields for farmers.

In this episode:

Ben Houlton, affiliate faculty member, former director of the John Muir Institute of the Environment at UC Davis

Maya Almaraz, program manager in the Houlton Lab and terrestrial biochemist

Iris Holzer, graduate student in the Houlton Lab

Derek Azevedo, executive vice president, Bowles Farming Company in Los Banos, California

George Dias, Specialty Granules in Ione, California

Amy Quinton: A spreader is traveling across a farm field in Los Banos, in California's Central Valley. This crop land is owned by Bowles Farming company. Derek Azevedo is its vice president. 

Derek Azevedo: This land has been under the stewardship of the same family for 160 years, and our charge is trying to figure out how to maintain its productivity and maintain its health and improve on all those things for the next 160. 

Amy Quinton: Which is why they're working with UC Davis researchers to take part in one of the largest experiments of its kind. It starts with this spreader. It's applying rock dust to the soil. Climate scientist Ben Houlton, an affiliate faculty member at UC Davis, says rock dust has the potential to store carbon. And lots of it. 

Ben Houlton: There's estimates that we could get four billion tons of carbon dioxide removed from the air each year, if you put these kinds of rocks on our global crop lands, which is about 11 percent of Earth's surface. 

Amy Quinton: Agriculture has a huge carbon footprint. But done right, it could be one of the world's biggest solutions to climate change. 

Theme: Climate models all agree that temperatures are going to increase. It’s going to be hotter; it’s going to be drier; fires are going to burn more frequently. Maybe this is never going to be the way it was again. We need to come up with ways to literally pull CO2 out of the atmosphere. How are we going to work together to solve a challenge like climate change?

Amy Quinton: Coming to you from our closet studios as we shelter in place across the Sacramento region, this is Unfold, a UC Davis podcast that breaks down complicated problems and discusses solutions. I'm Amy Quinton. 

Kat Kerlin: And I'm Kat Kerlin. 

Amy Quinton: This week, we unfold rock dust.  Last season's Unfold discussed how to sustainably feed a growing population. It's a task made much more difficult by climate change. 

Kat Kerlin: Last year, the Intergovernmental Panel on Climate Change, you know, the IPCC, came out with this major report that looked at agriculture, forestry and other land changes. It found nearly a quarter of our global greenhouse gases come from these sources. 

Amy Quinton: That's right. The report said it will be difficult to prevent temperatures from rising above two degrees Celsius without fundamentally altering the way the world produces food and uses land. 

Kat Kerlin: Agricultural greenhouse gases come from different sources. Cattle belch methane, which is a potent greenhouse gas. And fertilizers that help grow our food release nitrous oxide. 

Amy Quinton: A study by UC Davis researchers found in California, agricultural soils can release up to 41 percent of all of our nitrous oxide emissions. That said, Ben Houlton, who we just heard from a moment ago, says agriculture can also be a weapon in the fight against climate change. 

Ben Houlton: I don't think there's any industry on the planet that is more positioned to create negative carbon emissions. That means there's more carbon dioxide being taken up by the systems and being emitted than agriculture. 

Amy Quinton: Kat, he says it starts with the soil. Every time a farmer turns up or tills soil to grow food, not only can soil lose critical microbes that make it healthy, but carbon dioxide is also released into the air. 

Kat Kerlin: So that begs the question, if farmers keep their soil healthy, could they also keep carbon in the ground instead of the atmosphere? 

Amy Quinton: We know it helps. Practices like no-till farming or adding things to the soil like compost can restore soil health. And it's that last practice of using so-called soil amendments that UC Davis scientists are researching. 

Kat Kerlin: OK, compost is one thing, but rock dust? In this episode of Unfold, we're going to look at adding rock dust to croplands to see just how much carbon it can capture. 

Amy Quinton: These farming practices that can improve the rate at which carbon dioxide is removed from the atmosphere is often called carbon farming. So, Kat, you might think we'd start this episode on a farm. 

Kat Kerlin: But I guess I'd be dead wrong to think that a story on carbon farming would start on a farm? 

Amy Quinton: Yep. We have to start at the source where we get the rock dust that's going to be added to the farm. 

Kat Kerlin: And where was that? 

Amy Quinton: A mine in Ione, California, which is up in the foothills of Amador County. And coincidentally, that's near Carbondale, which is appropriate for this story, right? Carbon, carbon farming, Carbondale. Get it. OK, get ready for this? Listen. 

Kat Kerlin: Should I guess what this is besides a bunch of noise? 

Amy Quinton: This is the sound of rocks on a conveyor belt at Specialty Granules. The company mines this rock out of an open pit quarry. 

Kat Kerlin: And why are they mining rock? 

Amy Quinton: Well, mainly the rock is crushed into small pieces and sold to roofing companies to make shingles and other roofing materials. The rock in this case is metabasalt. 

Kat Kerlin: Metabasalt? 

Amy Quinton: Metabasalt. It's a fun word to say. So George Dias -- he's the site manager at the mine -- he drove us up to the top of a hill where you can get a bird's eye view of all the activity at the mine. And he explained how these rocks are crushed. 

George Dias: So those are boulders that they're bringing up and gets through a primary crusher, a jaw crusher, that brings that rock down to about a nine-and-a-half-inch rock. 

Amy Quinton: That's industry-speak for the size of the rock. 

George Dias: So it's two closed loop circuits that continue to crush and screen until we get her seven-eighths-inch minus material out the back. 

Amy Quinton: It's then milled into smaller sizes to use in roofing materials. But there is a by-product of all this crushing which George drives us to see. 

George Dias: This is dust. 

Amy Quinton: He points to several towering piles of gray material. He says some of it, the coarser material, is used in horse arenas. But most of it, this fine dust, is useless, a waste product. And it ends up in a landfill. Ben Houlton says while it may be the wrong size for roofing materials... 

Ben Houlton: It's just the right size to add back to soil, to rejuvenate the soil, grow our soils and do it in a way that starts to provide major benefits for the crops that we're trying to grow. And in fact, previous studies have shown that you can increase crop yields by 50 percent, adding these rocks back to the soil. 

Kat Kerlin: Amy, 50 percent is a lot. How does that work? 

Amy Quinton: Fifty percent is, of course, the high end of what it could yield. It turns out that this is a pretty special type of rock. 

Ben Houlton: So this rock is a kind of rock that produces some of the most nutrient-rich soils on the planet. It comes from volcanoes. And then this particular kind, it's very enriched in silica and vital nutrients for plants, especially potassium, which a lot of our growers need. 

Amy Quinton: So a little bit of science here. Silica helps plants form thicker cell walls. Ben says it prevents lodging. And lodging is when the stems of plants like wheat or grain bend over near the ground. 

Kat Kerlin: So that would make the plants difficult to harvest. 

Amy Quinton: Right. So thick cell walls and nutrients, good for plants and yields. 

Kat Kerlin: Okay. So how did these rock store carbon? 

Amy Quinton: We'll unfold that complexity in a bit. In a nutshell, though, as these metabasalt rocks, which are highly pulverized, as they weather and dissolve, they consume carbon dioxide from the air. 

Ben Houlton: We're going to see how much of the CO2 that is dissolving the rocks goes into a form in the soil that sticks around for a long time. 

Amy Quinton: So researchers want to know this: 

Ben Houlton: Can you accelerate that reaction to help us drive negative carbon emissions in a way that helps grow food more efficiently, given that we're going to be a 10 billion people by 2050? And we have to find a way to simultaneously grab 10 billion tons of CO2 from the air each year to meet the needs of the climate targets for the Paris agreement. 

Amy Quinton: By the way, since 2015, scientists have used the Paris agreement as a reference point for climate change targets. 

Kat Kerlin: Ben said in the beginning of the episode that at least theoretically, we could remove four billion tons of CO2 each year by adding rock dust, right? 

Amy Quinton: If it were spread across all of our global croplands. Right now, researchers are trying this out here in California, adding rocked us to 30 acres of different crop soils in Yolo County, the San Joaquin Valley and in the Imperial Valley of California. 

Kat Kerlin: That's still a big experiment. 

Amy Quinton: I caught up with the farmers and researchers in Los Banos in the San Joaquin Valley. 

Iris Holzer: Eighteen-point-three. What's that? Nine-point-one-five? 

Amy Quinton: UC Davis researchers unreal a tape measure on a six-acre field owned by Bowles Farming Company. Organic corn or tomatoes are typically grown here. 

Amy Quinton: Iris Holzer, a Ph.D. student, is measuring out large uniform plots or blocks on the field. That will be part of the experiment. 

Iris Holzer: And in each block, half the block is getting rock, half the block is a control. 

Amy Quinton: Iris is a woman who is downright passionate about soil. She explains how adding rock dust to this field can capture carbon. 

Iris Holzer: When the rock weathers and reacts with CO2, there are two different forms that the carbon can be stored as. 

Amy Quinton: And not to get too in the weeds here, but the carbon can form either carbonate or bicarbonate. If it's carbonate, then carbon will stay in the soil a long time, thousands of years or more. Iris says that has significant upsides. Researchers are happy with what they found in the soil so far. 

Iris Holzer: And the soils are very exciting.

Ben Houlton: We found carbonate. To explore and find little carbonate nodules. There's nothing like it on the planet. It's like discovering diamonds. I mean, I think that tells us that capacity for mineral formation will be good. So that we imagine there will be good carbon sequestration capacity, when we apply the rock. 

Amy Quinton: Several piles of rock dust, about six feet tall, line the farm field. A loader begins to shovel the piles into a spreader. As the spreader travels down the field, it spits out a light greenish gray coating of rock dust. Maya Almaraz, a biogeochemist and Ben's lab, has been working on this project for two years. 

Maya Almaraz: It's very exciting to finally see it go out. There's been a lot of planning and scrambling to get it done and, you know, in a somewhat short amount of time. And so it's really cool also to see it at such a large scale. So usually when we set up plots, they'll be maybe like 20 by 30 meters. And this is, you know, a whole acre being spread with a certain addition. 

Amy Quinton: Scientists are applying 16 tons of rock dust per acre. First, it will be applied to this organic cornfield. Ben Houlton says they'll then apply it to the soil of conventional alfalfa. 

Ben Houlton: We're kind of trying to figure out when you go from organic to conventional, are there distinct differences? And then when you go across globally important crops, crops that are important to California, are there any differences as well? Ultimately, we're hoping that these technologies can scale outside the state. So working on corn and alfalfa makes a lot of sense, if you want to create demonstrations that have meaningful impact. 

Amy Quinton: Alfalfa also has some of the highest nitrous oxide emissions in the world. This work is part of a much larger multi-campus collaborative effort researching carbon sequestration using other soil amendments, including biochar and compost. Maya Almaraz says these large-scale demonstration projects hold huge potential. 

Maya Almaraz: The only way to make these climate solutions be highly adaptable is to create these win-win situations where they have to be economical, they have to increase yields, they have to be fairly easy for farmers to do, and they have to have good science behind them that is verified. 

Amy Quinton: Those are some of the reasons Bowles Farming Company is taking part in this experiment. Vice President Derek Azevedo says he wants to improve this soil to help his crops, but it can be hard to know what works the best. 

Derek Azevedo: One of the challenges with soil amendments and soil health is that 90 percent of the people out there marketing and providing the information as it relates to soil health are people that are trying to sell you something. And so really sorting through here's something that's valuable versus here's somebody who's just trying to sell me something, another charlatan, is difficult. 

Amy Quinton: Derek says this demonstration comes with little risk. The rock dust was donated. The farm, like a lot of farms, already has a spreader, so they didn't need any special equipment. And then there's the potential to increase yields on an organic crop. 

Derek Azevedo: And if we have the opportunity to help sequester more carbon at the same time as improving our soil health and potential crop yields, I mean, those are all very, very useful and very powerful metrics that can create something that can sustain itself and build off itself. I mean, that's what we're here for. I mean, agriculture is a large industry and harnessed in the right way is one of the few industries that is big enough to make a difference. 

Amy Quinton: Researchers are measuring both carbon dioxide and greenhouse gases like nitrous oxide coming off the land. They'll be looking at how much water is stored in the soil after adding rock dust, and they'll be looking at what it could all mean for farmers' bottom line. 

Kat Kerlin: Amy, if we did this all across California croplands, what effect could it have on our carbon footprint? I asked Ben Houlton that very question. 

Ben Houlton: We did calculate that if you took some of these kinds of rocks and put them at a moderate level on California's croplands, we could get anywhere between 30 to 80 million metric tons of carbon dioxide capture each year. Right now, the state's emissions are somewhere around 440 million metric tons. That is a very big number that we could get and ultimately, I think that is where the potential lies. 

Kat Kerlin: So, Amy, would this be easy for farmers to adopt, especially worldwide? Is there even enough rock dust? 

Amy Quinton: Ben says there is enough rock dust in California. Some studies have shown that there is enough worldwide. But those same studies have also shown that it could get expensive and may not be worth it for the farmers. 

Kat Kerlin: So, do we have results? 

Amy Quinton: We have preliminary field results from here at UC Davis that showed a 30 percent increase in yields of organic corn by applying this type of rock dust to the soil. They saw a 12 percent increase in yields of conventionally grown corn. Scientists are still measuring greenhouse gases and yields on this large demonstration project and they hope to have answers soon. 

Kat Kerlin: And we'll let people know as soon as we find out. You can also visit our unfold website to find out more at 

Amy Quinton: I'm Amy Quinton. 

Kat Kerlin: I'm Kat Kerlin. 

Amy Quinton: Thanks for listening. 

Credits: Unfold as a production of UC Davis. It's produced by Cody Drabble. Original music for Unfold comes from UC Davis alumnus Damian Verrett and Curtis Jerome Haynes.