Crushed Stone Helps Farmers Capture Carbon

Dan Prevost grows corn, soybeans and cotton on about a thousand acres of farmland near the central Mississippi town of Raymond. Because the soil tends to get too acidic with the Deep South’s heavy rainfall, he does what many farmers do and buys crushed stone to control pH levels in the fields. So, when Prevost got an offer to get crushed rock for free, he liked the idea right away.

“That’s significant savings for me,” he told Newsweek.

It’s not just any rock. Prevost worked with a company called Eion to spread the volcanic rock olivine on his field. When rain falls, olivine and some other silicate volcanic rocks undergo a chemical process that absorbs carbon dioxide from the air as the rock wears down. Suddenly, Prevost’s standard farming practice of spreading rock dust became a potential climate solution.

“If we can do anything that makes sense financially and increases profitability on the farm while having environmental benefits, then I’m for it, 100 percent,” Prevost said.

The technique is called enhanced rock weathering, and studies show that if enough farmers use the right kinds of rock in the right ways, they could help fight climate change by sequestering massive amounts of planet-warming CO₂—all while improving their soil.

A study in the August issue of the American Geophysical Union’s monthly journal Earth’s Future found that rock dust applied to all global croplands could sequester more than 200 billion tons of CO₂ by the end of the century.

Climate scientists say that in addition to cutting greenhouse gas emissions, we will need that sort of large-scale removal of atmospheric CO₂ to keep warming below dangerous levels. While some high-tech (and high-price) approaches to atmospheric carbon removal are grabbing attention, Prevost’s plot shows that part of the solution could be as simple and cheap as crushed rocks.

Speeding Up Nature

Simon Manley is head of carbon for UNDO, a London-based company that works with farmers in the U.K. and U.S. on enhanced rock weathering. Manley explains the process as simply speeding along the natural carbon cycle in the erosion process by finely crushing silicate rock and exposing it to more rain, soil and air.

“That brings that carbon-cycle time frame down from tens of thousands of years to tens of years,” Manley told Newsweek. “It has the potential to have a meaningful impact for the task of removing what we need to from the atmosphere.”

UNDO uses basalt in its application and has found that five tons of the crushed volcanic rock can draw down a ton of CO₂. Manley said basalt and other similar rocks are abundant and the low-tech nature of the technique means there are few barriers to implementation.

“You pick it up with equipment which everybody’s used to, you know, diggers and loaders, you put it onto a truck, and you take it down the road to an agricultural site,” he said.

There is, however, a technological challenge when it comes to precisely measuring and thus monetizing the carbon removal results. You can’t manage what you don’t measure, as the old saw goes. So how can you measure the exact amount of CO₂ being removed by crushed rock?

“The crux of it is trust,” Eion co-founder and CTO Elliot Chang told Newsweek. “We ultimately need to build the trust that we’re actually quantifying that a ton means a ton, right?”

Eion has patented its process for tracking certain “fingerprint” trace elements left by the rock weathering. Chang said these are reliable indicators of the chemical process and provide a means of measuring the rate of CO₂ absorption.

“That kind of illuminates what’s happening in the soil in a way that hasn’t been done,” Chang said.

That sort of evidence of carbon removal allows companies to participate in carbon markets, promising an economic boost that can more quickly scale things up.
This month, Eion announced that the company had met the technical criteria for the carbon credits managed by the financial services company Stripe.

There is still more study needed to optimize enhanced rock weathering. The same study that projected a 200 gigaton potential for carbon removal also cautioned that not all stone applications are equal. Enhanced rock weathering seems to work best in hot, humid conditions and with finely ground materials and less well in cold, dry places.

Zero-Carbon Crops

Scaling up enhanced rock weathering will also require getting more farmers on board, and that could mean taking a more locally informed approach to climate communication. Prevost said it’s not easy to talk about global warming with his fellow farmers in Mississippi.

“That will definitely get an eye roll,” he said. However, he suggested, a deeper conversation about things like weather extremes, changing rainfall patterns and soil health will get a farmer’s interest.

Prevost said some farmers also feel a bit put upon when they’re blamed for environmental impacts such as nutrient runoff, erosion and emissions.

“I mean, agricultural activities, yes, they do contribute greenhouse gases, but at the same time, people have to eat,” he said. Prevost said proposed environmental fixes that involve additional cost or big changes to existing practices leave farmers “feeling a little jaded.”

What he likes about enhanced rock weathering is that it’s simple, cost-effective and helps farmers as they help the environment. He also uses other environmentally friendly practices such as cover crops and reduced tillage to reduce erosion and emissions.

“I’m very near a zero-carbon crop, if not below it,” Prevost said, “and that’s something I’m proud of, personally.”

But if enhanced rock weathering proves that it can deliver CO₂ reductions at a large scale, Prevost and other farmers could be growing a climate solution along with their crops.