Spreading rock dust on farms, a technique known as “enhanced rock weathering,” is one of the lesser-known carbon removal strategies. According to some estimates, the extremely simple technique could potentially lock up as much CO2 as planting a trillion trees. It might also boost crop yields, according to recent research.
Enhanced rock weathering is based on a naturally occurring process that already occurs on Earth. Raindrops naturally absorb carbon dioxide in the atmosphere when they fall, forming a diluted solution of carbonic acid (H2CO3). When that rain falls on igneous rocks such as basalt and peridotite, which are rich in silicate minerals, the carbonic acid dissolves the minerals into water-soluble ions. The “captured” carbon eventually flows into waterways, ultimately ending up at the bottom of the ocean where it is essentially sequestered forever. However, this is a very slow process that takes millions of years to affect the global carbon cycle.
Scientists for a couple of decades have been playing with the idea of somehow supercharging the process by using finely ground rock that has a high content of silicate minerals. One of the original suggestions in the early 2000s was to spread olivine – a green silicate mineral found in common igneous rocks – in forests, especially tropical areas, to neutralize soil acidity from acid rain while simultaneously drawing down carbon dioxide. However, olivine is typically high in toxins so the idea never really went anywhere.
In 2014, David Beerling, a biogeochemist at the University of Sheffield, proposed using basalt rather than olivine. Basalt is rich in pyroxene, a mineral that, like olivine, weathers quickly but does not contain toxic heavy metals. It’s also an abundant waste product of mining that has been piling up worldwide for decades. Beerling and his team released a study in 2016 that showed applying ground basalt over less than a third of all tropical land could dramatically reduce atmospheric CO2 and help neutralize ocean acidity.
Steve Long, a professor of crop sciences and plant biology at the University of Illinois, Urbana-Champaign, heard the idea and thought it might be useful outside of forests. As Long explains, “In the Midwest, we have 200 million acres of corn and soybean land, and all those farms are equipped with lime spreaders. So I said, ‘Why not try it here?’”
In fact, none of the early models had even considered farmland, according to Long. So, he and his team put it to the test at the Illinois Energy Farm on the University of Illinois Urbana-Champaign campus. Researchers repeatedly applied finely-ground basalt on twin fields at the Energy Farm for four years — one field with a corn/soybean crop rotation and the other with Miscanthus x giganteus, a perennial grass that is emerging as a productive bioenergy crop to replace fossil fuels.
The Illinois team calculated the CO2 reduction and weathering rate of the basalt by measuring the change in rare earth elements in the soil with the addition of basalt and comparing it to the calcium and magnesium in the system. The calculations showed that enhanced weathering reduced net carbon loss to the atmosphere by -42% in corn plots. In miscanthus plots, which already stored more CO2 than they emitted before the addition of basalt, enhanced weathering more than doubled carbon storage.
Paired with conservation tillage or cover crops, the basalt application could turn corn into a net carbon sink, according to the researchers. As an added bonus, corn and soybean yields increased +12-16% with enhanced weathering following improved soil fertility, decreased soil acidification, and upregulation of root nutrient transport genes.
This is just one of several studies to show the potential benefits of enhanced weathering for agriculture, as well as the planet. Investors and entrepreneurs have also caught wind of the idea and several companies are now offering basalt and similar treatments, such as volcanic rock and purified olivine. (Sources: University of Illinois Urbana-Champaign, AGU, Nature, Anthropocene Magazine)
