Grind up rocks, throw them on your farmland, and you’ll boost yields while also saving the planet. At least, that’s the message being sold by proponents of “enhanced rock weathering” (ERW), who claim that adding rock dust to soils enhances fertility and removes CO2 from the air. But what exactly is it and what are the potential downsides?
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, 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 believed to stay sequestered forever. However, this is a very slow process that takes millions of years.
Dozens of companies are now trying to sell sped-up, manufactured versions of this natural process. The most direct method is simply grinding rocks into a fine dust that reacts more easily with air and water. Others are experimenting with additives in their rock dust that speeds up the whole chemical process – even when rocks are ground down to dust, carbon removal via ERW can take a decade or more, depending on several factors.
One of those factors is the type of rock being used, which is also where some of the biggest risks are going to be for farmers. Critics note that long-term field trials tracking both carbon potential and accumulation of contaminants is currently lacking.
One of the earliest types of rock that was formally studied for ERW was olivine, a green silicate mineral found in common igneous rocks. It is a prime candidate for ERW because it weathers extremely fast. However, it is also high in toxins like chromium and nickel, raising concerns about long-term environmental contamination. Still, some companies are hawking olivine to farmers as a replacement for ag lime.
David Beerling, director of the Leverhulme Centre for Climate Change Mitigation at the University of Sheffield, U.K., oversees the longest-running enhanced weathering project in the world. Even he says that companies still need to resolve numerous outstanding uncertainties, which will only come through long-termr trials. “People want to know how this works if you keep doing it year on year … How does it affect yields? How does it affect soils? How does the carbon storage change over time?”
Crop studies conducted so far have been concentrated mostly in temperate zones, but they do suggest that silicate rock application can boost crop yields. Researchers credit this to minerals locked inside rocks that, when released via weathering, stabilize soil pH. In turn, this is expected to reduce the use of artificial fertilizers by improving soil health over time.
University of Illinois researchers, working with the Leverhulme Centre, wrapped up a five-year study last year looking at the potential of basalt rock amendments, which they’ve applied to corn and miscanthus (a perennial grass with potential use as a biofuel). The 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. Notably, the researchers say their basalt treatments increased corn yields by +12% and soybean yields by +16%. The full study is HERE. (Sources: MIT Climate, Mongabay, University of Illinois, AgFunder)
