Innovation: It’s weird to think of rocks, perhaps the most static and lifeless objects we come across, as bringing the soil back to life. And yet, the process of breaking down rocks (“weathering”) is foundational to soil health, the carbon cycle, and overall ecosystem balance. “Enhanced weathering” now presents one of the most promising carbon removal approaches out there, as well as one that could have a major impact on both crop yields and agricultural emissions. The numbers are mindblowing: “For example, applying 50 tonnes of basalt powder per hectare per year to 70m hectares of the corn belt of North America might sequester as much as 1.1bn tonnes of CO2 in the long run – equivalent to 13% of the global annual emissions from agriculture.” Though questions remain about the cost of incorporating enhanced weathering and the downstream environmental implications, enhanced weathering merits a deeper dive due to the potential co-benefits to agricultural productivity and ocean alkalization, its scalability to deployment, and its gigaton carbon removal potential.
Enhanced weathering builds on the natural process of weathering. This process starts with rain gradually breaking down rocks over long periods of time into rock grains and bicarbonates which move downstream into oceans. Enhanced weathering speeds this up by mining silicate rocks or acquiring them from industrial byproducts (like construction waste and steel slags) - basalt is the major one mentioned and it is considered the most sustainable - and then pulverizing them into a powder. This powder is then spread over large areas of land, where it interacts with plant roots and microbes to both stimulate biological plant activity and the chemical reactions we previously mentioned. The silicate powder provides key minerals, like calcium, magnesium, and iron for plants that they might not otherwise receive in biological inputs (like compost) or traditional fertilizers. In many ways, this mimics natural geological and biological interactions - though a lot remains unknown about the externalities of speeding this up. After the silicate powder interacts with rocks, water, and air, the resulting solid carbonate minerals either remain in the soil or flow into the ocean, where it slows ocean acidification.
If this idea sounds a bit cooky, the use of rock dust for altering soil properties is not a novel one. “Farming with rocks” is a widely available, low-cost soil amendment for many producers globally. During the Green Revolution, Brazil’s 180 million hectare Cerrado eco-region was unfit for farming due to naturally acidic soil (it is a tropical savannah). To change this, over 15 million tonnes of limestone were spread over the fields from the 1960s-1990s at about five tonnes per hectare. The region is a major contributor to Brazil’s soybean production while still being the predominant national source of cattle. Though this is not the most sustainable use of rock dust, it still shows the technical potential of such a practice and the possibilities - for restoration or expansion - that it unlocks for producers.
Given this theoretical framework, let’s review the recent research and field trials in this nascent field on enhanced weathering, particularly regarding the effects on agricultural productivity and potential carbon sequestration:
Early trials promise increased agricultural yields.
Enhanced weathering promises to increase yields and reduce the need for synthetic fertilizer use. At the Working Lands Innovation Center, they are testing basalt and wollastonite powders for enhanced weathering trials. Per former Director Ben Houlton (now Dean of Cornell College of Agriculture), their early results tied basalt and wollastonite to a 12% increase in corn yields. In a separate controlled environment study, the application of basalt led to a 20% increase in the yield of cereal grains. In Mauritius, sugarcane trials going back to 1961 added basalt powder to soils and increased yields 30% over five successive crops. In even more trials, Wollastonite applications increased soybean yields two-fold and increased alfalfa growth in terms of height, root biomass, and above-ground biomass dry weight. Olivine, another silicate, has also been tested as an effective substitute for agricultural lime, as has basalt to an extent. It’s pretty clear that enhanced weathering provides benefits to agricultural productivity, be it from increased resource efficiency, recovering mineral nutrients, and overall restoring acidified and desilicated soils.
As a carbon drawdown solution and “carbon farming” revenue stream, enhanced weathering is extremely promising, though the exact dynamics - and modeling - remains unknown.
Enhanced weathering will allow for plants to sequester more organic carbon and support greater biomass production. Based on field systems, existing scientific literature on weathering, and theoretical analysis, the application of silicate rocks to Chinese, Indian, American, and Brazilian cropland soil alone would remove more than 2 billion tons of CO2e each year. Even considering current carbon sinks in agriculture and discounting the potential 5-30% of CO2e released during mining, processing, transportation, and spreading, enhanced weathering is a gigaton solution that does not require land use beyond existing managed croplands nor compete for freshwater or land used for food growth.
Mycorrhizal fungi are particularly important in linking enhanced weathering’s inorganic carbon to the organic carbon cycling from plants. Technologies focused on fungi’s role in enhanced weathering could unlock its full potential - and help regenerate soils even more efficiently. One particular study showed rock-inhabiting fungi increasing olivine dissolution rates by 700% (h/t Twitter), significantly speeding up the enhanced weathering process. As we move from lab-based experiments into the biotic world, perhaps the potential carbon sequestration of enhanced weathering has been underestimated!
However, the operational cost needs to go down to make it palatable to landowners, while the sourcing and mining of billions of tons of silicate rocks need more clarity.
The operation costs of achieving a tonne of CO2e sequestered through enhanced weathering is between $52-480/tonne based on a U.K. analysis, not discounting higher crop yields and lower fertilizer needs. This varies based on the silicate material used: dunite could be competitive at $60/tonne, while basalt will need a $200/tonne to succeed at scale. If I’m a farmer, landowner, or organization looking into this space, while acknowledging the potential importance of carbon offset markets, I’m not sure incorporating enhanced weathering techniques is the right decision without the right policy incentives or upfront financing. The price is too steep currently, and the risks are unknown across a longer timeline. Mitigating the adoption risks for landowners will prove crucial for enhanced weathering to scale.
Furthermore, the release of trace harmful elements is an acknowledged risk that requires more research. Dunite, which is rich in nickel and chronium, might not be the ideal candidate. Though scientists focused on enhanced weathering don’t see the amount of silicate material available as an issue, sourcing the right one could result problematic. To sequester one billion tonnes of CO2e, more than 3 billion tons of basalt would have to be spread - which is equivalent to half of the current global coal production - and the spreading would be over one-fifth of global croplands. Finding such an amount of basalt, processing it, and spreading it presents an incredible logical challenge and high bill. Mining at this scale could have negative environmental and ecological impacts, as well as increasingly take a large chunk out of the carbon sequestered. Using waste materials or industrial byproducts is a potential solution too, such as steel slag, cement kiln dust, and mine tailings, but the risks to terrestrial, coastal, and marine environments remain unknown for these materials.
I’m excited to see capital flow into the space and researchers dive headfirst into finding the best way to do enhanced weathering. Our experience with spreading crushed rock, which is already a common practice, will allow the enhanced weathering supply chain to stack on top of existing agricultural and industrial supply chains. This potentially rapid transition could make it more attractive than any “technical” solution to climate change, while also having positive natural co-benefits to grasslands and croplands. To name a few, The Future Forest Company is doing interesting work spreading silicate material and biochar across forest floors to achieve 10x the carbon sequestration of reforestation. Project Vesta, a non-profit, is applying olivine to beaches across the Caribbean. Organizations like the Leverhulme Centre for Climate Change Mitigation are focused on scaling enhanced weathering within agriculture, so keep an eye out for their results. Enhanced weathering can even be applied directly to non-hazardous industrial waste - a novel way of thinking of our waste as being regenerative! I hope the focus on enhanced weathering as a regenerative process remains, as I believe that is what will truly unlock its potential - perhaps with the technical ingenuity that was done with liming on the Cerrado, but with a greater ecological purpose in mind.
In preparation for the holidays, we are preparing a gift guide of regenerative products to share after Thanksgiving and we will be showcasing indigenously owned/founded products next week. If you have any particular brands or promotions we should showcase (or founders we should interview), please send them my way!
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Thank you for this excellent piece Kevin. Incidentally, I've been using rock dust for 20 years or more on my veg beds and I'm very happy with the resultant fertility and crop health. Nevertheless, I am intrigued and receptive to the counter argument from some soil microbiologists and regen farmers that rock dust may be a bit of a distraction, and a costly one, in certain contexts - given the ability of healthy soils (in healthy diverse ecosystems) to mine rock in the subsoil extremely effectively. As you point out fungi are central to this mineral mining process and delivering those minerals in soluble form to plants at the rootzone.