For decades, farmers and gardeners have supplemented their soil with lime, volcanic ash, and crushed stone dust to improve its quality. Recent research studies suggest that the strategy could pull a meaningful amount of CO2 from the atmosphere if applied widely by farmers. Notably, the winner and a runner-up of the $100 million XPRIZE for Carbon Removal used the technique known as enhanced rock weathering (ERW) to demonstrate the potential for gigaton-scale carbon removal.
According to a 2020 study by the University of Sheffield (UK), the practice of layering soil with calcium- and magnesium-rich ground rock could capture enough carbon to account for between 5% and 10% of the United States’ annual carbon reduction goals for 2030. In India, the use of ERW could capture and sequester up to 40% of the nation’s Paris Agreement obligations.
There are “opportunities to align agricultural and climate policy,” the report states. What is ERW, and can home gardeners help by amending gardens and lawns with milled basalt and azomite soil supplements made with volcanic ash?
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Understanding Enhanced Rock Weathering
Nature has clever ways to clean the atmosphere. Since long before humans appeared, various types of rock — such as basalt and limestone — have served to capture CO2. As water passes over the rock, it dissolves and locks carbon in a hydrogen carbonate solution that runs off into groundwater.
The 2024 “State of Carbon Dioxide Removal” (CDR) report estimates that about 7 billion to 9 billion metric tons of CO2 must be removed from the atmosphere annually if the 1.5°C (2.7°F) Paris Agreement target is to be met. Currently, only 2 billion metric tons of CO₂ removal capacity is in place, met mainly by “conventional” methods such as reforestation.
Farmers have long supplemented soil with calcium-rich additives, including lime from ground limestone. Both the natural and human processes are examples of rock weathering. Humans enhanced the natural process by grinding stones to increase the surface area that water can reach, thereby increasing the amount of CO2 that can be captured and runoff into the soil.
A follow-up 2024 study published in Nature by the University of Sheffield demonstrates that the practice of layering soil with calcium- and magnesium-rich ground rock could capture enough carbon to account for between 0.16 and 0.30 gigatons of CO2 per year by 2050 in the United States alone, rising to 0.25-0.49 GtCO2 per year by 2070. This represents a substantial contribution, potentially accounting for 16% to 30% of the required reduction from CO2 removal technologies by 2050.
Field trials conducted in the U.S. Corn Belt over four years show that crushed rock can remove three to four metric tons of atmospheric carbon dioxide per hectare per year, while simultaneously improving crop yields. The results demonstrate that ERW practices had substantial co-benefits to soil health and crop production.
In India, the use of enhanced rock weathering could capture and sequester up to 40% of the nation’s Paris Agreement obligations. Analysis of crushed rock transfer between source states and recipient agricultural states for achieving these CDR trajectories reveals that within two decades, three states with pre-existing quarrying infrastructure co-located with basalt reserves (e.g., Wisconsin, Minnesota, and Michigan) will be the leading rock suppliers to adjacent farmland.
The benefits of ERW to soil are proven. Augmented soil is healthier in many cases and produces more crops. However, the carbon capture aspects of ERW need to be tested to confirm that it would be a good policy to encourage farmers to apply calcium- and magnesium-rich materials on a national or global scale.
The report also raises a significant concern. If farmers are encouraged to use ERW, will the mining and other production processes necessary to produce enough ERW material increase pollution and carbon dioxide emissions? The researchers note that iron and steel slag, byproducts of manufacturing, can be ground up and used to enhance farmland. The report argues early adoption of ERW can drive demand and innovation that relieves the potential environmental harm.
We have the materials; the challenge is how to collect, process, and get them into the ground at farms worldwide.
Commercial Development and Industry Growth
The enhanced rock weathering industry is rapidly expanding. Companies such as Microsoft and British Airways are already investing millions of dollars in the effort by purchasing carbon credits based on future emissions reductions.
In a major milestone, the world’s first verified enhanced rock weathering carbon credits were issued in January 2025 by InPlanet and Isometric. These credits demonstrate that it is possible to conduct scientifically rigorous MRV on open-systems carbon removal pathways.
The Carbon Business Council’s 2024 policy primer suggests that ERW prices could decrease to under $100 per ton, down from the previous estimated range of $160–$180 per ton. XPRIZE Carbon Removal winner Mati Carbon is developing systems to deploy rock dust in smallholder farms across countries in the Global South, including India, Tanzania, and Zambia.
Will ERW Make a Climate Difference?
Because ERW stores CO2 in the ground and keeps it there for millennia, the positive impact lasts much longer than some plans for direct-air capture. A 2024 study published in Environmental Science & Technology points to the need for comprehensive environmental monitoring to identify the potential negative consequences and positive co-benefits as ERW scales up.
China, India, and Brazil, three of the fastest-growing carbon-emitting countries, could eliminate enough CO2 using ERW to meet between 10% and 40% of their 2030 Paris Accord goals. And it is one of the few options someone could do at home.
Small farms, especially in developing economies, are the easiest target for ERW programs, the research argues. That raises the question, could gardeners join the carbon fight by adding milled basalt or azomite volcanic ash before planting each year?
A Home Garden Carbon Sink?
There is no scientific evidence that home gardeners can change the climate using ERW, but there is plenty of anecdotal evidence of the benefits of soil augmentation. As the cumulative impact of billions of people changing their travel patterns on CO2 emissions shows, small changes can add up to big global impacts. The problem is making the change last, and ERW’s long-term carbon sequestration ability makes small changes last.
If large and small farms are effective carbon sinks when farmers use ERW to supplement fields, we suspect individuals with large gardens could make an additional CO2 difference. After all, there are 90 million farmed acres in the continental U.S. and 40 million acres of lawn.
If you are a gardener, consider adding azomite soil supplements, milled basalt, or volcanic ash to the garden. It will improve yields and could help save the planet. If you have a large yard, adding one of these supplements to the grass each year will improve root health. And it may pull a little more CO2 from the air around us.
Here are several products that fit the bill in different sizes for practicing home ERW. The recommended amount of azomite soil per 1,000 square feet of garden or lawn is 2 to 3 pounds. For container plants, apply 1/2 to 1 teaspoon per inch of pot diameter.
Editor’s Note: Originally published on July 14, 2020, this article was substantially updated in July 2025.