Nine Facts about Enhanced Rock Weathering with Professor David Beerling

1. Enhanced rock weathering is a method for sequestering atmospheric carbon into the soil 

 A soil amendment refers to any material added to the soil to improve its physical or chemical properties. In a process called enhanced rock weathering, agricultural soil can be amended with a crushed silicate rock – usually basalt, a common volcanic rock. As crushed rock is applied to the soil, it interacts with crop roots, the soil chemistry, and microbial communities in the soil. These in turn help to break down the silicate rocks, releasing calcium and magnesium ions. Those ions go on to capture atmospheric carbon dioxide in the form of bicarbonate. This eventually drains out of the soil into watercourses and ultimately the ocean, where it’s stored for many thousands of years. 

2. The application of crushed rock for enhanced rock weathering is simple and straightforward 

 A simple and effective method of applying crushed rock for enhanced rock weathering is to use standard fertiliser spreading equipment. For example, when applied to soils in which bioenergy crops are grown, surface dressing the soil with crushed rock using a hopper will then kick off the physical and biological processes through which the rock is weathered and the carbon capture process can start. When applied to arable land, discs or chisel ploughs can be used to incorporate the crushed rock into the surface area of the soil. 

3. Enhanced rock weathering removes CO₂ from the atmosphere through ionisation 

 The silicate rocks used in the enhanced rock weathering process contain calcium and magnesium ions, which are cations (positively-charged ions). As the rocks break down, these cations are ripped out of their mineral lattices, therefore releasing positively charged ions which, once introduced into solution, must be counterbalanced by a negatively charged ion as per the laws of physics. These cations in the soil therefore attract bicarbonate – an anion (a negatively-charged ion) – which is CO₂ dissolved in water. Once the silicate rocks’ cations are released by weathering, they’re charge is then balanced by bicarbonate ions. The bicarbonate, CO₂ in soluble form, then drains down through the soil profile, and makes its way to the sea.  

4. Enhanced rock weathering can help to restore soil health and improve crop yield 

 One of the central challenges associated with the intensification of agriculture is that it degrades our soils. Intensive agriculture has resulted in soil acidification, and has depleted soils of key micronutrients which are important for maintaining soil health and facilitating crop production. As the silicate rock soil amendment weathers, it produces alkalinity which can reverse soil acidification, and releases micronutrients that are important for crop health, including silica. The rock weathering process also releases fertiliser nutrients which can then stimulate crop yields, like phosphorus and potassium. Large-scale field trials involving enhanced rock weathering processes in the US demonstrate increases in maize and soybean yields of around 10% and 15% respectively. Enhanced rock weathering therefore presents a technology which can capture carbon while also increasing crop yields and improving soil health. 

 5. Long-term trials are required to better understand the relationship between enhanced rock weathering and soil organic matter 

 A variety of field tests under a variety of conditions have sought to shed light on the relationship between enhanced rock weathering and soil organic matter, with differing results. In some cases, these trials have indicated a positive relationship between enhanced rock weathering and an increase in soil organic matter content, while other trials have shown no change in soil organic matter. This may be in part due to the difficulty of measuring changes in soil organic matter in acidic soils, especially in short timescales. There is a need therefore for more long-term field trials to better understand the relationship between enhanced rock weathering and soil organic matter. 

Studies involving the mineral basanite, a very fast-weathering mineral, in forests in China, have indicated an increase in soil organic matter following enhanced rock weathering. However, tests conducted in the Corn Belt in the USA don’t indicate any increase in soil organic matter or soil respiration (an indicator of soil organic matter loss) as a result of enhanced rock weathering treatment. This inconsistency of results indicates the need for further study to better understand the relationship between enhanced rock weathering and soil organic matter.  

6. There is an abundance of local and easily available sources of silicate rock for enhanced rock weathering 

There are multiple minerals which can be used as silicate rock soil amendments for the enhanced rock weathering process. Basalt as the leading candidate for the process as it is the most abundant surface rock in the world, and is already extensively mined around the world for aggregates and road building. Basalt is a byproduct from quarrying, as about 20% of the rock produced for infrastructure use is too fine to be used for aggregate or building, resulting in stockpiles of basalt building up across the world. In the USA, there are multiple enhanced weathering companies which have sprung up in order to exploit these stockpiles of crushed rock. These enhanced weathering companies optimise the allocation of those rocks to nearby farmlands to minimise the transport distance. In the UK, there are quarrying companies in Northumbria and Scotland which are stockpiling Basalt without realising how valuable this could be. 

Another source of silicate rock for enhanced rock weathering is basanite, which is mined across the world. Although basanite is fast-weathering, it doesn’t have the extra micronutrients discussed above which improve soil health and crop yield. Unlike basalt, basanite is not available in the billions of tons of reserves that you’d need to see a significant impact in the enhanced rock weathering process. Despite these seeming advantages which basalt holds over basanite, more research is needed to determine which silicate rocks work best which process. 

7. Key risks include contaminants using the process as a vector and societal issues associated with mining  

 Despite the minimal inputs required for enhanced rock weathering, it will be important to understand and avoid any environmental health risks associated with the process. For example, it is critical that no trace metal accumulation or other contaminants are introduced into the soil or onto crops through the enhanced rock weathering process. In addition to monitoring the impact of enhanced rock weathering on soil toxin concentrations, we need to better understand the effects of the application of silicate rocks on soil microbial activity. 

Beyond any potential environmental risks, enhanced rock weathering is associated with potential concerns over the public perception of increasing outputs from mining activities – especially in the UK, where negative cultural perceptions on mining may influence the uptake of enhanced rock weathering. Further afield, there are essential considerations around indigenous land rights when identifying new basalt quarrying locations in other countries.  

8. Enhanced rock weathering could be a cost-effective route for carbon in-setting in the UK food supply chain 

 One of the features of enhanced rock weathering as a business model is that all of the costs to the farmer are front-loaded. The costs involved in starting an enhanced rock weathering operation include the purchase of basalt, the transportation costs, and the application costs of the rock dust. As the rock starts to weather, and the soils to which it has been applied start to capture carbon, the potential to cover these upfront costs grows over time.  

In the USA, enhanced rock weathering operations have been funded through commercial companies which offer prepaid carbon removal purchases. These companies use the funds that they’ve raised to pay farmers to purchase, transport, and apply the rock dust. Once the process starts to capture carbon, these commercial companies can sell those carbon credits and recover their money. Once more evidence for the technology’s carbon capturing potential has been gathered, sustainable farming incentives for enhanced rock weathering in the UK may help farmers with the initial deployment. Following initial investment in rock distribution and application and infrastructure for soil carbon capture analysis, enhanced rock weathering could provide a cost-effective way to improve carbon capture within British agriculture, or as a potential route for farmers to sell carbon credits on a carbon removal platform. 

9. More evidence of enhanced rock weathering’s success and an effective funding model will help to improve uptake of the technology 

 Many farmers in Northumberland, Scotland and Wales are already using enhanced rock weathering to improve soil health, and increase carbon capture and crop yields. Growing the evidence base of the technology’s effectiveness will be crucial to increasing its uptake. Building a foundation of scientific evidence will make enhanced rock weathering better known and understood, which will increase its uptake amongst farmers. 

In order to achieve the large-scale use of enhanced rock weathering technologies in farms across the UK, a financing mechanism will need to be established. At the moment, quarries are providing basalt extremely cheaply, but farmers must still cover transportation costs, which can be a significant deterrent to uptake. It is also likely that prices of basalt will increase significantly with increases in demand and quarries realising the value of the commodity they are stockpiling. An effective funding model will therefore be crucial to increasing the uptake of enhanced rock weathering in order to see its benefits on a larger scale.