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Spreading crushed rock on farms is gaining traction as a method of removing carbon dioxide from the atmosphere. Now, two independent tests of the approach on farms in the US Midwest and in the UK have found it can also substantially increase crop yields.

鈥淚t鈥檚 very encouraging,鈥� says at the University of Sheffield in the UK, whose team tested the approach on an experimental farm in Illinois.

Silicate rocks such as volcanic basalt naturally react with CO2 dissolved in water to produce solid carbonates in a process geologists call weathering. The process can be accelerated if the rock is ground into dust to increase the surface area exposed to water. This has encouraged a number of companies to begin spreading large volumes of such rock dust on farms across the world in order to take advantage of 鈥渆nhanced weathering鈥� to capture more CO2.

But much about this approach remains uncertain, including how to accurately measure the amount of CO2 it removes and how the rock dust affects crops.

Between 2016 and 2020, Beerling and his colleagues compared a typical rotation of maize and soy in plots treated with basalt rock dust and nitrogen fertilisers to plots treated with fertilisers alone. They added 50 tonnes of crushed basalt per hectare each year to the plots, on which maize was grown for two years, then soy for one year, then maize once again.

They found yields of soy were 16 per cent higher in plots treated with the basalt. Maize yields weren鈥檛 so different between plots during the first two years of the rotation, but were 12 per cent higher in plots with basalt the year after soy was planted, which might suggest an interaction between the basalt and the nitrogen fixed by the soy.

The researchers attribute the boosted yields to improved soil pH as well as how plants responded to the phosphorus and potassium nutrients released as the rock dust weathered. 鈥淭he roots can sense it,鈥� says Beerling, adding that the nutrients and the improved uptake could help offset fertiliser costs.

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Based on the amount of magnesium and calcium dissolved in the rock 鈥� which is linked to the amount of mineralised CO2 鈥� the researchers estimate the treatment also captured between 6.7 and 14.3 tonnes of CO2 per hectare, a substantial amount if scaled across millions of hectares of farmland.

In another recent test, an enhanced weathering company called found similar changes in yield from a single-year trial growing spring oats at a farm plot in Newcastle, UK. Depending on how the seeds were planted, the researchers found a 9 to 20 per cent increase in yield on the plots treated with basalt dust and fertiliser compared with plots treated with conventional fertiliser alone. at UNDO estimates that the 18 tonnes of basalt added to each hectare would remove about 4.5 tonnes of CO2 per hectare over several years.

at Newcastle University in the UK, who collaborated with UNDO on the test, says the evidence for improved yields across different crops and climates could help get more farmers to sign on to try enhanced weathering. 鈥淔or it to work, we鈥檝e got to get the farmers on board and seeing the benefits to them,鈥� he says.