The centuries-old method of distributing crushed limestone on farmland can enhance crop yields by lowering soil acidity. While this practice is typically viewed as a contributor to greenhouse gas emissions, recent studies indicate that “liming” might actually assist in sequestering substantial amounts of carbon dioxide from the atmosphere.

“Liming can either act as a carbon source or as a carbon sink. Empirical evidence suggests it serves as a fairly effective carbon sink,” says Noah Pranabsky from Yale University. This revelation could motivate the global spread of limestone on farms, although liming does not produce this effect universally.

Each year, millions of tons of crushed limestone spread across fields are classified as emission sources. This classification arises because when alkaline materials dissolve in acidic soils, a significant portion of its carbon is released as CO2. However, this assessment is not entirely accurate. According to Tim Jesper Souhoff, also from Yale, the situation is more complex.

Today’s soils are highly acidic, resulting from extensive fertilizer use and pollution from fossil fuel combustion. Consequently, various other alkaline minerals in the soil dissolve and liberate carbon, even in the absence of limestone. “These CO2 emissions will happen regardless of whether lime is added or not,” Suhrhoff explains, emphasizing that this alteration of acidity can lead to unintentional consequences.

Suhrhoff advocates for a more accurate evaluation of emissions from this practice, suggesting that scientists should analyze CO2 emitted versus captured in scenarios both with and without liming.

As an illustration of this method, Suhrhoff, Planavsky, and their team focused on the Mississippi River basin, a region that gathers runoff from the majority of U.S. farmland. They calculated the net carbon impact of limestone applications from 1900 to 2015 across the area.

The researchers estimated emissions from the soil while utilizing geochemical models to assess how liming alters soil acidity. They also compared their model results with direct observations of alkalinity in the Mississippi, where limestone reacts with carbon dioxide.

By employing their innovative approach, the researchers determined that they had sequestered around 300 million tonnes of CO2 in the region, rather than enabling the release of hundreds of millions of tonnes of emissions. Souhoff presented these findings at the Goldschmidt Conference held in Prague, Czech Republic, on July 10th.

This practice can also be coupled with the increased use of crushed volcanic rocks on farmland to sequester even more CO2 from the atmosphere.

Wolfram Buss from the Australian National University suggests that while liming can act as a carbon sink, the success seen in the Mississippi River Basin may not apply universally. “There is a potential risk with lime application, which could result in net CO2 emissions in other systems, primarily due to the significant acidification of agricultural soils,” he warns.

The subsequent step is to determine where liming is most essential. “This presents the opportunity for us to foster improved crop yields, potentially leading to billions of tons of CO2 removal,” Planavsky remarks. Financial incentives can significantly aid low-income farmers who are unable to afford the optimal amount of lime needed for their crops.