For centuries, farmers have scattered crushed limestone across their fields to improve soil fertility by making it less acidic, leading to better crop yields. This time-honored practice, known as “liming,” was Until now, it was generally considered a source of greenhouse gas emissions. However, new findings suggest a surprising turn of events: liming can actually help remove large amounts of carbon dioxide (CO2) from the atmosphere.

From emission source to carbon sink

“Limization can be a carbon source or a carbon sink,” says Noah Planavsky of Yale University. Empirical measurements suggest that it is “a fairly effective carbon sink.” This new understanding could provide a powerful incentive to expand liming on more farms around the world.

Why was limestone previously seen as a problem? The millions of tons of crushed limestone that farmers spread on their fields each year have been counted as a source of emissions. The reason is that when the alkaline rock dissolves in acidic soil, much of its carbon is released as CO2. But Tim Jesper Suhrhoff, also of Yale, says this accounting is incomplete.

Why Traditional Accounting Is Incomplete

Today's soils are extremely acidic, largely due to the heavy use of fertilizers and pollution from burning fossil fuels. Suhrhoff explains that this acidity also causes other alkaline minerals in the soil to dissolve and release carbon, even without the presence of crushed limestone. “These CO2 emissions will occur regardless of whether you add lime to the system or not,” says Suhrhoff, emphasizing that the added acidity, not the liming itself, is responsible for the emissions. For a more accurate picture of emissions, it is necessary to compare the release and absorption of CO2 from the soil in scenarios with and without liming.

Surprising results of a study from the Mississippi River basin

As an example of this new approach, Suhrhoff, Planavsky and their colleagues examined the Mississippi River basin, which collects runoff from most of the farmland in the U.S. They calculated the net carbon impact of all the liming done in that region between 1900 and 2015.

The scientists used geochemical models and data on how interventions like fertilizers and liming change soil acidity to estimate emissions from soil. They also compared their modeling results to direct measurements of alkalinity in the Mississippi River, because limestone creates alkalinity when it reacts with carbon dioxide.

Using this new approach, scientists found that liming in this region – instead of generating hundreds of millions of tons of emissions – actually removed approximately 300 to 400 million tons of CO2 compared to a scenario where liming was not done. Suhrhoff presented this work at the Goldschmidt Conference on Geochemistry in Prague on July 10.

Potential and challenges for the future

Liming could also be combined with the growing practice of spreading crushed volcanic rock on farms—called enhanced rock weathering—to remove even more CO2 from the atmosphere. While liming can act as a carbon sink, Wolfram Buss of the Australian National University cautions that what worked in the Mississippi River basin won’t necessarily work everywhere. There are risks that liming could make it a net source of CO2 in other systems with significant acidification of agricultural soils.

The next steps are to identify where liming is most needed. “It opens up the possibility that we can incentivize something that will be good for crop yields and potentially give us billions of tons of carbon dioxide removal,” Planavsky says. Such financial incentives could be especially useful for low-income farmers who can’t afford optimal liming for their crops.

Ultimately, the discovery that liming can act as a carbon sink transforms an ancient agricultural technique into a strong ally in the fight against climate change, which also benefits the soil and crops. Think of limestone as a sponge that not only improves the soil for plants, but also silently absorbs invisible carbon smoke from the atmosphere, turning an ancient agricultural practice into an unexpected ally in the fight against climate change. Spring