A New Way to ‘Rock On’ to Fight Climate Change?

Maryland farmers currently dig deep—in their pockets—for limestone they spread on their fields to reduce soil acidity and help crops thrive. But crushed basalt rock that results from industrial processes might also be able to do the same job for free instead. And if it works, basalt won’t just improve soil pH, it would help reduce atmospheric carbon dioxide while reversing the harmful ecological effects of ocean acidification.

Now, University of Maryland environmental science and technology Professor Ray Weil, Lecturer Eni Baballari, and University of Maryland Extension Agent Mark Townsend are working with a Frederick, Md., farmer to test that big “if.”

Already, large corporations are betting on basalt to offset their carbon emissions with carbon credits, earned by paying to supply farms with crushed basalt rock or basalt dust. In Maryland, there is a ready supply of fine basalt dust because it is a byproduct of gravel and roof shingle manufacturing. But currently, there is no solid proof it works to raise soil pH or capture carbon. Plus, farmers need to know if its use has any downsides—could basalt rock somehow reduce yields or even contaminate soil?

Limestone is expensive, often $100 per ton, and mid-Atlantic farmers need to spread 3 to 4 tons per acre every few years to reduce the acidity of their soil for growing crops. In addition to the expense, limestone has environmental costs. Made of crushed calcium carbonate, it reacts with acidic soil to release carbon dioxide into the atmosphere. And carbon dioxide is one of the major greenhouse gases causing global warming.

Basalt, on the other hand, is made of silicate and contains no carbon. When it reacts with acidic soil, it draws carbon dioxide from the atmosphere, and turns it into bicarbonate ions dissolved in groundwater. That groundwater can make its way into streams, rivers and eventually the ocean, where bicarbonate helps stop the rising acidity that’s robbing corals and shellfish of the carbon they need to build their shells.

The reaction of limestone and basalt with acidic soil is part of the natural process of weathering; but in nature, it happens over millions of years, while farmers and carbon credit seekers need these reactions to happen over just a few years. They know crushing limestone into a powder speeds up the weathering process, but no one knows for sure if it works as well for basalt powder. So Maryland farmers reached out to Weil and Extension agents when the carbon-credit brokerage startup Lithos-Carbon offered to deliver tons of basalt rock dust for free.

“They weren’t getting many takers,” Weil said, “and the farmers wanted to know what to think of this offer that sounded too good to be true.”

One of them, a soybean farmer from Frederick, agreed to let Weil conduct experiments on his farm. Over the past year, Weil and researchers in his lab have been testing four different treatments in 16 plots to compare the effects of applying basalt dust or traditional agricultural limestone. They are measuring the rate of change in soil pH at different soil depths, and monitoring the effects on soil structure, water holding capacity, infiltration rate, etc. They are also measuring any impacts to plant growth, yield and nutritional composition of both soy and wheat crops. Perhaps most important, they are monitoring bicarbonate in the water that leaches from their study plots to confirm whether basalt dust actually removes carbon dioxide from the atmosphere and produces valuable carbon credits.

It is a complex research project involving sophisticated deep soil sampling and collection of leaching water from 32 locations in the farm field. The team is also planning laboratory studies to more clearly understand the chemistry and processes under varying conditions.

Weil expects the study to take a couple of years, as neither product changes soil pH overnight. So far, during the first growing season of the study, there was little change in the pH of the basalt rock-treated soil. But that doesn’t mean it won’t eventually work.

Both farmers and the carbon credit market are surely eager for answers, and when the final results are in, they will be applicable to a wide swath of central Maryland and southern Pennsylvania with similar types of soil, Weil said.