Grinding rocks into a soil additive? It may be closer than you think

Do you have one of those rocky fields? If you do, you know the routine. In the spring, pick up anything bigger than your fist and carry it to the field edge or rockpile. That might save breaking a disc blade, or a fertilizer shank, or getting picked up by the combine at harvest and wreak havoc on its insides. Then, after spring through fall tillage and a winter of freezing and thawing, there they are again, thick as ever, or so it seems.

What if you had a machine that would pick them up, grind them to dust and spread the dust back onto the field? And get paid to do it. Sound far-fetched? Maybe not.

All soils, you know, begin with a rock. Rocks are the source of all the phosphorus, potassium, calcium, and other minerals that feed plants (but not carbon, nitrogen and oxygen). These minerals become available to plants and soil organisms as the rocks dissolve from acids produced by rainwater interacting with carbon dioxide, and from acids produced by the soil organisms and plant roots. These dissolved nutrients are recycled by plants and soil biota to eventually, over millions of years, turn rocks into productive soils with a load of stored nutrients called humus.

Dissolving bare rocks with rainfall is so slow as to be unnoticeable. Most soil begins with grindings from glaciers or a deposit of volcanic ash which gives rainwater massive surface areas to work on. The eruption of Mount St. Helens in Western Washington in 1980 deposited a four-inch layer of ash over much of the Columbia basin, now mixed into the existing soil to weather away and produce nutrients for hundreds of years.

So if you grind rocks and spread the dust, you add minerals to soil that will weather over the years and give your soils a lasting nutrient boost.

A number of institutions are studying the feasibility of adding rock dust to soils to enhance fertility, raise pH and absorb carbon dioxide (CO2) from the atmosphere. Removal of CO2 occurs when rainwater reacts with CO2 in the soil to form carbonic acid. The carbonic acid reacts with silicates minerals in the soil and pulls CO2 from the atmosphere forming bicarbonates with calcium or magnesium silicates. These bicarbonates raise soil pH, and when leached from the soil and wind up in the ocean, ocean pH is raised. Since pH is falling in our soils and the ocean, this is a win-win situation.

Raising soil pH and adding nutrients through rock dust has been found to raise corn yields by as much as 20 percent .

The University of Illinois, in a five-year study at its Energy Farm, found that four applications of rock dust, 20 tons per acre per year, reduced net carbon loss to the atmosphere from a corn plot by 42 percent. Yield increases of 12 percent for corn and 16 percent for soybean were observed . The benefit from the weathering of the rock dust is expected to last for many years

Five tons per acre per year sounds like a lot of rock, but many farmers apply two to four tons of another rock (limestone) to raise pH one point, and do it every three or four years.

Rock dust additions work best in wet, warm climates that speed up weathering. But the University of California, Davis, and Cornell University found the storage of CO2 by added rock dust worked well even in the dry California climate.

Besides rock dust, experiments with ground concrete have been shown to produce similar results. Good way to keep used concrete out of landfills.

Entrepreneurs have been emboldened by the positive result of rock dust to benefit the land and the climate, with dozens of companies around the world now offering farmers free rock dust while they sell carbon credits to large corporations. In 2023, Microsoft became the first company to buy carbon credits “based on enhanced rock weathering … from Scottish company UNDO.”

Grinding rock, of course, and hauling it to a field takes lots of energy. But, surprisingly, it uses less energy — up to half as much — than high-tech CO2 capture methods such as direct air capture using big fans and pumping the CO2 underground, or growing bioenergy crops, burning them while capturing the CO2 emissions and storing them.

As I said in the beginning, it may be a bit far-fetched, but someday we may see a farmer grinding field-strewn rocks into powder for a soil additive, and getting paid for it by some carbon credit program. Never underestimate the inventive ability of a farmer.

Jack DeWitt is a farmer-agronomist with farming experience that spans the decades since the end of horse farming to the age of GPS and precision farming. He recounts all and predicts how we can have a future world with abundant food in his book “ World Food Unlimited .” A version of this article was republished from Agri-Times Northwest with permission.

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