In 1845, the German chemist Justus von Liebig declared that plants derive their minerals from rock weathering, not from some mystical "humus" in soil. He was half right. Plants do get their calcium, magnesium, and phosphorus from dissolved rock. What Liebig missed was the invisible workforce doing most of the heavy lifting: billions of microorganisms that crack open minerals faster than chemistry alone ever could.

The Underground Energy Economy

Every year, plants pump 55 to 60 gigatons of carbon underground—roughly 20% of everything they photosynthesize. That's not waste. It's payment to an ecosystem of bacteria and fungi that extends plant roots by orders of magnitude and accelerates the breakdown of bedrock into soil.

The scale is almost incomprehensible. The energy flux moving belowground equals up to 83,000 terawatt-hours annually, six times the electricity produced globally from fossil fuels. Most of that energy feeds mycorrhizal fungi, which colonize the roots of nearly all land plants. In exchange for sugars, these fungi deliver water, nutrients, and—scientists now understand—dramatically enhanced mineral weathering.

This arrangement has implications far beyond plant nutrition. When microbes accelerate rock weathering, they also accelerate carbon sequestration. The same chemical reactions that liberate calcium and magnesium from silicate rocks also consume CO₂, locking it away as stable bicarbonate ions that eventually wash into oceans and remain there for millennia.

Two Fungal Strategies, Two Different Impacts

Not all mycorrhizae weather rock equally. Arbuscular mycorrhizal (AM) fungi evolved over 400 million years ago and partner with most crop plants and grasses. Ectomycorrhizal (EM) fungi arrived much later—during the Cretaceous period when atmospheric CO₂ ranged between 1,100 and 1,700 parts per million, more than double current levels.

EM fungi evolved in a high-CO₂ world and built themselves to weather rock aggressively. They receive 7 to 30% of their host tree's photosynthate, compared to around 10% for AM fungi. They use that carbon budget to produce powerful organic acids that dissolve minerals. Field studies consistently show that EM trees intensify weathering by a factor of 1.9 to 2.6 compared to AM trees.

Researchers using radioactive carbon tracers have directly tracked the process: photosynthate flows from tree to fungus, the fungus secretes acids that attack calcite-containing rock grains, and calcium dissolves into soil solution. Trees growing in soils with crushed limestone show dramatically accelerated weathering when colonized by EM fungi—and dramatically increased CO₂ consumption.

Intriguingly, as CO₂ levels have fallen from Cretaceous highs to near-present levels, calcium dissolution rates have halved for both fungal types. The implication is unsettling: the natural weathering thermostat that has regulated Earth's climate over geological time may be losing sensitivity just when we need it most.

Engineering the Process

If fungi can accelerate weathering, can bacteria do better? That's the question driving recent experiments with Bacillus subtilis strain MP1, a bacterium isolated from agricultural soils. MP1 produces oxalic and citric acids that chelate silicon ions and forms biofilms on feldspar surfaces, dramatically increasing mineral dissolution.

In mesocosm experiments—essentially large, controlled pots—adding MP1 to crushed silicate rock increased soil weathering rates more than sixfold. Soil inorganic carbon jumped 20% compared to untreated controls. When researchers moved to field trials across eight soybean farms, the results held: MP1 treatment led to gross accrual of 2.02 metric tons of inorganic carbon per hectare annually.

The mechanism is surprisingly straightforward. Bacillus bacteria excrete organic acids that lower pH around rock particles. The acidified microenvironment accelerates mineral dissolution. Released calcium and magnesium ions then react with CO₂ dissolved in soil water, forming stable carbonate minerals. The process is fastest in the first 24 hours after microbes contact minerals, slows after five days, and approaches a constant rate after 45 to 102 days.

The Australian CarbonLock project, running from 2023 to 2026, is taking this further by combining waste rock from mines with beneficial microbes and organic matter. Mine waste is typically free and uncontaminated—just crushed rock with high surface area. Adding microbes and compost creates habitat and provides bioavailable nutrients to drive activity. Early indications suggest mineral solubilization rates could increase by orders of magnitude compared to spreading rock dust alone.

The Economics of Carbon Removal

Enhanced rock weathering currently costs $300 to $350 per metric ton of CO₂ captured. That's expensive compared to forest planting, but competitive with direct air capture technologies. The cost comes largely from grinding rock to powder and transporting it to fields.

Microbial enhancement could change the economics. If bacteria or fungi increase weathering rates sixfold, you need one-sixth as much rock dust for the same carbon removal. Transport costs drop proportionally. Processing costs might rise slightly for microbial cultures, but fermentation scales well.

There's another economic angle: co-benefits. Rock dust increases soil pH, raises cation exchange capacity, and boosts calcium and magnesium availability—all improvements farmers would pay for independently. Unlike carbon forestry, enhanced weathering doesn't take productive land out of cultivation. It enhances the land already being farmed.

Global croplands cover approximately 12 million square kilometers. Even partial adoption of microbially-enhanced rock weathering could sequester gigatons of CO₂ annually while improving soil fertility—a rare convergence of climate and agricultural interests.

The Permanence Problem Solved

Carbon removed from the atmosphere only matters if it stays removed. Trees burn, soils erode, and organic carbon decomposes. Enhanced weathering sidesteps these problems by converting CO₂ into mineral carbonates and dissolved bicarbonate ions. Once formed, these compounds remain stable for thousands of years. Ocean bicarbonates persist for millennia before eventually participating in the geological carbon cycle.

This permanence sets enhanced weathering apart from most biological carbon removal strategies. It mimics—and accelerates—the same silicate weathering feedback that has regulated Earth's climate over hundreds of millions of years. We're not inventing a new process. We're tuning an ancient one, with the help of organisms that have been doing this work since before dinosaurs walked the Earth.