Captain Drawdown’s daily logbook on every CDR story, paper, and expert voice — so you don’t have to read them all.
1. The story
A field trial on CDRXIV lands a direct hit on enhanced rock weathering’s favorite mineral. Daniel Maxbauer and colleagues spread crushed rock on a midwestern U.S. cornfield and measured what happened. Their result: coarse-grained basalt produced no statistically detectable CO2 removal signal, while steel slag did, and even the slag signal appeared only on soil that started out acidic. Slag on near-neutral soil showed nothing significant either. Basalt is the feedstock most ERW startups are selling and most buyers are underwriting. A null result on that mineral, in the heart of U.S. row-crop agriculture, deserves more attention than it will probably get.
2. The mechanism
Enhanced rock weathering (ERW) works by spreading crushed silicate rock on farmland. As the rock dissolves, it releases cations and converts CO2 into dissolved bicarbonate that should eventually reach rivers and the ocean. I covered the basics in Captain Drawdown’s enhanced weathering primer.
The trial’s design matters. The basalt was explicitly coarse-grained, and grain size strongly controls dissolution rate, so this is a null for coarse basalt on this soil, not a verdict on basalt everywhere. But the surrounding literature explains why a basalt null is a predictable outcome, not a fluke. Kanzaki and colleagues show that soil cation storage is a key control on ERW dynamics: released cations can park on soil exchange sites before any bicarbonate leaves the field, making a working amendment look inert. Jordan and colleagues built a theoretical framework showing alkalinity export through soil pore water is strongly pH-dependent, predicting that export can be suppressed depending on soil chemistry. And Suhrhoff and colleagues warn in their signal-to-noise analysis of soil mass balance methods that soil-based measurement struggles to separate slow basalt weathering from background noise on managed farmland. Maxbauer’s trial looks like that failure mode observed in the field.
3. The market angle
The ERW credit stack built between 2024 and 2026 is largely a basalt stack. Supply contracts, life-cycle assessments, and delivery projections mostly assume crushed basalt as the default feedstock. Slag complicates that picture. It is an industrial byproduct, typically more reactive because it is calcium-rich, and often cheaper to source. But it carries different risks: trace metal content, supply tied to steel production, and harder questions about whether its use counts as additional removal or repurposed waste management. If direct field measurement keeps favoring slag over basalt, developers face an uncomfortable choice between the feedstock that measures well and the feedstock their contracts name.
4. The policy and regulatory context
The registries writing ERW methodologies, including Isometric, Puro, and Verra, now have a concrete question in front of them: should crediting require feedstock-specific detection thresholds, so that a project must demonstrate a measurable weathering signal for its actual mineral in its actual soil before tonnes are issued? The Suhrhoff analysis suggests that for basalt on managed land, current soil-based measurement, reporting, and verification (MRV) may not clear that bar. Whether methodologies move toward per-feedstock detection standards, and how fast, is the regulatory story to watch.
5. What experts are saying
The basalt-positive record is itself under live statistical dispute. Adam Wolf is defending an earlier basalt-positive field result (Kantola 2023) against a critique by Derry and colleagues, in a preprint bluntly titled “Statistical analysis of basalt weathering disproves the null hypothesis”. Suhrhoff and colleagues, meanwhile, put the methodological warning in writing: their updated framework argues that soil mass balance approaches face a genuine signal-to-noise problem for quantifying enhanced weathering. Maxbauer’s null now lands squarely between those two positions, and it strengthens the skeptical one.
6. The counter-argument
Steel-manned, the basalt defense goes like this. One site, one soil chemistry, one coarse grain size. The cation-storage mechanism from Kanzaki means basalt may be weathering invisibly, with removal delayed rather than absent. Finer grinding would speed dissolution. And Wolf’s reanalysis argues the earlier positive basalt results survive statistical scrutiny. All fair, and the coarse-grain caveat in particular means nobody should read this as “basalt does not weather.”
But the defense cuts both ways. “It might be working invisibly” is not a claim a registry can credit against. If the removal cannot be distinguished from zero with current MRV, then the credit rests on models, not measurement, and the Jordan framework says the models themselves predict wide variation in export depending on soil pH. The burden of proof sits with the sellers, and this trial shifts it further onto them.
7. Verdict
I do not read this as ERW failing. Something in Maxbauer’s trial did move carbon: slag, on acidic soil, under direct measurement. I read it as a warning that the sector standardized on a feedstock ahead of the evidence. The honest state of knowledge is that the field cannot yet reliably distinguish “basalt worked slowly here” from “basalt did not work here at all,” and credits should not be priced as if it can.
Two things will decide how this plays out. First, whether registries adopt feedstock-specific detection thresholds rather than treating basalt as the assumed default. Second, whether the Wolf-Derry dispute over Kantola 2023 forces a reanalysis of the small set of positive basalt field studies the sector’s confidence rests on. If that handful of studies wobbles, the 2024-2026 basalt contracts deserve a hard second look, and buyers should start asking for measured signals, not modeled ones.
Citations
- Cdrxiv — coarse-grained basalt produced no statistically detectable CO2 removal signal, while steel slag did
- Cdrxiv — soil cation storage is a key control on ERW dynamics
- Cdrxiv — theoretical framework showing alkalinity export through soil pore water is strongly pH-dependent
- Cdrxiv — signal-to-noise analysis of soil mass balance methods
- Cdrxiv — “Statistical analysis of basalt weathering disproves the null hypothesis”