One of the persistent concerns about enhanced rock weathering (ERW) is trace metals. You’re spreading crushed rock on farmland — what happens to the nickel, chromium, and other metals in that basalt?

A new preprint on CDRxiv offers some of the most detailed field data yet. Short answer: no detectable increase in harmful metals after two years.

The study

Researchers from Yale and collaborators applied 20 tonnes/hectare of iron- and aluminum-rich basalt to hayfield and pasture soils on a working Vermont dairy farm. They sampled agricultural soils (0–15 cm) twice before application and four times after, spanning fall 2022 through spring 2025. They also tracked a riparian corridor hydrologically connected to the treated fields.

Results

  • No increase in extractable Ni, Cr, Pb, Al, Mn, Zn, Cu, or Cd in treated agricultural plots versus controls.
  • Forage yields unchanged. Tissue metal concentrations in hay didn’t differ between treated and untreated plots.
  • Surprising downstream effect: Riparian soils showed 60–80% declines in extractable Ni, Al, and Pb, coinciding with a rise in Ca:Al ratios — consistent with basalt-derived solutes actually reducing metal mobility.

In other words, the basalt weathering products that moved downslope appear to have sequestered trace metals rather than mobilized them.

Why this matters for ERW deployment

Trace metal risk is a legitimate concern that critics raise — and rightly so. Iron-rich basalts contain nickel and chromium at concentrations that warrant monitoring. This study, conducted at realistic application rates on a real farm over multiple seasons, provides strong evidence that agronomic-rate applications pose minimal risk.

The paper also makes a practical MRV recommendation: track soil pH and Ca:Al ratios as indicators of metal availability, and consider hydrologic connectivity when designing monitoring programs.

It’s one study, one soil type, one basalt source. But it adds to a growing body of evidence that ERW can be deployed responsibly — provided you monitor properly and choose your feedstock carefully.

Source: Zacharias et al. (2026), CDRxiv preprint #463

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