Compost Keeps Locking Cobalt in Soil a Decade After Farmers Stop Spreading It

Cobalt rarely makes headlines, yet the silvery metal sits at the heart of the modern green economy. It hardens the superalloys in jet engines, stabilizes the lithium batteries in electric cars, and threads through the electronics that define contemporary life. That same strategic value, however, is driving more cobalt into the environment than ever before, and soils are where much of it ends up. A new study from Italian soil scientists now shows that one of agriculture’s oldest tools, compost, can keep this potentially toxic metal locked away in the ground for years after the last application, offering a surprisingly durable shield against contamination.

The research, published in the journal Discover Soil by Francesca Pedron and Gianniantonio Petruzzelli of Italy’s National Research Council’s Institute of Research on Terrestrial Ecosystems in Pisa, tackles a question that has been largely ignored: what happens to a soil’s ability to trap cobalt long after compost spreading has stopped? Most studies of organic amendments focus on soils that are actively being treated. The Italian team instead examined soils from a single Tuscan farm where compost applications had ceased either five or ten years earlier, comparing them with a neighboring field that had never received compost at all.

Cobalt occupies an unusual position among trace metals. In tiny amounts it is essential, serving as the metallic core of vitamin B12 and supporting crop growth, but above certain thresholds it becomes harmful to plants, disrupting nutrient transport within cells and producing symptoms such as leaf yellowing, premature leaf drop, reduced photosynthesis, and stunted biomass. It enters soil from natural events such as volcanic eruptions and forest fires, and from human activities including mining, smelting, road traffic, and the manufacture of cobalt-containing alloys and chemicals. Material flow analyses indicate that its dispersion in the environment is increasing as demand for batteries and green technologies accelerates, and recent machine-learning maps of European soils have revealed rising cobalt concentrations in some agricultural areas.

Whether that rising cobalt actually threatens crops and groundwater depends on a set of chemical reactions collectively called sorption, the transfer of dissolved metal ions from soil water onto solid soil surfaces. A soil with strong sorptive capacity holds cobalt immobilized, keeping it away from plant roots and aquifers. A soil with weak sorption lets the metal remain in solution, where it can be taken up by crops or leached away. The researchers hypothesized that compost-induced changes in soil organic matter and acidity would persist long enough to maintain enhanced cobalt sorption for a decade or more, and their experiments were designed to test exactly that.

The three soils came from the plough layer of a farm in the Province of Pisa, in a flat alluvial plain with a Mediterranean climate. Two of them had received green compost, produced from garden trimmings, crop residues, and other plant waste, for eight consecutive years at a rate of five megagrams of dry matter per hectare per year. On one plot, applications stopped ten years before sampling; on another, five years before. The third plot had never been treated. Texture, iron content, and total cobalt concentration were nearly identical across all three soils, but the chemical fingerprints of compost were unmistakable. Organic matter had risen from 1.15 percent in the untreated soil to 2.08 percent in the ten-year-old plot and 3.13 percent in the five-year-old one, while pH had shifted from a slightly acidic 6.25 to near-neutral values around 7.0. Cation exchange capacity, a measure of the soil’s ability to hold positively charged ions, climbed in parallel.

Those differences translated directly into cobalt retention. In laboratory batch experiments, the team shook one-gram soil samples with cobalt nitrate solutions of increasing concentration for twelve hours at controlled temperatures, then measured how much metal disappeared from solution. Fitting the resulting curves to the Langmuir isotherm model revealed maximum sorption capacities of 222 millimoles per kilogram in the five-year soil and 196 in the ten-year soil, against just 141 in the untreated control. The shape of the isotherms, an L-type pattern, showed that at low cobalt concentrations sorption rose almost linearly, indicating that high-affinity binding sites dominate initially; only as those sites fill do weaker domains take over, flattening the curve toward a plateau.

The mechanism behind this enhanced retention lies in the chemistry of compost-derived organic matter. Humified materials introduce abundant carboxyl, phenolic, and hydroxyl functional groups that form stable complexes with divalent cations such as cobalt. The near-neutral pH produced by composting amplifies the effect in two ways: it reduces competition between cobalt ions and hydrogen ions for reactive surfaces, and it encourages the deprotonation of those functional groups, making them more negatively charged and chemically reactive. Experiments that varied the ionic strength of the background solution added a further clue. Sorption in the untreated soil dropped by 40 to 48 percent as ionic strength increased, a signature of weak, electrostatically held outer-sphere complexes. In the compost-amended soils the decline was far smaller, 18 to 22 percent in the five-year soil and 28 to 36 percent in the ten-year one, pointing to a predominance of inner-sphere complexes, in which cobalt coordinates directly and strongly with organic and mineral surfaces.

Thermodynamic analysis reinforced the picture. Across temperatures from 298 to 318 kelvin, the Gibbs free energy of sorption was negative in all soils, confirming a spontaneous process, but the values were most favorable in the compost-amended plots. Enthalpy changes were positive, indicating an endothermic process that likely reflects the energy cost of partially dehydrating cobalt ions and forming specific surface complexes, and entropy changes were negative, consistent with metal ions becoming immobilized on ordered surface sites. The distribution coefficient, a ratio that summarizes how strongly a soil partitions cobalt between solid and solution phases, told the same story: differences among the soils were largest at low cobalt concentrations, precisely the range most relevant to real agricultural contamination, and converged only as high-affinity sites approached saturation.

Perhaps the most striking evidence came from desorption experiments, in which previously loaded soils were challenged with a harsh 0.5 molar calcium chloride solution designed to displace loosely held metal. In the untreated soil, between 1.6 and 24 percent of the sorbed cobalt was released, depending on loading. In the ten-year compost soil the released fraction ranged from 1.2 to 18 percent, and in the five-year soil from just 0.6 to 11 percent. At low cobalt loadings, desorption was minimal in all soils, confirming that the first metal ions to arrive occupy high-affinity sites and are effectively locked in place. The cobalt that resisted even this aggressive extraction can reasonably be considered non-bioavailable, meaning it is unlikely to reach plants or soil microorganisms.

The authors are careful to note the limits of their work. The compost’s own cobalt content was never measured because Italian law does not require it, so a complete mass balance was impossible, although total soil cobalt did not differ among the three plots, suggesting the compost added no detectable metal. Laboratory conditions, with fixed temperatures and equilibration times, cannot fully reproduce the complexity of field soils, including microbial activity, and the results apply specifically to the soils studied rather than to all soil types. Long-term effects will also depend on compost composition, climate, and farming practices. Even so, the central finding stands: in Mediterranean soils, where organic matter decomposes slowly enough to persist for decades, a single decade of composting left a chemical legacy that still suppressed cobalt mobility ten years later. As cobalt demand surges with the ecological transition, the study suggests that the humble practice of returning organic waste to farmland is not merely a fertility tool but a long-term instrument for keeping a critical, and potentially toxic, metal out of the food chain.

Subject of Research: Long-term cobalt sorption and desorption behavior in aged compost-amended Mediterranean agricultural soils

Article Title: Cobalt sorption in aged compost-amended soils

Article References: Pedron, F., & Petruzzelli, G. (2026). Cobalt sorption in aged compost-amended soils. Discover Soil, 3(1), Article 109. https://doi.org/10.1007/s44378-026-00273-2

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00273-2

Keywords: cobalt, compost, soil organic matter, sorption, desorption, Langmuir isotherm, heavy metals, soil chemistry, Mediterranean soils, bioavailability, trace metal contamination, sustainable soil management