Water erosion is not an abstract threat in this part of the world. Roughly 40 percent of Moroccan land is threatened by the process, and the Rif Mountains, which occupy just 6 percent of the national territory, contribute about 60 percent of the sediment reaching the country’s reservoirs. According to Morocco’s National Watershed Management Plan, annual soil losses exceed 20 tonnes per hectare per year in the Rif proper and fall between 10 and 20 tonnes per hectare per year in the Pre-Rif transition zone where the El Harrar catchment lies. The study’s authors, Bahija Filal, Mohammed Abahrour and Mostafa Hmamouchi, note that these figures reflect the interaction of steep dissected relief, fragile clay and sandstone geology, sparse natural vegetation and intensive agricultural exploitation concentrated on hillsides, a combination that makes the region one of the most erosion-prone landscapes in the Mediterranean.

The team’s approach centered on the soil erodibility factor, known in erosion science as the K factor, which expresses a soil’s inherent susceptibility to particle detachment and transport by rainfall and surface runoff. Unlike rainfall erosivity or slope steepness, the K factor is a property of the soil itself, determined by texture, organic matter content, structure and permeability. The researchers drew on the Wischmeier nomograph, a method developed in the early 1970s that relates these four soil characteristics through a texture parameter calculated as the product of silt plus very fine sand content and the complement of clay content, combined with USDA-based codes for soil structure and permeability classes. Structure codes range from very fine granular to blocky arrangements, while permeability codes span from rapid infiltration exceeding 60 millimeters per hour to very slow rates below 1 millimeter per hour.

To apply this framework across the watershed, the team digitized seven major soil units from the 1:50,000 Taza Soil Map into ArcGIS Pro: Lithosols, Regosols, weakly developed alluvial soils, weakly developed colluvial soils, Vertisols, calcimagnesic soils and Fersiallitic soils. Because the El Harrar watershed forms part of the larger Oued Lahdar basin, the researchers were able to assign previously published physicochemical data and K-factor values from that basin to the corresponding soil classes identified on the map. The matching procedure was verified through field observations and high-resolution satellite imagery, though the authors acknowledge that relying on legacy analytical data means the assessment operates at the soil-unit level rather than from fresh laboratory measurements of individual samples.

The resulting erodibility map reveals a striking spatial divide. Calcimagnesic soils, which cover 45.15 percent of the watershed, show the lowest erodibility with a K value of 0.23, thanks to relatively high organic matter content of 2.01 percent and clay content of 38.5 percent. At the opposite extreme, Fersiallitic soils occupying 23.78 percent of the catchment on the slopes of Jbel En nehir register the highest erodibility at K = 0.46, reflecting their lower organic matter of 0.95 percent and reduced clay fraction of 28.4 percent. Between these poles, moderate erodibility characterizes colluvial soils and Lithosols covering 14.64 percent of the area, while high erodibility marks Vertisols, alluvial soils and Regosols that together span 16.44 percent, distributed along stream banks and in the southern portion of the watershed.

Statistical analysis of the seven soil units confirmed the physicochemical logic behind these patterns. Organic matter showed the strongest negative correlation with the K factor at R = −0.70, indicating that soils rich in organic matter are substantially more resistant to particle detachment. Clay content followed with R = −0.43, consistent with the role of clay particles in promoting cohesion and structural stability. The silt plus very fine sand fraction showed only a weak negative correlation at R = −0.19. The authors emphasize that these coefficients, calculated from representative values for just seven mapped soil units, should be read as exploratory indicators of soil-unit-level trends rather than causal proof, and they call for additional field sampling and laboratory work to strengthen the statistical evidence.

Field observations provided tangible confirmation of the mapped patterns. In areas classified as highly and very highly erodible, the researchers documented rill erosion, gully erosion, bank erosion and slope degradation, aligning the intrinsic K-factor map with visible on-the-ground erosion features. The observed K range of 0.23 to 0.46 sits within the spectrum reported for other watersheds in northern Morocco, including values of 0.00 to 0.58 in the El Maleh catchment and 0.27 to 0.46 in the Ghrala watershed, though it is narrower than the ranges of 0.10 to 0.82 and 0.11 to 0.82 reported for the Amzaz and Asra watersheds respectively. The authors attribute these differences to variations in parent material, soil composition, topography and land cover that directly shape how erodibility is distributed across landscapes.

The findings carry practical weight for land managers grappling with soil degradation in the Eastern Pre-Rif. Because organic matter emerges as the single most influential property governing erodibility in this catchment, conservation strategies that boost soil organic content, whether through organic amendments, vegetation cover restoration or reduced tillage, would directly strengthen the structural stability of the most vulnerable soils. The authors also recommend protecting and rebuilding vegetation on cultivated slopes, constructing and maintaining terraces where agriculture persists on steep terrain, and incorporating soil erodibility maps into territorial planning and environmental decision-making to ensure that limited conservation resources are directed toward the areas where they will yield the greatest benefit.

Beyond its regional significance, the study contributes to a broader scientific conversation about how intrinsic soil properties interact with climatic and anthropogenic pressures to determine erosion outcomes. Previous research has shown that increasing organic matter improves aggregate stability, strengthens bonding between particles and enhances infiltration, all of which reduce the ease with which rainfall and runoff can detach and transport soil. Clay content can similarly benefit cohesion and structural stability, though its contribution depends on clay mineralogy and interactions with other soil properties. By mapping the K factor and linking it statistically to the physicochemical characteristics that underlie it, the El Harrar study offers a template for watershed-scale assessments elsewhere in the Mediterranean and beyond, demonstrating that understanding why soils differ in their resistance to erosion is as important as measuring how fast they are being lost.

The authors conclude that the soil erodibility map of the El Harrar watershed provides a scientific basis for incorporating soil physicochemical information into erosion risk assessment and land management strategies in the Eastern Pre-Rif. As Mediterranean countries confront intensifying erosion pressures under changing climate and land-use regimes, the ability to identify priority areas for soil conservation at the soil-unit level, rather than relying solely on modeled soil-loss rates, represents a meaningful advance in the toolkit available to safeguard one of the region’s most threatened natural resources.

Subject of Research: Mapping the spatial variability of soil erodibility based on physicochemical properties in the El Harrar watershed, Eastern Pre-Rif, Morocco

Article Title: Mapping soil vulnerability based on the soil erodibility factor (K) in the El Harrar Watershed (Eastern Pre-Rif, Morocco)

Article References: Filal, B., Abahrour, M., & Hmamouchi, M. (2026). Mapping soil vulnerability based on the soil erodibility factor (K) in the El Harrar Watershed (Eastern Pre-Rif, Morocco). Frontiers in Soil Science, 6, Article 1891092. https://doi.org/10.3389/fsoil.2026.1891092

Image Credits: AI Generated

DOI: 10.3389/fsoil.2026.1891092

Keywords: soil erodibility factor, K factor, water erosion, El Harrar watershed, Eastern Pre-Rif, Morocco, organic matter, clay content, GIS mapping, soil conservation, Mediterranean soils, Wischmeier nomograph

The stakes are considerable. Soil maps built on harmonized terminology are indispensable for capturing spatial variation in soil cover, supporting land-use planning, and integrating national data into international frameworks. Because soil surveys across Europe were conducted independently, converting national legends into a common classification has become a continent-wide challenge. The World Reference Base, or WRB, does not seek to replace national systems; it serves as a reference frame for correlating soils internationally. But as the new study, published in the Spanish Journal of Soil Science, demonstrates, a straightforward one-to-one conversion of Portuguese soil units into WRB categories is simply not possible for most of the country’s classification hierarchy.

The researchers, Manuel Madeira, Samuel Guerreiro, Pedro Arsénio, and Vasco Florentino, focused on roughly 47,555 square kilometers of southern and central Portugal, spanning the Algarve, Alentejo, the Lisbon metropolitan area, and part of the Central region. The area’s climate is predominantly Mediterranean — hot, dry summers and mild, wet winters — with annual rainfall ranging from 400 to 1,000 millimeters. Geologically, it encompasses the ancient Iberian Massif with its Precambrian and Paleozoic metamorphic and igneous formations, alongside younger Meso-Cenozoic sedimentary basins. This diversity of parent materials, relief, and climate produces a wide array of soils, and it is precisely this complexity that the 1974 classification struggled to capture in terms compatible with modern diagnostics.

The Classification of the Soils of Portugal is a hierarchical system with five categorical levels: orders, suborders, groups, subgroups, and families. Its ten orders include Incipient soils, Litholic soils, Calcareous soils, Mollic soils, the evocatively named Barros — shrinking and swelling clay soils — Argilluviated soils, Podzolized soils, Halomorphic soils, Hydromorphic soils, and Hydromorphic organic soils. The system was conceived before sufficient data and modern processing tools existed, and its definitions lack the quantitative diagnostic criteria and thresholds that the WRB demands. The 1:25,000 soil map built on it relies on scarce, non-georeferenced reference soil profiles, and surveyors of the era applied subjective judgments about soil depth and parent material that varied from one mapper to another.

To test how well the old system maps onto the new, the team compared the classification of 186 legacy reference soil profiles under both systems. The results were striking. No CSP order achieved a perfect one-to-one match with any WRB Reference Soil Group. The strongest correspondence belonged to the Barros order, whose shrinking and swelling clay soils matched Vertisols in 83 percent of profiles. Among suborders, Colluvial soils corresponded entirely to Regosols, and Humic Litholic soils matched Umbrisols in 78 percent of cases. Everything else fractured. Lithosols, defined vaguely as soils with effective depth under 10 centimeters, matched Leptosols in only 24 percent of map units, with the remainder splitting among Cambisols, Regosols, and other groups.

Some mismatches proved scientifically revealing. The CSP’s Argilluviated soils — soils with clay-enriched subsoils — corresponded to Luvisols in only 40 percent of cases, because the criteria for defining an argillic horizon differ from those for the WRB’s argic horizon, and because lithic discontinuities in sedimentary parent materials were not considered in the old system. Podzolized soils matched true Podzols in just 23 percent of map units, since the Portuguese definition allowed soils without the characteristic ortstein layers to be included, and no oxalate-extractable aluminum and iron determinations were ever performed to confirm spodic horizons. Calcareous soils corresponded largely to Cambisols rather than Calcisols, because most lacked a true calcic horizon, their carbonates being simply inherited from the parent rock.

Recognizing that direct translation was failing, the researchers turned to soil-forming factors — the classical pillars of pedology. They framed the legacy soil profiles within landscape units defined by lithological groupings, relief classes derived from a digital terrain model, and climate classes based on temperature data from 1981 to 2010. This environmental framing dramatically improved correspondence. Landscapes of granite and related rocks proved to be 73 percent Regosols with 27 percent Cambisols, matching the near-exclusive dominance of Non-Humic Litholic soils in the old map units. Diorite landscapes were 77 percent Luvisols, mirroring the dominance of Argilluviated soils. On mafic rocks, Vertisols occupied 60 percent of the landscape, echoing the Barros order’s 60 percent share.

The environmental approach also exposed errors in the old maps. In schist landscapes, the 1:25,000 map showed only Argilluviated soils and Lithosols, yet the profile data revealed that Luvisols, Cambisols, Regosols, and Leptosols all occur there — meaning both Lithosols and Argilluviated soils were overrepresented. Similarly, in sandy coastal landscapes, Podzols turned out to be far less extensive than earlier maps suggested, occupying mainly plains while only Arenosols appear on strongly sloping terrain. Relief proved to be a powerful discriminator: Regosols and Leptosols increase from undulating to steep areas, while Planosols, Stagnosols, and Solonetz appear only in plains or gently undulating landscapes.

Beyond assigning Reference Soil Groups, the WRB system allows soils to be refined with principal and supplementary qualifiers — descriptors such as Leptic, Skeletic, Calcaric, Vertic, Eutric, or Clayic that capture specific properties. The team found that many of these could only be tentatively assigned, because the CSP never quantified soil depth, coarse fragments, abrupt textural differences, or exchangeable aluminum. Still, clear lithological patterns emerged: the Leptic qualifier tracks parent material, declining from Regosols to Cambisols to Luvisols on the same rock type; the Calcaric qualifier clusters on non-compact limestones; the Vertic qualifier on mafic rocks; and the Clayic qualifier on clayey and fluvial materials. Even the old map’s soil phases offered clues — thick soil phases reliably indicated the absence of the Leptic qualifier.

The study concludes that at least 24 of the WRB’s 32 Reference Soil Groups likely occur in Portugal, with 22 on the mainland alone, underscoring the exceptional diversity of Mediterranean soils shaped by varied parent materials, landforms, and centuries of human activity. The proposed methodology offers a viable path toward a unified national 1:100,000 soil map in the WRB system, but the authors are candid about its limits: with no funding for new fieldwork, legacy data alone cannot resolve every ambiguity, and decades of land-use change — irrigation expansion, olive groves, forest plantations, wildfires — may have altered soil profiles since the original surveys. They argue that Portugal now faces a choice: patch the 1974 classification or rebuild it with quantitative, internationally correlated criteria. Either way, the conversion exercise is more than a bureaucratic translation — it is an opportunity to reimagine the country’s soil geography for the era of digital soil mapping and EU-wide harmonization.

Subject of Research: Converting Portugal's legacy genetic soil classification to the World Reference Base for updating national soil mapping

Article Title: Converting the genetic-based national soil classification to the World Reference Base: a challenge for updating soil mapping in Portugal

Article References: Madeira, M., Guerreiro, S., Arsénio, P., & Florentino, V. (2026). Converting the genetic-based national soil classification to the World Reference Base: a challenge for updating soil mapping in Portugal. Spanish Journal of Soil Science, 16, Article 16368. https://doi.org/10.3389/sjss.2026.16368

Image Credits: AI Generated

DOI: 10.3389/sjss.2026.16368

Keywords: soil classification, World Reference Base, Portugal, soil mapping, legacy soil data, Reference Soil Groups, digital soil mapping, pedology, Mediterranean soils, soil-forming factors, Vertisols, Luvisols

The study, led by Shikma Zaarur of the Hebrew University of Jerusalem together with colleagues at the Agricultural Research Organization’s Volcani Institute, the Geological Survey of Israel and Ben-Gurion University of the Negev, was designed to simulate the relentless potassium drain of intensive agriculture. The team filled ten-liter pots with three contrasting alkaline soils from Israel: a clayey soil from the coastal plains, a loess sandy loam from the western Negev, and a red sandy soil from Rehovot. Peruvian alfalfa, one of agriculture’s most potassium-hungry crops, was sown in April 2024 and harvested sixteen times over roughly thirteen and a half months, with each cut removing potassium from the system. All essential nutrients except potassium were supplied through drip irrigation, so potassium was the only element expected to become limiting in the unfertilized treatments. A parallel set of pots received potassium chloride fertilizer, split into two applications.

Soil potassium exists in a hierarchy of reservoirs that differ enormously in size and accessibility. The smallest, potassium dissolved in the soil solution, represents only about 0.1 to 0.2 percent of total soil potassium, but it is the only pool plants can absorb directly. The exchangeable pool, one to two percent of the total, consists of potassium adsorbed onto clay surfaces and some interlayer sites, and it is what the standard ammonium acetate extraction measures. Below that sits interlayer potassium, locked between the sheets of 2:1 clay minerals such as illite and illite-smectite, which can account for up to ten percent of total soil potassium. Finally, structural potassium, hosted in micas and potassium feldspar, makes up 90 to 98 percent of the total and is considered almost inert. Fertilizer recommendations worldwide treat the exchangeable pool as the soil’s capacity to feed a crop, effectively ignoring everything beneath it.

The mass-balance results shattered that assumption. In the unfertilized pots, cumulative potassium uptake reached about 7.3 grams per pot in both the clayey and loess soils and 4.4 grams in the red sandy soil, while the initial exchangeable potassium plus the small background input from irrigation water could account for only a fraction of those amounts. Total uptake was 2.0 to 3.9 times greater than the estimated supply from exchangeable potassium and natural inputs. Non-exchangeable pools supplied 67 percent of uptake in the clayey soil, 50 percent in the loess, and a striking 74 percent in the red sandy soil. Direct measurements of total soil potassium at the end of the trial confirmed the depletion: total potassium fell from 88 to 75 grams per pot in the clayey soil, from 75 to 65 in the loess, and from 31 to 28 in the red sandy soil, with the rhizosphere cores showing even greater losses than the mass balance predicted.

To trace where the missing potassium came from, the researchers exploited a geochemical fingerprint. Rubidium substitutes for potassium in mineral structures because the two ions carry the same charge and nearly the same radius, but different minerals incorporate rubidium in different proportions, giving each mineral source a characteristic potassium-to-rubidium ratio. As plants depleted the exchangeable pool, its potassium-to-rubidium ratio shifted dramatically toward the signature of the fine clay fraction in all three soils. In the loess soil the ratio plunged from 3664 at the start to 284 by the eighth harvest; in the red sandy soil it fell from 547 to 154 within the same period. The exchangeable pool was converging on the composition of the clay minerals themselves, indicating that illite and illite-smectite interlayers, not potassium feldspar, were the dominant suppliers.

The mineralogy of each soil also governed how much the crop responded to added fertilizer. In the clay-rich soil, where abundant mixed-layer illite-smectite buffers the soil solution, fertilization produced only a marginal and statistically insignificant biomass gain of 5.3 percent across sixteen harvests. The loess soil responded with a significant 5.5 percent increase, but only from the eleventh harvest onward, once the plants had drained the readily available potassium. The potassium-poor red sandy soil responded earliest and most strongly, with fertilization boosting cumulative biomass by 13.5 percent, significant at most harvests from the third cut onward. This pattern mirrors decades of puzzling field observations in which crops on alkaline soils showed little or no yield response to potassium fertilizer despite soil tests suggesting deficiency.

Perhaps the most provocative finding concerns the speed of mineral weathering. Using a first-order approximation, the team calculated the illite dissolution rate needed to sustain the observed potassium release. If all the non-exchangeable potassium had come from illite, between roughly 17 and 34 grams of the mineral would have had to weather in each pot over a single year. That translates to dissolution rates of about 10 to the minus 11 to 10 to the minus 12 moles per square meter per second, depending on assumed grain size, which is two to three orders of magnitude faster than natural illite weathering rates and far above laboratory rates measured at neutral pH. The implication is that intensive cropping creates a biological sink that pulls mineral dissolution forward, accelerating weathering of potassium-bearing clays to rates relevant to a single growing season, a phenomenon usually assumed to require millennia.

The mechanism likely lies in the rhizosphere. Alfalfa, with its enormous biomass and repeated cutting schedule, can remove around 200 kilograms of potassium per hectare annually. As roots strip potassium from the soil solution, chemical equilibria shift, and the researchers suggest that root exudation of organic acids may further enhance potassium release from interlayer sites. Consistent with this, recent work has shown that alfalfa roots can trigger significant potassium release from micaceous interlayers within just four months. The exchangeable pool in the study soils did not simply drain to zero; instead it stabilized at a dynamic minimum, a steady state where the rate of mineral release matched the rate of plant removal. That minimum differed for each soil and was controlled by mineral composition and plant demand, meaning the exchangeable test captures only a snapshot of a moving target.

The practical consequences are substantial. If exchangeable potassium tests underestimate plant-available potassium by a factor of two to four in these alkaline Mediterranean soils, then fertilization recommendations built on those tests risk both unnecessary applications on clay-rich soils, where the mineral buffer is strong, and potentially misleading deficiency diagnoses elsewhere. The authors note that the degree of underestimation is likely governed by the abundance of potassium-bearing clay phases, particularly illite and illite-smectite, which are common in eastern Mediterranean soils partly because of mineral-rich dust inputs. Long-term field experiments in Israel, spanning roughly thirty years, have similarly reported little response to potassium fertilization in cotton and wheat grown on soils with comparable clay mineralogy, lending field-scale support to the greenhouse findings.

The study does not mean potassium fertilization can be abandoned. Plants in the unfertilized pots eventually developed classic deficiency symptoms, including chlorosis, necrotic leaf margins and stunted growth, and tissue potassium concentrations fell below the roughly 1.1 percent sufficiency threshold for alfalfa by the end of the trial in all soils. Rather, the message is that the exchangeable test reflects only one slice of a dynamic system in which clay minerals act as both sink and source, fixing potassium when supply is abundant and releasing it when demand is high. Accurate potassium management, the researchers argue, requires methods that account for the mineralogical buffering capacity of soils, ensuring that potassium is applied where it is genuinely needed and conserved where the ground itself can keep feeding the crop.

Subject of Research: Non-exchangeable potassium release from clay minerals sustaining plant potassium uptake in alkaline Mediterranean agricultural soils

Article Title: Non-exchangeable K sustains plant K uptake in alkaline Mediterranean soils

Article References: Zaarur, S., Erel, R., Bier, A. K., Rivlin, A., Nahari, I., Morag, N., Yeshno, E., & Naon, L. (2026). Non-exchangeable K sustains plant K uptake in alkaline Mediterranean soils. Plant and Soil. https://doi.org/10.1007/s11104-026-09118-x

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09118-x

Keywords: potassium, soil fertility, clay minerals, illite, illite-smectite, mineral weathering, alfalfa, fertilization, Mediterranean soils, exchangeable potassium, K/Rb ratio, plant nutrition

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