About 15% of world’s cropland polluted with toxic metals, say researchers
By: Evan Kudla
Peer-Reviewed Journal Article: https://www-science-org.proxy.lib.umich.edu/doi/10.1126/science.adr5214
News Article: https://www.theguardian.com/environment/2025/apr/17/about-15-world-cropland-polluted-toxic-metals-say-researchers
Background Information
The quality of soil is of vital importance, with it providing “the basis for nearly 95% of food consumed by human beings,” and as development continues to improves and population keeps growing, food production will need to increase between 35 to 56% by 2050 (2). While many factors are a threat to soil resources globally, including soil erosion, fertilizer and pesticide overuse, and pollution from industry, soil pollution from toxic metals is another factor that can lead to decreased crop yields and unsafe food (3).These toxic metals are non-degradable, which can allow them to accumulate in the soil over the span of many years (4).
Toxic metals in the soil come from both geogenic (natural from Earth) and anthropogenic (human-caused) sources. When looking at geogenic sources, it’s important to discuss the different layers of soil, which can be seen in Figure 1. Toxic metals can be commonly found in bedrock (natural soil parent materials), parent rock (basalt, shale), and some minerals (pyrite, sphalerite). These can contain elevated levels of toxic metals such as As, Cu, Cd, and Ni due to the high affinity of sulfur in the compounds for these different metals (5). These toxic metals can be released from these soil parent rocks from geological weathering and soil-forming processes and can accumulate in the soil by binding electrostatically with clay minerals or complexing with organic matter (6). Anthropogenic sources of toxic metals began at the start of the Bronze Age with the start of mining, which transfers large amounts of rock with high metal concentrations to the surface, which can ultimately lead to soil pollution from causes like runoff and irrigation of crops with polluted water (7). Other anthropogenic sources include products like certain fertilizers, paints, batteries, and industrial infrastructure like cables and even buildings (8).
Figure 1: Different Layers of Soil (9).
Finally, it’s important to understand that the concentrations of toxic metals in soil can also depend on a variety of outputs, where main outputs include processes like leaching, surface runoff erosion, plant uptake, and crop harvest. Toxic metals can also move between soil layers, such as from the C to the O layer, from the “plant-pump effect,” where as water is drawn up from plants from deeper soil layers, it can pull up toxic metals (10).
Peer-Reviewed Journal Article:
The article’s main goal was to systemically investigate the impact of 7 major toxic metals on soils across the globe: As, Cd, Co, Cr, Cu, Ni, and Pb. This was done by compiling datapoints from across many different studies and using these datapoints to convert into 10 km x 10 km toxic metals soil concentration grids that they then compared to agricultural thresholds (AT) and human health and ecological thresholds (HHET) for toxic metals concentration. After using their extremely randomized trees (ERT) machine learning model, they found that between 14 to 17% of surface soils in croplands had at least 1 toxic metals that exceeded the AT for crops. Cd had the highest global excessive rate (9%), with it being found most prevalently in places like northern India, Pakistan, Bangladesh, southern China, among others, and was due to both anthropogenic factors and geogenic enrichment. Ni and Cr saw exceedance rates mostly in the Middle East, subarctic Russia, and Eastern Africa areas, As in areas such as Southeast Asia, West Africa, and central South America, and Co in areas like the Democratic Republic of the Congo. Most of these high exceedance areas are due to both the geogenic background as well as mining activities (11,12).
Figure 2: Distribution of toxic metals in soil, with the color code showing the maximum probability of exceedance among the seven metals (1).
Figure 2 shows the probability of the soil exceeding the agricultural threshold, with red having at least one metal exceeding the threshold and blue being below the threshold. The areas most at risk include the Middle East, northern and central India, and southern China. The article found there to be a “metal enriched corridor” going across southern Europe, the Middle East, South Asia and southern China. The article proposes a likely explanation for this is that these regions are home to many different ancient civilizations, including the Roman Empire, Persian culture, and ancient India. These areas not only have geogenic sources of toxic metals but due to the development of these early civilizations, anthropogenic sources have played a huge role in the accumulation of toxic metals in the soils of this metal enriched corridor.
The article also investigates specific potential factors that may cause differing levels of toxic metals exceedance, including climate, topography, irrigation, and mining. From the machine-learning data, it was able to positively correlate toxic metals exceedance to monsoon climates in places like India, with these regions having more than double the global average amount of toxic metals exceedance. Heavy precipitation during the wet season leads to heavy weathering to release these toxic metals, while evapotranspiration during the dry season can concentrate these metals near the surface. Conversely, the cold and humid hemiboreal climate zone was negatively correlated, with it having less than half the average amount of toxic metals exceedance due to weaker weathering and plant-pump effects (13). Mainly, increased rainfall leads to higher toxic metal exceedance and increased frost leads to lower toxic metals exceedance. Additionally, topography factors like hilly mountain areas with steep slopes were also found to be positively correlated due to increased rock exposure and erosion factors, with increasing elevation and slope increasing the toxic metals exceedance (14). Finally, areas with intensive mining and surface irrigation also have more than twice the amount of toxic metals exceedance due to previously described mining effects and contaminated irrigation water.
Figure 3: Full details of each part is above, but overall is showing factors of climate, topography, and irrigated and mineral-rich regions on toxic metals exceedance (1).
Overall, the article found that a variety of factors related to soil metals exceedance, including weathering, plant-pump effects, mining, irrigation, climate, and topography. The main limitations of the study can be attributed to limited sample data in developing and remote areas, specifically in northern Russia, Central India, and Africa, and so data in those regions have higher levels of uncertainty. For the data 10 km x 10 km grids, they were based on average toxic metals concentration and don’t necessarily reflect on site-specific toxic metals concentration. In the future, toxic metal enrichment areas are only expected to grow from the switch to “green” technology, such as increasing demand of electric vehicles and wind turbines, and so the article concludes by urging necessary policy measures to protect global soil resources.
News Article
The news article by Sinéad Campbell in the Guardian starts by summarizing the main finding of the article that between 14 and 17% of cropland is contaminated with at least 1 toxic metal. It explains briefly that the data analysis was done through the collection of data from different regional studies as well as using machine learning. The article then uses quotes from a Dr. Liz Rylott, who is a professor of biology at the University of York. This was done as a source of credibility to help explain and emphasize the dangers that these toxic metals have on our health and environment as well as to emphasize how this problem is only going to get worse as the need for critical metals to fuel green technology increases. The article also briefly explains how these toxic metals can come from both natural and anthropogenic sources and how the toxic metals in the soil can decrease crop yields and contaminate water and food crops, with up to 1.4 billion people potentially impacted. There is also a brief mention that Cd is the most widespread toxic metal. It ends with the statement that toxic metals soil contamination is independent of borders, it impacts lower income countries disproportionately, and that cooperative action amongst nations is required to address the issue.
Analysis
Overall, I think the article does a subpar job at effectively conveying the journal article. First, I don’t like how in the sub header of the article it mentions As and Pb containing soils, as the study found Cd to have the highest exceedance, followed by Ni and Co. I feel like this was done as a way to grab people’s attention and doesn’t represent the emphasis in the journal article. I do think the article does a good job at explaining why and how these toxic metals are bad for us and the increasing issue that they pose for us in the future. The conclusion that toxic metals are independent of borders and that we need to work together is a decent summary emphasizing how it is a global problem of importance. My main issue with the article is that it leaves out a major portion of the study by not talking about the different factors that can influence toxic metals exceedance. All it mentions are that they come from both natural and human sources, which I feel is way too drastic of an oversimplification. I think it would have been beneficial to mention the metal enriched corridor from ancient civilizations as well as some basic information on how factors like climate, topography, and specific human activity like mining and irrigation can influence toxic metals exceedance in soil. I also thought interviewing someone that didn’t take part of the study worsens the credibility of the article and doesn’t allow for the researchers to emphasize what they found to be most significant about their study. Finally, I think that it would have been extremely helpful to include an image from the study of the distribution of toxic metals exceedance so people could see the areas of higher toxic metals exceedance.
References
(1) Deyi Hou et al., Global soil pollution by toxic metals threatens agriculture and human health. Science 388, 316-321(2025). DOI:10.1126/science.adr5214
(2) Food and Agriculture Organization of the United Nations,“Healthy soils are the basis for healthy food production” (FAO, 2015).
(3) Food and Agriculture Organization of the United Nations,“Saving our soils by all earthly ways possible” (FAO, 2022).
(4) D. Hou et al., Nat. Rev. Earth Environ. 1, 366–381 (2020).
(5) Food and Agriculture Organization of the United Nations and United Nations Environment Programme, Global Assessment of Soil Pollution (FAO and UNEP, 2021).
(6) B. J. Alloway, Ed., Heavy Metals in Soils: Trace Metals and Metalloids in Soils and their Bioavailability (Springer Dordrecht, ed. 3, 2013).
(7) A. A. Meharg, C. Meharg, Environ. Sci. Technol. 55, 7757–7769(2021).
(8) J. O. Nriagu, J. M. Pacyna, Nature 333, 134–139 (1988).
(9) “Soil Horizons,” Science Facts (2023); https://www.sciencefacts.net/soil-horizons.html
(10) M. Imseng et al., Environ. Sci. Technol. 52, 1919–1928(2018).
(11) British Geological Survey, World Mineral Production 2016–2020 (BGS, 2022).
(12) G. Gunn, Ed., Critical Metals Handbook (John Wiley & Sons, 2013).
(13) D. Hou et al., Global soil pollution by toxic metals threatens agriculture and human health, Dryad (2025); https://doi.org/10.5061/dryad.83bk3jb2z.
(14) W. E. Dietrich, J. T. Perron, Nature 439, 411–418 (2006).
(15) S. Campbell, “About 15 of world’s cropland polluted with toxic metals, say researchers,” The Guardian (2025); https://www.theguardian.com/environment/2025/apr/17/about-15-world-cropland-polluted-toxic-metals-say-researchers
Hi Evan, this is an interesting topic and a good summary! I'm familiar with aquaponics/hydroponics for sustainable food production but this is probably very susceptible to water quality and access, which is becoming a bigger issue with climate change. I've worked in remediation on soils polluted with heavy metals but typically only on industrial sites or near busy roadways, so thinking about food production in these spaces never occurred to me. You also make a really good point about the natural sources of these heavy metals becoming a bigger problem because of anthropogenic actions, so even if humans aren't directly introducing heavy metals to the environment their actions are still allowing more heavy metals to be produced/released.
ReplyDeleteI think the article mentioning lead and arsenic in the subheader is almost click-baity, since these are the heavy metals/contaminants that the public is most familiar with/worried about. I wonder if studies like this would encourage a better look at the public health implications of these other heavy metals, which the public might not currently be conscious of.
I also noticed that the heavy metals include some of the "RCRA 8" (arsenic, barium, cadmium, chromium, lead, mercury, selenium, and silver) but not all and include others. The RCRA 8 are going to be heavily monitored in the US but might not be in other countries, so maybe there's a lack of data available. Do you think the choice of heavy metals in this study were chosen based on availability of data or prioritization?
Hi Hunter, in the journal article itself, they don’t specify their reasoning for choosing the seven toxic metals that’s they did. However, in the supplemental material, they say that they chose the seven that they did because they represent important soil pollutants due to their toxicity, widely observed exceedance, and extensive anthropogenic activities that cause influx into the soil. They specifically say that mercury does meet the criteria as the seven toxic metals studied, but that since it has a much different transport mechanism due to its volatility, it wasn’t included. My guess is that the other metals you mentioned weren’t included because they were less significant to cropland soil pollution. For instance, I looked up that Ba has very low soil mobility and would only be released more under fairly acidic conditions (https://doi.org/10.1016/j.apgeochem.2017.05.002).
DeleteHi Evan, great analysis! You did an excellent job summarizing both the peer-reviewed article and the news piece, and your analysis clearly highlights the disconnect between the two. I really liked how you pointed out that the news article focused too heavily on attention-grabbing details, like As and Pb, rather than accurately representing the data showing Cd as the most prevalent contaminant. Your critique about the omission of important contextual factors—like the metal-enriched corridor and the influence of climate and topography—was insightful and showed a strong understanding of how these factors shape soil contamination patterns. I also appreciated your comments on credibility, noting that the journalist interviewed someone outside the study rather than a contributing researcher. First, would you happen to know how the “plant-pump effect” influences the vertical movement and accumulation of toxic metals in different soil layers, and could this process vary depending on the type of crop grown? Also, given that monsoon climates were found to have higher toxic metal exceedance, do you know what specific mechanisms during wet and dry seasons contribute most to the mobilization and concentration of these metals in agricultural soils?
ReplyDeleteHi Cody, the plant pump effect is when plants absorb water from deeper layers of the soil with their roots, they can uptake toxic metals as well from those lower layers, and can redistribute the toxic metals to the top layers of the soil when parts of the plant like leaves fall off and decompose. I would definitely say that the type of metal is very impactful on if it can be uptake by plant, with factors like mobility and solubility being major influences. I would say that for different crops, the further the roots go down into the soil, or more water uptake required by the crop, the more likely that the crop could uptake toxic metals that originate from these lower soil layers. There is definitely complex chemistry involved on whether these toxic metals remain in roots or get taken to other parts of the plant. For the monsoon climate, basically the article stated that the high precipitation and then subsequent high evapotranspiration are leading factors.The heavy rainfall causes heavy weathering and subsequent release of toxic metals from parent rock, and that evapotranspiration brings these metals close to the surface that are then left behind when the water evaporates.
DeleteHi Evan, really interesting and detailed work! I agree that the news article would've benefitted from an interview from one of the researchers, and that they didn't quite seem to understand what was most important from the study's findings. I know that human causes of cadmium are normally from things like batteries and other electronic waste, but I was wondering what the Ni and Co were mainly from. Also, I couldn't quite find what the plant pump effect is - the paper only seemed to say it existed, but did not elaborate on how it works. I thought it was very interesting that ancient civilizations could have influenced this type of pollution that we see today as well, and it is fun that history and science can have this unique type of collaboration. Finally, since green technology is expected to make the metal pollution problem worse, what policy-making and on-the-ground strategies do you think are viable in terms of reducing the effects?
ReplyDeleteHi Kevin, the plant pump effect is essentially when plants take in water from lower soil layers with their roots, that also taken in some toxic metals as well. When parts of the plant like the leaves fall off and decomposes, they can redistribute these toxic metals to the top layers of the soil. For policy making, I do know that it is crucial for there to be cooperative effect as toxic metals can travel across long distances. The journal article doesn’t mention specifics, but I do know that recycling end-of-life batteries and having a closed loop process is one approach.
DeleteHi Evan, great analysis! I was surprised that the news article was so short. I agree that the new article would benefit with the addition of a map of the toxic metals instead of only a link to the journal article because most readers are unlikely to click on an external link. The news article ends with a call to action saying these toxic metals have 'no borders' and are a global problem, but the article fails to sufficiently explain how toxic metals get into the soil which undercuts the strength of the call to action. There is a large part of the journal article that focuses on the sources of the toxic metals in the sources.
ReplyDeleteThe researchers used machine learning techniques in their study, I wonder if the continued advances in AI technology could improve the results of this study combined with an effort to fill in the data gaps.
Hi Evan! I really enjoyed reading your analysis of this peer-reviewed article. I agree that the news article did not properly discuss the major findings of the study overall which resulted in a lower rating.
ReplyDeleteYou briefly discussed green technologies like electric vehicles' and wind turbines' ability to reduce carbon emissions, but they increase demand for metals such as lithium, cobalt, and nickel. Mining and processing these metals can enrich soils with toxic metals, especially near extraction sites. Did the authors propose any mitigation strategies to address the increasing soil metal contamination associated with the growing demand for these technologies? Was there any discussion around current or emerging solutions, such as the use of cyclodextrins, for sequestering to remove heavy metals from the environment?
Hi Evan, great review! It is disappointing that the article was given a title that did not properly encompass what the original literature was communicating. It is also disappointing that the article skipped out on some key points about the different factors that cause toxic metals exceedance. I do have some questions about the toxic metal exceedance and how climates affect this quality. How is it that areas of frost have deceased correlation of toxic metal exceedance? What is the science behind this?
ReplyDeleteHi Sydney, the specific reasons that areas of frost have decreased correlation of toxic metals pollution exceedance is that the ground is frozen for several months of the year, which prevents any movement of the toxic metals from the lower soil levels to move closer to the surface. It also leads to less plant pump effect (toxic metals taken in by plants when they take in water with their roots) as well as slower chemical weathering of parent rocks that release these toxic metals, as these reactions are in part slowed due to decreased temperature.
DeleteHi Evan, this was a very good summary! This was particularly interesting because my article was also about toxic metals. I think this is a very interesting topic and I love seeing the maps showing how the metal concentrations in soil vary worldwide. I also agree with your analysis of the news article. One thing I wonder is what are some of the specific measures that can be taken to stop this toxic metal contamination of soil or to make it better. Is there a way to take metals out of the soil? I also wonder if there are any specific metals that are more problematic than others for the soil.
ReplyDeleteHi Amy, what I found was that toxic metals that are higher in solubility and mobility pose more of a threat because there is increased likelihood of these toxic metals getting into the top layers of soil. For instance, Cd has very high solubility and mobility, and this metal was found to be the toxic metal of highest exceedance in the soil. Cd can pose threats certainly in areas with water intensive farming like in southern China and growing rice. The journal article didn’t really mention any specific measures that could be taken to decrease toxic metals pollution exceedance contamination or remove them from the soil. Definitely being extremely vigilant in regulating mining and maybe implementing recycling methods to limit amount of mining required are some ways to focus on limiting toxic metals contamination.
DeleteHello Evan, I agree with you that it is unfortunate when articles bait there viewers by referencing things they already know about like Pb and As instead of what is crucially more important/relevant ie Cd, Ni, and Co. At the very least they could have put Cd then Pb but I digress. Your summary was fantastic along with the first figure was very helpful in understanding this problem. This is my first time seeing anything about heavy/toxic metals in farm soil as usually it is referenced to be in drinking water where I have seen it. I know you say mining leads to higher heavy metal concentration but I have a hard time understanding how that correlates with farm soil as from my understand the two aren't usually nearby each other or does it simply being disturbed caused the heavy metals to get released. Also do they ever say why the monsoons and heavy amounts of rain cause high contamination vs frost as I would expect these to have a similar problem with the erosion leading to release of metals.
ReplyDelete