Soil testing
For the first time, scientists have produced a detailed global map of toxic metals in soils, revealing that the problem may be far larger and more widespread than previously recognized.
The new analysis, published in Science, combines nearly 800,000 soil samples from around the world with machine-learning techniques to identify hotspots of heavy metal contamination, including arsenic, cadmium, lead, and nickel.
The findings suggest that 14% to 17% of the world’s agricultural land may be contaminated with potentially harmful levels of at least one toxic metal.
Worse still, the researchers estimate that between 900 million and 1.4 billion people are living in regions of elevated risk.
“This is a wake-up call,” says Matthias Wiggenhauser, an environmental scientist at ETH Zürich. “The extent of the problem seems to be larger and more global than many of us had thought.”
The contamination comes from a combination of sources, both ancient and modern. Human societies have been mining, smelting, and working metal for millennia.
These activities have left a toxic legacy in the soil, especially in regions with long industrial histories.
At the same time, natural processes such as the erosion of mineral-rich bedrock continue to release toxic elements like cadmium and arsenic into the environment.
To create the global map, researchers led by Deyi Hou at Tsinghua University compiled 1,493 regional studies and analysed 796,084 soil sample points.
Then they applied machine-learning algorithms to extrapolate metal concentrations in places with no direct measurements.
The result is a predictive map of global soil metal contamination, divided into 10-kilometer-wide grid squares.
The most widespread culprit? Cadmium.
Found in zinc ores and used in rechargeable batteries, pigments, and coatings, cadmium topped agricultural safety thresholds in about 9% of soil samples. High levels were found in regions across Asia, Africa, and Latin America.
Importantly, researchers caution that total soil concentrations don’t always equate to direct health or crop risks.
Factors such as soil pH, climate, and the chemical form of the metals affect whether the contaminants can be absorbed by plants or enter the human body. Still, experts agree the study is a vital starting point.
“This is a monumental effort,” says Andy Meharg, a biogeochemist at Queen’s University Belfast. “It provides an essential global baseline that should help guide future research, risk assessments, and policymaking.”
The findings come at a time of renewed international focus on soil health.
As demand rises for critical metals used in clean energy technologies, the researchers are urging governments to conduct more widespread soil sampling, especially in under-surveyed areas like sub-Saharan Africa, and to prioritise soil pollution in environmental regulations and development agendas.
“Soil is often forgotten,” says Hou. “But without healthy soil, we can’t produce safe food, and we can’t protect human health. It’s time to take this threat seriously.”
By Jed Thomas
IET 36.3 May
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