13 June 2024

Metals in fertilisers impact global soils

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According to a study by Duke University, uranium, cadmium and chromium make fertilisers from the USA and the Middle East particularly impactful on soil quality

by Matteo Cavallito

Mineral phosphate fertilisers show the presence of toxic metals and this phenomenon is widespread worldwide. This is the conclusion of a study by a group of scientists from Duke University in Durham, North Carolina.

The investigation, explains a statement from the university, is based on a new method for detecting the spread and impact of these contaminants on soil, water resources and the food supply. The list of metals includes cadmium, uranium, arsenic, vanadium and chromium.

Isotopes as ‘fingerprints’

To assess toxic metal contamination, researchers have focused on a peculiar indicator: strontium isotopes, i.e. different versions of the same chemical element that differ in the number of neutrons in the nucleus. According to Avner Vengosh, chair of the Division of Earth and Climate Sciences at Duke University’s Nicholas School of the Environment, these isotopes can be seen as a ‘fingerprint’ that can reveal both contamination and its source.

Indeed, chemical analysis of each fertiliser shows a unique isotopic mixture that corresponds to the phosphorus-rich rocks from which it came.

“We measured strontium isotopes in both phosphate rocks and fertilizers generated from those rocks to show how fertilizers’ isotope ‘fingerprint’ matches their original source,” said Robert Hill, the study’s lead author and a PhD student at Duke University.
“Given variations of strontium isotopes in global phosphate rocks, we have established a unique tool to detect fertilizers’ potential impact worldwide.”

The study

To assess the reliability of isotopes as indicators of trace elements in fertilisers worldwide, researchers analysed 76 phosphate rocks and 40 fertilisers from major production areas. The scientists also collected samples from mines, commercial sources and the Tidewater Research Station, an experimental field in North Carolina.

“Consistent with the concentrations in parent phosphate rocks, phosphate fertilizers from the U.S. and Middle East have substantially higher concentrations of U, Cd, Cr, V, and Mo than those in fertilizers from China and India,” says the study published in the journal Environmental Science & Technology Letters.

Fertilisers from China and India, on the other hand, show higher concentrations of arsenic. The isotopes in phosphorous products, the researchers explain, “directly mimic the composition of their source phosphate rocks, with distinctive higher ratios in fertilizers from China and India relative to phosphate fertilizers from U.S. and Middle East.”

The impact of fertilisers

According to the researchers, the statement says, the high concentration of uranium, cadmium and chromium would make phosphate fertilisers from the US and the Middle East more impactful on soil quality than products from China and India. The study once again highlights the problem of the massive use of these chemicals in agriculture and its consequences for the climate and the environment.

In the crosshairs, for example, is the widespread use of fossil sources in the extraction and production of essential elements and substances such as hydrogen and ammonia that form the basis of nitrogen fertilisers. The latter, in particular, contribute to the contamination of watercourses as well as causing eutrophication (the excessive enrichment of nutrients) and generating health risks for animals and humans.