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News Story
4 December 2020

The hidden reality of soil pollution calling plants to the rescue

An agroecological farm in Nicaragua.

Photo: Henrik Haller.

Millions of people live and grow food on contaminated soil, often unsuspectingly of its health risks and without taking any measures to avoid exposure. Some farmers know that their soil is polluted, but their need to make a living leaves them little choice.

More than 22 million hectares, an area larger than Uruguay, are known to be affected by soil pollution globally. However, the real number is likely to be much higher, since most polluted sites are not registered and in many countries, soil pollution monitoring is still in its infancy.

Common causes of soil pollution in agricultural soil include industrial activity and waste disposal, but also excessive use of agrochemicals like herbicides and insecticides as well as from application of impure phosphorous fertilizers, resulting in health problems that range from cancers (arsenic, asbestos, dioxins), to neurological damage and lower IQ (lead, arsenic), kidney disease (lead, mercury, cadmium), and skeletal and bone diseases (lead, fluoride, cadmium).

Henrik Haller, a senior researcher and teacher at the ecotechnology department of Mid Sweden University, is convinced that phytoremediation, a way to remedy the polluted soil with plants and associated soil microbes, can help us avoid these health hazards. If carefully done, phytoremediation can provide a way to safely produce food on polluted soil, restricting food chain contamination, while gradually reducing the pool of pollutants in the soil.

“In an ideal world, we wouldn’t cultivate polluted agricultural fields, or better still, the land wouldn’t have been polluted in the first place. But this is not the case, and we need to adapt to reality by developing ways to safely produce food even in polluted soil and do low-cost sustainable soil clean-ups prior to food production in these places,” says Henrik Haller.

For the last 18 years, he has been running an agroecological farm in Nicaragua, where he has been experimenting with phytoremediation, including microbial degradation of diesel fuel and extraction of organochlorine pesticides.

Henrik Haller is sampling soil at his farm.

Photo: Henrik Haller.

Why phytoremediation?

Excavation and removal of the contaminated soil, blending with clean soil from off- and on-site or covering polluted soil with clean all are different soil remediation options, but these require heavy machinery and their costs range from $1000 – $100,000 per acre, which are hard to get if you are a smallholder farmer in a low-income country.

Hence, for many farmers affected by soil pollution, phytoremediation offers a sensible option. What’s more, phytoremediation can be combined with the cultivation of cash crops, including edible crops, which increases its economical appeal.

“The threat soil pollution poses to human health doesn’t only depend on the concentrations present in the soil, but also on the type of crops that are grown. Some crops can accumulate a lot of pollutants in their edible part, even if concentrations in the soil are low (the hyperaccumulators). Other crops may exclude the pollutant and produce edible parts that are perfectly safe to eat, even when grown in highly polluted soil (the excluders),” says Henrik Haller.

Hyperaccumulators are well-studied and include many edible plants such as amaranth, sunflower, rapeseed, Indian mustard and cabbage. However, a lot less is known about the “excluders” and exactly how they work. For instance, some plants may exclude some metals while absorbing others. A promising candidate is a corn that has been grown in soil contaminated by electronic waste and yet produced corn that was safe to eat.

This eco-friendly soil clean-up method is yet to become mainstream and the knowledge about which crops are safe to grow is limited. Typically, farmers don’t choose the crops based on whether they accumulate pollutants or not. Clearly, phytoremediation will require capacity development and farmer-researcher collaboration.

Another area that needs attention is regulation and control. Government authorities often overlook soil pollution, so effective preventive policies are lacking. In a recently published review article in the journal Chemosphere, Henrik Haller explores opportunities and risk for growing food in polluted soil and reviews conditions and policy options for soil pollution reduction in Europe, China and Central America.

He finds that in Europe, governments have banned food production on large surface areas of land considered too contaminated to safeguard the population’s health. But in many countries in the Global South sufficient soil pollution regulation and environmental law enforcement are lacking. As a result, huge contaminated land areas with unknown concentrations of numerous pollutants are used for food production.

What are the options?

Undoubtedly, this topic and the method need more research and understanding. However, Henrik Haller outlines three strategies that may be employed to minimize human exposure to pollutants when growing food in polluted soil.

  1. Remove pollutants from the soil and/or degrade them prior to food production by using plants that hyperaccumulating the pollutants in its biomass. This may be appropriate for areas where there is a polluter with clear liability and sufficient funds exist to endure an income loss until the levels of pollution are deemed safe for crop production.
  2. Protect the food chain from contamination prior to remediation by using plants-excluders, which don’t absorb pollutants in an edible part. This is an interim solution since no remediation takes place. However, the promotion of excluding plants may be a suitable emergency response to the many poor people who have no choice but to grow food on polluted soil.
  3. Extract and degrade pollutants while producing a pollutant-free edible part. This can be done by using plants that accumulate a high concentration of pollutants in its tissue, but don’t transfer it to its edible part or by combining “edible excluders” with “hyperaccumulators”. This strategy is suitable at sites where a delay and reduction in income generation cannot be endured, but where there is enough resources for thorough monitoring to ensure that no pollutants enter the food chain.

 A worthwhile pursuit

 Addressing and preventing soil pollution is an urgent matter, not only because of the health concerns, although these provide substantial grounds for action, but also because of the scarcity of soil resources against the backdrop of continuously increasing human population. However, considering the ongoing trends, it might be inevitable that more and more farmers will be forced to cultivate in contaminated areas. That is why phytoremediation needs more attention in research and practice.

Henrik Haller admits that phytoremediation isn’t risk-free, but the health hazards associated with such food production can be avoided if managed responsibly. In this context, developing robust monitoring and regulation of soil pollution as well as ensuring its diligent implementation and enforcement is essential.

Food production with phytoremediation would also be knowledge and data-intensive, and much remains unknown about which pollutants accumulate in which food crops in different growing conditions.

But the method itself offers a promising low-cost solution that small-scale farmers, who represent the vast majority of the world’s farmers, can apply to clean their soil and halt negative health effects that come with it, without further agricultural land expansion and interruptions in food production.

Henrik Haller wishes to soon start a project in which he can test the bioaccumulation and translocation patterns of the 40 most common pollutants in the 40 most common food crops.