How Pesticides Are Harming Soil Ecosystems | Civil Eats

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The first year after Jason Ward began transitioning his newly purchased conventional farm to organic production, he started seeing more earthworms in the soil beneath his corn, soybeans, and wheat fields. By the third year, he had spotted numerous nightcrawlers—big worms reaching up to eight inches long—on his 700-acre farm in Green County, Ohio.
With conventionally farmed land, “anything synthetic is hurting the natural ecosystem of the soil,” said Ward, whose acreage is now largely certified organic. “As you transition away from that, the life comes back.”
By life, Ward means the rich diversity of insects and other soil invertebrates—earthworms, roundworms, beetles, ants, springtails, and ground-nesting bees—as well as soil bacteria and fungi. Rarely do conversations about the negative impacts of pesticide use in agriculture include these soil invertebrates, yet they play a vital role in soil and plant health and sequestering carbon. Worms eat fallen plant matter, excrete carbon-rich casts and feces, cycle nutrients to plants, and create tunnels that help the soil retain water. Beetles and other soil insects feed on the seeds of weeds, or prey on crop pests such as aphids.
But those critical functions are jeopardized by more than a billion pounds of pesticides used in the U.S. every year, according to a new peer-reviewed study. Compiling data from nearly 400 laboratory and field studies, researchers at the Center for Biological Diversity, Friends of the Earth, and the University of Maryland found that pesticides harmed beneficial soil invertebrates in 70.5 percent of cases reviewed . Studies conducted in the field alone, however, resulted in fewer significant negative impacts (about 50 percent of cases reviewed).
“What this study really drives home is that pesticide use is incompatible with healthy ecosystems, across organisms, pesticide classes, and a whole set of different health outcomes, including death,” said Kendra Klein, senior scientist at Friends of the Earth and co-author of the study. “We have to be talking about pesticide reduction in conversations about regenerative agriculture.”
Herbicide use has risen steadily in the U.S. in past decades, particularly on genetically modified crops. Recent USDA surveys show 98 percent of soybean acres and 97 percent of corn acres are sprayed with herbicides with known health and environmental impacts, including glyphosate (Roundup), atrazine, and dicamba. Neonicotinoid insecticide use has also risen in recent decades as a seed treatment for field crops, even though pesticides in this class are implicated in colony collapse disorder in bees and potential endocrine disruption in humans. The U.S. lags behind the world’s largest agricultural producers, including Europe, China, and Brazil, in banning harmful pesticides, according to a 2016 study that found that more than a quarter of all agricultural pesticides used in the U.S. are banned in Europe.
The researchers reviewed studies covering 275 unique species and 284 different pesticides or combinations of pesticides available in the U.S. Insecticides, unsurprisingly, produced the largest negative impact on soil invertebrates (75 percent of cases), followed by fungicides (71 percent), herbicides (63 percent), and bactericides (58 percent). The pesticides either directly killed the organisms studied or significantly harmed them by impairing their growth, for example, or decreasing their abundance and diversity. The earliest signs of pesticide impact (e.g., structural changes and biochemical biomarkers of harm) were observed most frequently in soil organisms, followed by reproductive harms, mortality, and impacts on behavior, growth, richness and diversity, abundance, and biomass.
The findings add further evidence to the role that pesticides may be playing in biodiversity decline and the “insect apocalypse,” and they raise critical questions about the ability of soil to capture and store carbon if pesticides are killing or harming the very organisms that perform those vital functions.
Ecotoxicologist Ralf Schulz, a professor at the University of Koblenz-Landau, wasn’t surprised by the results but urged caution in their interpretation. Field-based studies, which account for one-third of the cases reviewed, showed fewer significant negative effects. One possible reason: Lab studies often use higher pesticide concentrations, while uncontrolled environmental variables could provide some buffering capacity for pesticide effects in the field. It’s important to evaluate whether lab studies tested pesticide concentrations at levels that would be found in the field, Schulz said, but the researchers noted that was beyond the scope of their paper. Schulz’s own research has found that pesticide toxicity has more than doubled for many invertebrates since 2005.
“It’s a very important study, but it’s not so easy to interpret the 70 percent negative effects directly,” Schulz said. One shouldn’t assume that means ‘in 70 percent of soils we have problems,’” he added, “because that could be wrong.”
However, Richard Smith, an agricultural ecologist and associate professor of natural resources and the environment at the University of New Hampshire, said that—on the contrary—the findings could be “somewhat conservative.”
“It paints a really good picture of the general negative effects of pesticides on soil invertebrates,” he said, “but it doesn’t necessarily tell us the degree of [harm].”
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Soil Invertebrates ‘Routinely Ignored’
The U.S. Environmental Protection Agency’s (EPA) risk assessment process for approving pesticides requires manufacturers to test for potential harmful effects on aquatic insects and the European honeybee, which is used as a surrogate for other terrestrial invertebrates, but not soil-dwelling organisms, said Nathan Donley, environmental health science director at the Center for Biological Diversity.
But the honeybee “is not adequately representative of a lot of really important insects and arthropods,” Donley told Civil Eats. “EPA thinks that terrestrial invertebrates fall in one of two categories—pollinators and bird food—but they do so much more than that. This is such an important group of animals that is being routinely ignored.”
The study results underscore the need to include soil organisms in any risk analysis of a pesticide that could contaminate soil, both Donley and Klein said. The risk assessment should also take into account the important functions these organisms provide, such as decomposing dead plants and animals, regulating pests and diseases, and sequestering carbon in the soil.
The Center for Biological Diversity and Friends of the Earth filed a legal petition, supported by 67 public health, environmental justice and other organizations, urging the EPA to include a robust assessment of the harms to six classes of soil organisms, beyond the European honeybee.
Bringing Pesticide Reduction into Regenerative Agriculture
Some of Smith’s published research has focused on the impact of seeds coated with insecticides and fungicides on weeds and below ground invertebrate communities. He found that much of the chemicals end up in the soil and aren’t taken up by the crops.
“We’re also finding that it travels quite a bit in the soil, and it resides in the soil for longer than folks suspected . . . As we’re looking at the data coming from these studies, [it shows] that even a small amount is having an impact on the soil communities.”
Klein says the researchers’ major motivation for conducting the invertebrates study was to call attention to the need to include pesticide reduction in the discussion about regenerative agriculture practices, such as no tillage and cover crop use, which have become popular in many food and agriculture circles for their soil health and climate benefits, but often include the use of herbicides.
The U.S. Department of Agriculture (USDA) under Biden is preparing to reward farmers who take up regenerative practices through a public carbon market and other incentives. Private companies such as Indigo Ag are also developing private carbon markets, while a number of “Big Food” players—ranging from General Mills to McDonald’s and Danone—are ramping up plans to incentivize regenerative practices in their supply chains. It worries Klein that pesticides aren’t a larger part of these conversations.
“So often the role of pesticides in harming soil health is left out,” Klein said. “That leaves the growing field of regenerative agriculture open to co-optation by pesticide companies. We’re seeing some of the worst actors trying to ride the momentum of soil carbon sequestration to identify new markets to sell their products,” she said, pointing to Syngenta, Croplife, and Bayer.
The practice of planting cover crops such as cereal rye and legumes is increasingly encouraged by soil health experts as a vital practice for sequestering carbon, retaining water, and increasing farm resiliency to climate change. On conventional farms, however, cover crops are often used in conjunction with reduced tillage, meaning that they’re getting “burned off” or killed with Round Up and other herbicides rather than being tilled into the soil.
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Some farmers, like Ward in Ohio, however, use mechanical means, or a “roller crimper” to kill off cover crops like cereal rye, and they plant their soybeans or other cash crop into the residue of the rye. Mowing or roto-tilling the cover crop (but not the soil) are other means organic farmers use for mechanically removing cover crops, according to Rodale Institute Midwest Organic Consultant Léa Vereecke.
Klein points to research showing that organic farms sequester more carbon than conventional farms, but there is also evidence that organic farming practices can run counter to sequestering carbon in soil, because it tends to require a lot of tillage.
For example, Ward, told Civil Eats that in some fields he is constantly tilling the soil to manage weeds. Three days after he plants, he’s in the field running a rotary hoe, and then he’s back three days later. “That’s one of the biggest downfalls—the fact that you do have to keep working that soil over and over and over again to get good weed protection,” said Ward.
Some organic farmers are working to dramatically reduce their tillage, but Vereecke doesn’t think “there is such thing as a 10-year-long rotation that doesn’t involve any tillage and is 100 percent organic.” Still, she points to research showing that if tillage is done wisely, optimal soil health and some carbon storage is obtainable. “Science is now showing us that herbicide use is more harmful than tillage,” she added.
There are no easy answers, Smith said. “There are tradeoffs in every aspect of agriculture, and I’m not sure that we have really figured that balance” between pesticide reduction and carbon sequestration. “But we’re working toward it.”
Meg Wilcox is a freelance writer based in Boston focused on solutions-oriented stories about the ways people are fighting climate change, protecting the environment and making our agriculture systems more sustainable, including by addressing poverty. Read more >
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