Experts are raising alarms that gardeners may be inadvertently killing earthworms with soil conditioners marketed as “organic.” They are calling for tighter regulation on these products, which can poison the invertebrates.

Charles Darwin recognised the critical role of earthworms in enhancing soil structure and fertility, dedicating his final book to them.

He added: “It may be doubted if there are any other animals which have played such an important part in the history of the world as these lowly organised creatures.”

Despite their importance, some gardeners seek to remove worm casts from their lawns. These casts, which are worm excrement, can be considered unsightly, especially when squashed and spread over the surface.

Numerous products available to gardeners and greenkeepers claim to combat these casts, reducing the need for manual removal. However, many of these products contain saponins, substances highly toxic to earthworms. Some are marketed as “organic soil conditioners” without mentioning their lethal effect on worms. Others claim to “irritate and deter” worms, driving them deeper into the soil, without disclosing the harmful active ingredients.

Experts point out that soil conditioners are lightly regulated and do not undergo the rigorous risk assessments required for pesticides, despite their potential toxicity to garden creatures.

Earthworms play a crucial role in breaking down dead plant material into nutrients, contributing to healthy soil, which is essential for a thriving garden and broader ecosystem. The Royal Horticultural Society highlights that worm casts are beneficial for the soil and can be used as a nutrient-rich potting medium, discouraging their removal.

Worm populations are believed to be declining, though studies are limited. Recent research suggests UK worm numbers may have dropped by about a third in the past 25 years.

Prof. Dave Goulson, a biologist at the University of Sussex, has written extensively on gardening in harmony with invertebrates and is investigating these soil products. He said: “Dozens of products are being sold to gardeners and groundskeepers, especially [at] golf courses, to combat worm casts. Many contain saponins, found in one scientific study to be ‘highly toxic’ to earthworms.

“The products are widely marketed online, with some bulk products obviously aimed at professional greenkeepers, other smaller bottles aimed at gardeners. The saponin usually seems to derive from ‘tea seed’, so is an organic product, but that doesn’t mean it is harmless: botulinum toxin and cyanide are organic. Groundskeepers should not be allowed to poison earthworms while pretending to ‘condition’ the soil. It seems to be pesticides by stealth.”

Campaigners are urging the government to examine these products and suspend their use until their effects on earthworms and other wildlife are fully understood.

Nick Mole, a policy officer at Pesticide Action UK, said: “Fertilisers and soil conditioners aren’t subject to anywhere near the same level of scrutiny as pesticides. They don’t appear to go through a risk assessment process to ascertain if they are harmful to non-target species such as earthworms, which is highly concerning given how widely they are used.

“Any substance that is intentionally released into the natural environment has the potential to cause harm, even those labelled as organic. Now that the alarm bells have been sounded the UK government must act quickly to suspend use of these products while they investigate further, including conducting thorough risk assessments looking at the impact of these products on earthworms and other wildlife, including aquatic species. Or will the government sit on its hands for another 10 years, as it did with neonicotinoids and bees?”

A Department for Environment, Food and Rural Affairs spokesperson said: “Decisions on the use of pesticides are based on careful scientific assessment of the risks. Pesticides are only allowed on to the market if they meet strict environmental requirements and pose no threat to human health.”



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