Earthworms are classified as 'keystone species' because of their central role in maintaining soil health. They do this by converting organic waste into obtainable nutrients, which plants can absorb. In addition, their tunnelling aerates the soil and allows surface water to drain away, preventing compaction. Earthworms are therefore central to soil structure and tilth. Their vital role in maintaining healthy soil is widely recognised, but it is becoming increasingly clear that some farming practices are preventing them from thriving.
People have long realised the importance of earthworms. Aristotle noticed how they could turn the soil, so he called them “the intestines of the earth”. Charles Darwin had a fascination with earthworms. He spent 39 years studying their habits and biology. While Darwin is best known for defining evolution, his 1881 book “The Formation of Vegetable Mould Through the Action of Worms”, gave the world its first proper understanding of these modest creatures.
There are thousands of species of earthworm, which are divided into three main groups; those that dwell on the soil surface in compost or leaf litter (eg. red wrigglers Eisenia fetida), those that feed and cast within soil and those who construct permanent burrows from which they visit the surface to obtain food, often at night (eg. common earthworm, Lumbricus terrestris).
Worms dramatically increase soil fertility because they eat about one and a half times their own body weight each day. As a result they are constantly defecating. Worm casts, or the poop they leave behind, have been found to contain up to five times more available nitrogen, seven times more available phosphates and eleven times more available potassium than the surrounding soil. Worm casts also change the chemistry of soil by introducing beneficial bacteria.
Farmers have often observed that farmyard manure, or dung, seems to have more long-lasting benefits than artificial fertilizers. This is true and the reason is that dung encourages earthworms by providing them with just the sort of healthy food they love. Earthworms thrive on high DM applications of humus, compost, dung or plant matter.
Tillage farming, especially when conducted without break crops, has been shown to have negative impacts on worm populations. A 1996 study found that Earthworms “were reduced by 70% compared to previously undisturbed sod after five years of plowing”. The same study found that only 11-16% of the original earthworm populations survived 25 years of annual tillage. This fact led to the development of 'no-till' seed drills in the US. No-till has succeeded in increasing the number of earthworms slightly, but their numbers in tillage fields are still much lower than in grassland.
Earthworms depend on a balance between soil temperature, moisture content, pH, salts, aeration and texture. High concentrations of chemical fertilizers, especially urea, can kill worms outright. Fertilizers and pesticides increase soil acidity which can have detrimental impacts on worm populations. Even moderate use of agro-chemicals provides earthworms with enormous challenges through bio-accumulation, where residues build up over time.
Chemicals that were considered harmless to worms have recently been shown to have detrimental effects, even at sub-lethal levels. Growth rate and fertility of worms were affected by sub-lethal doses of a range of chemicals, both in laboratory conditions and through field trials. Dozens of studies have now shown that low levels of farm chemicals will impact worms. A HYPERLINK "https://www.hindawi.com/journals/aess/2010/678360/"review of these studies found that the “results of earthworm ecotoxicological tests may be confounded with different properties of soils such as organic matter, water holding capacity, pH, cation exchange capacity, Carbon/Nitrogen ratio, and clay content”. This could account for the blind spot which led research bodies to overlook the cumulative impacts of chemical inputs.
Some individual chemicals are already under review for their impacts, such as neonicotinoids, which have been linked with bee deaths, and a number of suspected endocrine disruptors, which affect foetus development in humans. A study published in February found that earthworms died from exposure to just 0.01% glyphosate solution. Considering the fact that glyphosate is the world's most commonly used herbicide, and is usually mixed at 1% solution, the cumulative impacts of this alone could be great.
The possibility that farm chemicals are impacting soil health through their effects on earthworms will have ramifications in the future. For now the chemicals lobby is determined to protect this multi-billion dollar industry and they insist their products are safe. Ecologists are worried that we are putting too much pressure on earthworm populations, without which we would find ourselves unable to grow food.
At present our most common earthworm species are not threatened with extinction, but their decline under intensive agricultural practices is apparent. Farming bodies may soon have to come up with earthworm-friendly inputs for crop production because, like bees, without the ecosystem services that worms provide, we might not be able to feed ourselves at all.