The value of earthworms
What’s happening under our feet?
TEXT Paul Donovan
Charles Darwin, one of the world’s greatest naturalists, spent many years studying earthworms. He came to the conclusion that “no other living thing has had such a profound impact on history as has the earthworm”. They were, he concluded, “Natures ploughs” and were an integral part of the soil’s macro-fauna. A lot of modern-day scientists have gone even
further, and suggest earthworms are the most influential animal to have changed the earth, and not us humans.
Earthworms are vital in breaking down detritus, putting nutrients back into the soil, spreading nutrients about, helping with drainage, soil aeration, providing habitats for other soil-dwelling organisms, and breaking down the soil’s structure. Recent studies have found that even relatively poor soil can sustain in excess of 250 000 earthworms per acre, and fertile soil could hold seven times that amount.
One of the most important ways in which earthworms benefit the soil, though, is their ability to digest just about every type of organic plant material. With the aid of a chemical in their stomach called drilodefensins, they can digest and break down even the most poisonous of plant leaves. Drilodefensins are thought to be unique to earthworms and counteract polyphenols, which is an antioxidant produced by plants that inhibits the enzyme activity that aids digestion in many other species of herbivore.
Earthworms have been around for approximately 120 million years, and it is fair to say that without them the soil we try and grow seeds and plants in would be very different, resembling clods of impacted, clay-like material in which just about nothing could grow.
There are approximately 2500 different species of earthworms, each of which lives at a specific soil level. Some remain within the top few centimetres, while others live at lower levels. This means that the soil is receiving nutrients and being worked at every layer.
The earthworm’s role
The role of the earthworm can be categorised as follows:
Biological: This is one of the worm’s most important functions, as they recycle large pieces of organic matter into micronutrient-rich humus. This humus is then pulled into the soil where it is either ingested by the earthworm or breaks down naturally. You may be interested to hear that an earthworm can process up to five times its weight in food per day. In other words, 1000 earthworms could munch their way through 1.5kg of waste per week. Imagine what that does for your compost heap!
Chemical: Earthworms consume minute soil particles that are broken down and then excreted in the form of castings. Theses casts are five times richer in available nitrogen, seven times richer in available phosphates, and 11 times richer in available potassium than the surrounding upper 150mm of soil. In good soil, an earthworm can produce up to 4.5kg of casts per year.
Physical: Their burrowing activity greatly influences the composition of the soil. Not only do they move microbes such as bacteria around, but they also take nutrients from the surface level to deeper down in the soil. They also break down clumps of soil into smaller particles, and their tunnelling helps with water absorption, preventing runoff, and also aerating the soil. The tunnels may also create microhabitats for other arthropods.
Earthworms can pick up soil contaminants such as pesticides and fertilisers, as well as hormones and antibiotics found in animal manure. Some of these they can breakdown, while others will remain in their gut. Findings have shown that earthworms are extremely adaptable to contaminants and pollution in the environment, and actually reproduce better in contaminated soil. This means they can play a major role in cleaning up contaminated soil. As they break down these harmful contaminants, many elements are released back into the soil and made available as nutrients for plant growth.
The great value earthworms have to the soil has seen numerous enterprises throughout the world breeding them for commercial gain. With the application of manure, detritus and mulch, relatively unproductive soil can quickly be brought back to fertility again, and sustain good plant growth. But it needs the service of earthworms to help break these down and spread them throughout the soil.
Did you know? South African earthworms can be huge. One specimen found in 1967 was 6.7m long, although a more realistic size is a lot smaller than this.