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Earthworms: The saviour of our soil?

Topsoil is created by the physical and chemical weathering of rock into sand, silt and clay. It is estimated that this process builds topsoil at the rate of one inch every 500 years. Such a timescale effectively makes soil a non-renewable resource in our lifetime. Perhaps it was for this reason that President Franklin Delano Roosevelt stated “The nation that destroys its soil destroys itself.”

We continue to destroy our topsoil at an alarming rate. About a third of the world’s topsoil has already been degraded and if current rates of degradation continue unabated the world could be without topsoil in around 60 years.

While farming more sustainably may put the brakes on some of the destruction, what is really needed is a way of speeding up the recovery. One possible option may have been staring us in the face for more than a century – earthworms.

Charles Darwin spent time researching earthworms and their role in soil formation. Through some backyard experiments he concluded that topsoil levels could be increased in only a matter of years rather than centuries, primarily as a result of the digestive work of earthworms. Although there is much anecdotal evidence to support Darwin’s conclusions, there is scant scientific research to be found on it. There are, however, studies showing that earthworms are important actors in the regeneration of compacted soil, speeding up its repair.

Earthworms have two key requirements for good health. First, they need dead plant material to feed on. If crop residue is removed, earthworms lose their food source. We can increase organic matter not only with crop stubble but also with green manure crops, crop rotations to build up organic matter, or even permanent pasture which allows organic matter to die and decay.

Second, like so much of the natural world, earthworms need to be left alone to do their work. They do not react well to soil disturbance. Soil tillage greatly affects earthworms. It destroys their tunnel systems and is a particular issue in the autumn when breeding is taking place. Heavily compacted soil is difficult for earthworms to move through. Tillage also stimulates drying the surface soil and greater temperature fluctuations between day and night, both unfriendly conditions for earthworms. Earthworms need moist soil and are inactive when it is dry. A further negative impact of tillage is that it brings earthworms to the surface where they are subject to predators such as birds.

Several studies have shown significantly higher earthworm populations in no-till soil versus soil that has been ploughed. Although the research remains patchy, it is clear that earthworms do not co-exist happily with modern agriculture. Follow the whole story here