One of Earth’s most useful creatures

SPRING is a busy time of the year in my vegetable garden.

There is a bit of a lull at present because most of the hardy species have been planted and it’s still too early in my cold corner of the country to think about putting in anything frost sensitive. But I’ve been spending a lot of time weeding, digging and raking.

This activity has brought me into close contact with earthworms and, gardening being a relatively contemplative activity, I’ve been thinking about them. We tend to take worms for granted but they really are fascinating creatures. In fact many naturalists have spent their lives studying them. Charles Darwin devoted 39 years to his earthworm study. He said: “It may be doubted whether there are many other animals which have played so important a part in the history of the world as have these lowly organised creatures.”

He was referring, of course, to the earthworm’s role in creating and maintaining the fertile soils on which so much of life depends. An earthworm is basically a tube within a tube. The inner tube is its digestive tract and the outer tube, which is composed of segments called anulli, is what it uses to move through the soil. Each segment is covered in microscopic bristles with which it can hold on to the side of its burrow while it extends its front end and retracts its tail to move forward.

While it’s doing this it eats the soil in front of it, passing it through the inner tube and extracting and digesting plant material and excreting the inorganic matter. To make the whole process of burrowing easier the worm secretes a lubricating slime.

This lifestyle benefits the soil in my vegetable garden in several ways. Firstly it converts organic matter like dead roots or buried leaves into the nitrogen, potassium and phosphates that plants need to grow. Secondly the burrows both aerate and drain the soil. In fact a moving worm acts like a piston, driving air ahead of it. Finally the slime is also rich in nitrogen and its stickiness helps bind together soil particles to create what are called aggregates.

To appreciate the value of all this you have to understand the density of the worm population. There have been various studies done on this and there’s quite a variation in the conclusions but it seems likely a healthy Irish field that has had little or no artificial fertiliser applied will hold between one and two million earthworms in every hectare. This means, if it’s a pasture field, the weight of the worms will exceed the weight of the livestock in the field. It also means Darwin was quite right because without these ‘lowly organised’ and hidden helpers the fertility of that field would decline rapidly and the farmer would soon go out of business.

There are many species of earthworm in Ireland and some of them have a habit of moving about on the surface of the ground on wet nights — Americans call them ‘night-crawlers’. The experts argue about why they do this but the most likely explanation is that it’s easier to find a mate on the surface than it is underground. Their sex lives are complicated —- they are hermaphrodites but not self-fertile so they need to mate.

I always thought that the notion that if you cut a worm in half it would survive as two worms was a complete myth. But it’s partially true. Many species have the ability to re-grow missing segments, particularly segments near the tail end. The more I learn about them the more interesting they become.

* [e-mail]