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Scientific Name: Lumbricus terrestris

picture of an earthworm

In the concluding chapter of his book, 'The Formation Of Vegetable Mould, through the Action of Worms' (1881), Charles Darwin wrote:
"When we behold a wide, turf-covered expanse, we should remember that its smoothness, on which so much of its beauty depends, is mainly due to all the inequalities having been slowly levelled by worms. It is a marvellous reflection that the whole of the superficial mould over any such expanse has passed, and will again pass, every few years through the bodies of worms. The plough is one of the most ancient and most valuable of man's inventions; but long before he existed the land was in fact regularly ploughed, and still continues to be thus ploughed by earth-worms. 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 organized creatures."

Where conditions are ideal with plenty of organic matter present, it is estimated that there can be up to 50,000 earthworms to the acre, and these will bring about 12 tons on material to the surface in a year. Where the last ice age extended in North America 10,000 years ago, all of the earthworms were wiped out, so all of the species found there now were brought mainly in the rootballs of imported plants since European settlers moved there. This is an exception where the introduction of an alien species has been a positive addition to an ecosystem.

They feed on partly decomposed organic matter in and on the soil, this is broken down further and after excretion the 'castings' are acted on by the soil micro-organisms. This means that the earthworm is an important part of the recycling of nutrients in the soil. In addition their burrowing aerates the soil and improves drainage, helping other soil organisms and plant roots.
There are usually three types of worms found in different layers of healthy soil. In the top layer the red worms feed on the litter. Lower down in the top soil different species move to the surface to reach the litter. The deep burrowing worms are much larger rising to the surface during the night and use their mouths to drag leaves down to feed on. Over-cultivation of the soil removes the litter layer so the worms have nothing to work on and the numbers will decrease. The older cultivation method of rotating crops and leaving a fallow year allowed the litter to be replenished and the more recent no till system with compost spread on the surface gives plenty organic matter for the worms to work on.

The worm casts contain higher levels of nitrogen, phosphorous and potassium than the surrounding soil - the products of digested organic matter and trace elements brought up from deeper levels. Calcium carbonate is also added and this can make acid or alkaline soils more neutral. This 'fertilizer' is distributed throughout the soil and research has shown that each worm can produce about 150g per year - much better than a dry or granular fast-acting chemical fertilizer which has no humus, leaches away and may repel the worms. Some gardeners collect worm casts to use as a fertilizer and soil conditioner.
Recent studies have also found that enzymes excreted in the casts act as growth promoters on plants.

Many earthworms dwell in the top few centimetres of the soil, but the larger species will go down nearly 2m. They need moisture as they absorb oxygen and give off carbon dioxide through their skin, so they burrow deeper if the soil dries out or freezes. However, they will drown in waterlogged soil so the increasing periods of prolonged wet conditions may harm the population - they are forced to the surface and birds exploit these conditions to make easy pickings of them. Usually they come to the soil surface at night when temperatures are lower and the humidity is higher - also the birds are not about. The small amounts of mucous they leave as they move around acts as a scent trail which guides them back to their burrows; heavy rain washes it away and this is why they can be found on the surface after a shower as they are lost. Most soil types are suitable except very acidic conditions, and coarse sand.

They can reproduce when 3 - 6 weeks old, making a cocoon in the soil from which live worms emerge. The common view that cutting a worm in half makes two is not true, they can only regenerate the final segments at the rear end, usually both severed parts die.
Lumbricus terrestris is the usual species found in gardens and the average specimen is about 10 to 20cm long. However, recently in Widnes, Cheshire a gardener came across a worm which was 40cm long and weighed 26g. Its reward for surfacing and revealing itself was to be taken to the Natural History Museum where it was killed and preserved. It was given the name Dave by the finders stepson. The usual lifespan is up to six years and predated by birds, frogs, rats, badgers and many other animals.

Unfortunately the worm population of Northern Ireland and Co Down in particular, is under great threat at present from the New Zealand Flatworm. This interloper feeds exclusively on earthworms and can reduce the numbers to below detectable levels in the right conditions. The scent trails mentioned above are used by the flatworms to find the worms.

To increase the worm population, adding organic matter provides food and adding lime to acidic soil will help. They are said to like mint and planting it is supposed to attract them to the area.
Turning over the soil too often has a detrimental effect, so the no-dig method of gardening is beneficial and they will do the incorporating of organic matter for you - this is part of the Permaculture technique of growing. The Brandling worms (Eisenia fetida) sold as fish bait are not suitable as they need the warmth of the wormery or compost heap.

Some people, usually those maintaining putting and bowling greens, do not welcome the worms because of their 'casts' which spoil the perfect playing surface they are trying to achieve. They can make the surface slippery as well and are the perfect medium for germinating weed seeds. A top-dressing of coarse sand has been shown to reduce the production of casts - I suppose they don't like eating sand or abrading their skin on it, any more than we do! An application of about 4cm split over three to four months should have some effect, ie. six or seven applications of 0.5cm every two weeks.
Where soil compaction is a problem on a playing field, earthworms have been brought in to improve the situation.

Earthworms are very important when recycling organic waste. They populate every compost heap and special wormeries or composting toilets mean we can keep all of the humus and nutrients for use at home. (Though you might have to watch out for animal welfare groups, recently in New Zealand a special study had to be carried out to see if the worms are traumatized by the ordeal, fortunately it was found that they are not.)
To find out how to set up your own wormery for composting kitchen waste check out this video:-