I can guarantee that when I am taking a group of children (and sometimes adults) for a garden course, or a composting event, that within three minutes I will get at least one blood curdling scream from the participants; and it's not always from the girls.

The screams are not a response to mixing up a horror film with the DVD on the water cycle, or because they are being attacked by a chainsaw wielding lumberjack, or even because I have asked them to do the washing up.

No, the screams usually start when the first worm appears in a spadefull of soil. By the end of the session though, all the irrational fears associated with the wriggly guest have usually been dealt with and the screamer ends up proudly display a gyrating worm on the palm of their hand.

Like pigs on land and crabs on the shoreline, earthworms are the scavengers of the soil, making food out of everything that has lived and died. They pull leaves and vegetable matter, remains of animals, insects and organisms under the soil, where, with the help of sand and earth particles, they grind them up and extract any nutrients.

The worms' glandular secretions, along with nitrogen, are mixed up within the food in the process of eating. The end result is very fine worm cast which is rich in lime, minerals and vegetable matter. Worm casts contain seven times more nitrogen, three times more potash and two times more phosphorous than ordinary garden soil. Casts also contain glandular fluid from the worms innards which are essential to the growth of plants. The casts are made of the finest particles, which makes them easily absorbed by roots. The average worm will eat its own weight every day, and excrete its own weight in humus every day.

Worms are known to exist mainly below the earth for periods of up to 14 years, reproducing themselves hundreds of times each year. In a good soil, worms can produce up to ten tonnes of worm casts every year per acre. Not only that, their extensive tunnelling allows rain and air to penetrate the soil, preventing it becoming dry, compacted and devoid of oxygen. It is less laborious and more economical to let the worms do your digging. The results are far superior to those achieved by backbreaking spade work. In fact, when a garden is dug, all the good and beneficial work that the worm has already done for you is destroyed

Although it has taken a long time to prove, the earthworm does plays a vital role in our garden. These little creatures are the tireless cultivators of our productive border soil and the magicians in our compost heap. Bowling green caretakers and cricket pitch groundsmen curse them for their casts, but vegetable growers, farmers and flower nurseries praise them.

Darwin, in his theory of the species, wrote that the weight of cattle which could be fed on any given pasture land was in direct relation to the number of worms contained within it. A survey in the New Forest by the NFU in more recent years concluded that "worms are a measure of soil fertility". Chemical fertiliser manufacturers will not agree, but the humble earthworm is an essential tool in the production of good, productive soil. If the entire worm population perished, then all farmland would become barren without the humus content that they provide. We should teach our children not to scream when they find an earthworm, but rather to shout for joy.

JOBS TO DO THIS WEEK

* At last you can venture out into the vegetable garden and start planting the first crops. Onion sets are available in most garden centres, and can be pushed into well-drained soil. Some people prefer to snip the papery tops off with a pair of scissors as this prevents the birds pulling them back out of the ground. If you prefer, you can sow drills of the seed in the place that you want them to grow. When they sprout you can thin them out roughly and then carry out a final thinning in another month's time.

* It is a good time to plant mint in the herb garden. Mint roots can be very invasive so, as a precaution, I always plant it in a bucket and then sink the bucket into the ground.

* Give roses a good prune now as this will tidy them up as well as stimulate this year's growth.

READER'S QUESTION

Q - Anthony from North Yorkshire wants to know what the small black flies that swarm out of his compost heap are, and are they detrimental to his compost?

A - It sounds as though you have a colony of sciarid flies living in your bin. These are small flies that live just below the surface of the soil, or compost.

In fact, it is an indication that your heap is decomposing well and turning into a friable, crumbly mixture.

Apart from being a nuisance, they do not cause any harm to your soil and the larva, which are small worms, might actually help to break down the waste for you.

Trouble may occur though if you use this soil for sowing seeds as the worms might take a liking to the roots of your tender young seedlings.

* Brigid presents Ask about Gardening every Sunday on BBC Radio Cleveland 95FM from 1-2pm.

Questions will be answered on the day by e-mailing cleveland.studios@bbc.co.uk, texting 07786200995 or phoning 01642-225511.

Alternatively, send questions to brigidpress@hotmail.com or The Clow Beck Eco Centre, Old Spa Farm, Croft-on-Tees, Darlington, DL2 2TQ