The Importance of Earthworms

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We all like to have nice gardens with healthy plants and the only way to ensure this state is to have a healthy soil-food-web. That means a soil that is teeming with microbes, beneficial fungi along with many soil creatures including big populations of earthworms.

The easiest way to determine that you have good healthy soil is by the number of worms you see when the soil is opened up. No worms, in a moist soil, means you have a problem and until rectified you will struggle to have healthy plants and gardens. Note that I say moist soil because when the soil becomes too dry, too wet or too cold you will be lucky to see any worms, even if you do have good worm populations.

When temperatures drop or soils get too warm or dry, worms know what to do. If it starts getting chilly, they may tunnel deep into the soil before it hardens. They may also coil into a slime-coated ball and go into a sleep-like state called estivation. It’s something like a hibernating bear.
Once conditions improve to the worm’s liking, up they come to work your gardens for you.
There are 3500-4000 species of earthworms around the world and nearly 200 species have been identified in New Zealand. They are full of calcium, protein, fibre and vitamins, making them a valuable food source for many mammals, reptiles and fish. Earthworms vary in size, on average from no more than 1 centimetre to about 3 metres in length. One of the world’s largest earthworms, the Giant Gippsland Earthworm (Megascolides australis), is found in Australia. It has an average length exceeding 1 metre. However, the longest recorded earthworm was a South African giant specimen (Microchaetus rappi), measuring around 7 metres in length.
An interesting site to find out about worms and other aspects of gardening is hosted by Lincoln University.
The following is some of the information they provide on earthworms: Pastures commonly support the biggest populations of earthworms as they usually contain large amounts of organic matter and are infrequently disturbed by cultivation events.
Numbers of earthworms can commonly range between 7 and 12 million per hectare under a productive pasture in New Zealand. This corresponds to 1 to 3 tonnes of earthworms per hectare and means that the weight of earthworms below a pasture is similar to the weight of the grazing animals supported above ground!
Earthworms are hermaphroditic, which means they each have both male and female organs.
It is an old wives’ tale that cutting an earthworm in half will make two earthworms; one part may survive, but it is much more likely that both parts will die.
The excreta produced by earthworms is known as casts. As soil passes through an earthworm’s body, some of the nutrients are converted into forms that are more readily available for plant uptake. So casts are generally rich in plant-available nutrients like nitrogen, phosphorus, and potassium.
As well as consuming soil, earthworms also help to break down and thereby recycle organic materials (such as dead herbage and dung). Scientists have calculated that each year through their activities they are responsible for burying around 6 tonnes of pasture litter per hectare.
As they burrow through the soil, earthworms create burrows and channels that help to loosen the soil, allowing air to circulate and roots and water to penetrate the soil more easily.
In New Zealand an incredible 25-30 tonnes of soil per hectare per year has been measured by scientists as being deposited by earthworms on the soil surface in the form of casts. But we must remember that some earthworms deposit their cast material both within and upon the soil surface, so the total amount of soil that they turn over in a year is even higher!
There are four main types of earthworms that you might find examples of in your garden:

Compost dwellers. Like to live in high organic matter environments such as compost heaps, but will not usually survive in soil unless it contains very high amounts of organic matter.
Soil surface dwellers. Feed on decaying roots, shoots, leaves and dung and live near the soil surface (0-15 cm depth). Important in mixing plant litter into the soil.
Topsoil dwellers. Most common earthworms in New Zealand; live in the top 20-30 cm depth of soil. Burrow through soil, eating and excreting it. Tend to eat more soil than organic matter.
Subsoil dwellers. Tend to live in permanent burrows up to as deep as 3 metres below soil surface. Drag food such as leaves into their burrows from the soil surface. Often larger than other earthworm types.
What affects earthworms?
• Temperature (they don’t like it too hot or too cold)
• Moisture (they don’t like it too wet or too dry)
• Food availability/type (some sources of organic matter are of better quality/contain more nutrients than others)
• Soil type and texture (soil organic matter is a good food source; sand can be abrasive to the earthworm’s skin)
• pH of soil/organic material (most earthworms prefer a pH closer to neutral)
• Seasonality (numbers decline during summer when it gets hot and dry)
• Land management (cultivation, fertiliser application, irrigation)
• Predators (such as birds and flatworms)
• Toxic substances (such as fungicides, fumigants, chemical weed killers, sprays and chemical fertilisers such as General Purpose, Rose etc and Nitrophoska)

How do I encourage earthworms in my garden?
Maintain soil pH between 5.8 and 6.3 by adding lime periodically to the soil. Limit the amount of cultivation where possible but, if cultivating, avoid machines that pulverise the soil and the earthworms contained therein. Limit the use of harmful pesticides etc as mentioned above.
Irrigate the soil during dry periods to maintain earthworm activity, and increase organic matter in the soil (a good food source for earthworms) by incorporating composted material and animal manures.
You can breed worms in a worm farm such as a Worm-a-Round. I have two of these units in operation and my well composted gardens are now full of worms as a result. You simply put your kitchen wastes into the worm farm to feed the worms and gain worms, worm pee and worm casts for your gardens while turning your scraps into a valuable garden resource.
Lime your gardens regularly with a fast acting lime such as Rapid Lime. (which I will be discussing in a few weeks time). Apply animal manures and compost as mulches along with Magic Botanic Liquid as a soil drench makes moist garden soils a paradise for big worm populations.
Cultivation of soil such as digging should be kept to a minimum. Instead, cover bare soil with animal manures and compost (in that order) and then plant your seedlings into the compost.
If you need to dig a garden to loosen the soil or to add in compost this is best done when the soil is dry and the worms are not present.
Total avoidance of common fertilisers and chemical sprays is a must.
When plants such as annuals and vegetables are finished either cut them off at ground level or pull out with the minimum of soil disturbance. This also applies with weeds. Not only do these actions give minimal disturbance to worms but it also leaves the webs of beneficial fungi intact to the great benefit of all other existing plants.