Earthworms have never been considered of major importance in the soil management of orchards in Australia. However, orchardists and dairy farmers in Europe are very conscious of the benefits from earthworms and their populations.
Soil biologists are generally more interested in animals or microflora that are pests, which cause diseases or affect the nutrition of plants.
There may be a million species of soil animals, and many kinds of unknown interactions between them and the soil environment. We know little of the ecology and effect of roots and microflora in soil.
It is hard and difficult work to collect reliable samples of animals, roots or microflora, and to follow their seasonal activities in soil, to interpret the data, and to understand the mechanisms.
Earthworm—the best known soil animal
Probably the best known soil animal is the earthworm. Worldwide there are about 1800 species.
The most common and beneficial earthworm found in gardens and orchards is the red worm, which belongs to the family Lumbricidae (lumbricides).
All of the lumbricides in Australia have been introduced from Europe. The early settlers probably brought them accidentally in pots with plants.
Most Australian native species are ‘lazy’ and have no beneficial effects. Reference is therefore made to the European species.
Earthworms are silent, headless, bi-sexual creatures without eyes, and have a voracious appetite. They exist throughout the world—from the cold soils of Iceland to the tropical islands of Indonesia—and come in sizes from the microscopic to upwards of one metre.
What do earthworms do?
Earthworms mostly eat and burrow.
Humans consume about four percent of their body weight (in dry weight) of food per day. An earthworm eats 11 to 14 percent, which in wet weight converts to about its own body weight every 24 hours.
Chemical effects
Earthworms are omnivorous, so their food includes fresh and decaying plant and animal matter and soil.
The nutrients from the food are extracted for the worm’s use and the balance is excreted as casts. But it is how worms affect the soil they eat and live in, that is important to orchardists and gardeners.
A skin excretion of slime, to assist in the worm’s movement, and casts are responsible for some of the nitrogen in the soil. But most of the nitrogen comes from earthworms when they die.
Depending on the worm population density, earthworm carcasses can release 18 to 90 kilograms of nitrogen per hectare annually.
With a nitrogen to phosphorus to sulphur ratio of 12:1:1, dead worms would also contribute a further 2 to 9 kilograms per hectare of phosphorus and sulphur annually.
As earthworms burrow and feed through the soil, they set free bound-up substances, like phosphates, by breaking down the large complex molecules into more simple plant-available forms.
When the chemical analysis of the excreted material is compared with the surrounding soil, castings contain five times more nitrate, seven times more phosphorus, three time more exchangeable magnesium, one and a half times more calcium, and eleven times more potassium than the surrounding soil.
Castings also have a neutral pH and have a higher base exchange capacity (ability to react with nutrients) than the surrounding soil.
But the casting content is dependent on the organic matter and mineral content of the soil that the worms ingest. Worms cannot make something out of nothing.
Since earthworms produce their body weight in casts every 24 hours, a well worm-populated orchard or pasture, can turn over 70 tonnes of soil per hectare between spring and autumn.
They can consume the leaf litter from an orchard in autumn in three months. In doing so, they can have a significant phytopathological effect on some harmful fungi, such as black spot (scab) of apple and pear.
This fungus overwinters in fallen apple and pear leaves on the orchard floor where fruiting bodies form. If the leaves are not consumed by earthworms and microflora, spores are dispersed from these fruiting bodies when temperature and humidity conditions are right for an infection period in spring. Young leaves and fruit on the trees are then infected.
It has also been observed that earthworms eat directly the fruiting bodies of the brown rot fungus formed on dead stone fruit prunings on the ground.
Physical effects
Earthworms improve soils physically by tunnelling through soil, by mixing organic litter into the soil and by producing casts.
They also improve nutrient cycling by mixing fertilisers with soil.
The microbial activity, either within the earthworm’s gut or in the casts, produces gums (possibly polysaccharides) or fungal hyphae, which stabilise the casts. Calcium may also be partly responsible, since calcium stimulates earthworm activity.
In temperate Australian soils, which generally have low amounts of surface litter and are dry through summer, earthworms are inactive throughout summer.
However, in a non-cultivated peach block at the Tatura Research Institute, where large amounts of organic matter have been added annually for 10 years, and the soil kept wet by irrigation, earthworms remained active throughout summer.
The entire surface soil (0–200 mm) appeared to be casts and probably had passed through earthworms many times.
Earthworms produce their own tunnels to live in and move around, as opposed to some animals which live in cavities formed by other animals or roots.
Soils, where earthworms are very active, have an extensive network of inter-connecting horizontal and vertical tunnels, generally continuous to the surface.
The earthworms exude mucus, which stabilises the walls. The soil lining the tunnels is not compressed, because earthworms ingest rather than push their way through soil.
They tend not to enter subsoils unless the subsoil is disturbed. This is probably due to poor aeration and lack of food.
The tunnels are stable and may persist for years after the earthworms have left. This may also be due to the oriented clay particles and concentration of humic materials, iron and calcium, which have been shown to line the tunnels.
These stable inter-connecting tunnels are important for movement of water, air and growth of roots.
The tunnels, which are 1 to 10 mm wide, depending on the size of the earthworm, increase the macro-porosity of the soil.
The tunnels increase water infiltration, drainage and aeration. For example, at the Tatura Research Institute, an irrigated peach block with an ample supply of food and water, had 2000 earthworms per square metre of topsoil, compared with 150 worms per square metre in local orchards without ample food and water.
In the same research block of peach trees, the water infiltration was 80 times, and the macro-porosity four times that of the local orchards.
We also found that roots used old earthworm tunnels and root channels (called biopores), and could penetrate harder soils with, than without tunnels and biopores.
Earthworms are also emerging as a highly efficient tool to remove pesticides and pollutants from soils and combat climate change by reducing greenhouse emissions from landfill waste.
Recent studies found that earthworms removed heavy metals, pesticides and organic micro-pollutants from soil, a technique known as vermire-mediation.
You may now realise how valuable these slippery, ground-dwelling creatures are.
Yes, the humble earthworm does not deserve to hang on a hook or be drowned.
See photos and other great articles in the May-June 2012 issue of Tree Fruit magazine
