Life in the soil is largely dependent on organic material from plants – from above ground (leaves, branches, stems) and below (roots). The soil contains a teeming mass of life – not only roots but bacteria, fungi, protozoa, and a range of multicellular animals of which earthworms are the largest, most abundant and widespread.
In his 1881 book The formation of vegetable mould through the action of worms, with observation on their habits, the English naturalist Charles Darwin described the role of earthworms in soils. He noted, for instance, that lumbricid earthworms drag leaves into their burrows. In the garden of his home in Kent is the ‘worm stone’, which he said gradually ‘sank’ into the earth through the burrowing and casting of earthworms, with an annual casting depth of 2–5 millimetres. He documented the important role of earthworms in mixing mineral and organic matter in the upper layer of soil, improving its fertility and ability to hold water and support better plant production. His book triggered the study of soil biology.
Friends of the earth
In 1881 Charles Darwin wrote of earthworms, ‘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.’ 1 By watching worms he recorded how, among other activities, they mix mineral and organic matter in the upper layer of soil, improving the fertility.
Burrowing and casts
Earthworms have an important role in breaking down soil and complex chemicals in plant litter and other organic matter. Their burrowing improves soil drainage, conditions for roots, aeration and soil structure.
Worm casts on the soil surface are a mix of soil and plant material, rich in microbes and many nutrients. They aid the formation of small clumps of soil (aggregates), which allow the passage of air and moisture through the soil. The number of casts varies according to the seasons, peaking in winter, but they can amount to 25–100 tonnes per hectare a year.
Earthworms help form topsoil by depositing their casts on the soil surface. In one study, sticks and stones left on the surface were buried by worm casts, at a depth of almost 1 centimetre per year.
In New Zealand it is only introduced earthworms such as Aporrectodea caliginosa that leave casts on the soil surface. In native forests, where there are only native earthworms, there is no surface casting.
Weight and number of earthworms
In native forests the weight of earthworms has been found to be greater than that of all other animal groups in the area, including birds and mammals.
In grazed, fertile pastures the earthworm weight may also exceed that of the grazing animals. The mean weight of earthworms in some of New Zealand’s best agricultural soils ranges from about 640 to 3,000 kilograms per hectare. Compare this with 15 sheep per hectare, weighing about 800 kilograms in total.
There may be up to 1,000 earthworms per square metre, or 10 million per hectare. Numbers are at a maximum in spring, and are usually lowest after summer droughts.
In less intensively farmed and less productive hill country, there may be only a few hundred worms per square metre, including a few native species such as the large Octochaetus multiporus. Of course, the number and weight of grazing animals on hill-country pasture is also less.
Farming: impact on numbers and diversity
When farmland is cultivated, earthworm numbers decline through both physical harm and change in the types and amount of plant material they feed on.
Studies in Canterbury showed that worm numbers dropped from about 900 per square metre (9 million per hectare) under pasture, to less than 200 per square metre (2 million per hectare) under long-term arable cropping. There was also a decline in species diversity, with up to five species (Aporrectodea caliginosa, A. trapezoides, A. rosea, Lumbricus rubellus, Octolasion cyaneum) under pasture, but only two (A. caliginosa, A. trapezoides) under arable cropping.
The weight of earthworms fell from about 9,000 kilograms per hectare under pasture to some 200 kilograms per hectare after more than nine years’ arable cropping. The potential benefits of breaking down organic matter and improving soil structure are also severely reduced.