"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."
Charles Darwin, 1881
Nearly 200 different earthworm species have been found in New Zealand.
Most of these species are native to New Zealand, but around a dozen
species were accidentally introduced from Europe by the early settlers.
The native earthworms are in general much bigger than the introduced
species, with some native species reaching 30 cm or more in length
and having a diameter larger than your middle finger. Although these
species used to be widespread in New Zealand, nowadays they tend
to be largely confined to areas where the soil is less frequently
disturbed, such as in the forests, in old gardens and in the hills
and mountains.
The species that were introduced from Europe now dominate in our
gardens and agricultural land. They were inadvertently introduced
by the early European settlers, who brought plants and shrubs with
them.
The settlers' ships also used soil as ballast and this was off-loaded
at the ports once no longer required. The soil contained many earthworms
and they gradually spread out from the ports. Indeed some farmers
deliberately introduced earthworms to their land. after seeing the
benefits from the presence of such earthworms.
Earthworms are one of the most important groups of organisms that
live in the soil, since as they burrow their way through the soil
they also help to improve soil structure and fertility. They make
holes that help with root growth and allow air, water and fertilisers
to enter the soil. Their activities can also reduce erosion, increase
plant growth and help to provide food for important micro-organisms.
It is therefore wise to encourage the growth of these beneficial
creatures in the soil as they will help to maintain your soil for
you.
Their main requirement is a good supply of organic matter in the
soil which acts as their main food source. Some species prefer to
feed on dead plant roots, dead leaves, dead herbage or animal dung,
which gets partially broken down as it passes through their guts.
Other species preferentially graze upon the soil particles themselves,
while still others are said to feed upon fungi and small microorganisms
present in the soil.
After the soil has passed through an earthworm's gut some of the
soil nutrients become more available to plants and the structure
of the soil also becomes more stable.
Some earthworms are more suited to living in the soil while others
prefer to live in compost. Soil dwelling earthworms will not survive
in compost and likewise compost dwelling worms will usually not
survive for very long in soil.
In general, earthworms survive and multiply where there is a plentiful
food supply, sufficient (but not too much) moisture and where the
temperatures are neither very hot nor very cold. Addition of lime
has been shown to encourage earthworms as most of them prefer more
alkaline conditions.
For soil dwelling earthworms, soil management practices that increase
the amount of organic matter in the soil usually encourage earthworm
activity due to the increase in the amount of food available to
the earthworms. Consequently, as the addition of mulches, farmyard
manure or crop residues to the soil increases the amount of organic
matter in the soil, such practices also cause earthworm numbers
to increase. Similarly, where the addition of fertilisers to soil
results in an increase in plant production, a simultaneous increase
in earthworm numbers is found, since the fertiliser indirectly increases
the soil organic matter, the food source for the earthworms, by
increasing the plant production.
Overall, a much larger population of earthworms is usually supported
under a pasture than under a crop, since higher amounts of organic
matter are returned to the soil under the pastoral system. Therefore,
where practicable, crop residues should be returned to the soil
in order to encourage a large population of earthworms in cropped
soils. Such crop residues not only act as a food source for the
earthworms, but also help to conserve water in the soil and provide
protection for the earthworms from extreme temperatures.
Incidentally, the old wives' tale that if you cut a worm in half
you will end up with two worms is not actually true!! Only one part
may survive and even then it depends upon where the earthworm is
cut whether it is able to regenerate the missing part of its body...
For more information contact:
Dr Trish Fraser
The Earthworm Hole
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