SA Grain July 2017

Earthworms and its

role in soil

arthworms are cylindrical, segmented worms from the

phylum

Annelida

. They live in the soil, feeding on living

and dead organic material. More than 2 000 earthworm

species have already been identified, of which 300 species

have been recorded in South Africa.

Depending on the species, a mature earthworm’s size can vary

from 10 mm to 3 m. Earthworms are further classified into three

groups according to their behaviour and habitat namely epigeic,

endogeic and anecics.

Epigeic earthworms do not dig tunnels but live on the soil surface

where they feed on rotting plant and animal material.

Endogeic earthworms live within the soil where they form horizontal

tunnels in the top 10 cm to 30 cm of the soil. Endogeic earthworms

ingest soil in order to absorb nutrients from degraded organic mate-

rial present in the soil.

Earthworms in the anecic group dig deep within the soil during

the day but surface during the night to feed and deposit their

casts. Anecic earthworms form deep, permanent, vertical tunnels

in which they move to the surface to obtain plant material on

which they feed.

Earthworms cause physical, chemical and biological changes to

the soil profile, which affects the habitat and activities of other

organisms present in the soil system. They recycle dead plant

material to compost and improve nutrient availability by pulling or-

ganic material deeper into the soil.

By ingesting and digesting organic material to form humus, nutri-

ents are made available for plants. Their burrowing activity improves

soil structure, as well as aeration and the drainage ability of the soil.

Earthworm populations are dependent on both physical (tempera-

ture, moisture, aeration and texture) and chemical properties (pH)

of the soil, as well as the availability of food and the ability of the

species to reproduce and distribute.

They are less abundant in disturbed soils and are typically only ac-

tive when enough moisture is present. Although earthworms vary

in their preferences, the majority prefers a neutral to slightly acidic

environment.

For example, fertilisers containing nitrogen create acidic conditions

fatal to earthworms. Expired earthworms can often be seen on the

surface of soil treated with DDT, lime sulphur or lead arsenate.

On the other hand, by adding organic matter on a regular basis,

their food and nutrient requirements will be taken care of and mois-

ture content will be optimal as organic material retains moisture.

Biological factors such as predators can also play a role in the suc-

cess of an earthworm population because a wide variety of animals

such as rats, birds, foxes, moles, snakes, frogs, snails, toads, ants

and beetles feed on them.

A mature earthworm possesses a clitellum (belt-like swelling)

that forms part of its reproduction system containing both male

and female reproductive organs (

Photo 1

). Even though each earth-

worm possesses both male and female organs, they cannot fertilise

themselves.

Therefore, two earthworms are needed for reproduction. Copu-

lation between two earthworms takes place at night on the soil

surface. Sperm from each worm is deposited in the other so that

both can produce a cocoon containing fertilised eggs. Each

cocoon produces one to 18 earthworms during favourable condi-

tions. Under field conditions, the average lifespan of an earthworm

is four to eight years.

Earthworms are wonderful little creatures that will flourish within

the right conditions. Therefore, before introducing earthworms

to farming or garden soil, conditions should be favourable for the

survival of the earthworms. For instance, cultivated soil from older

gardens are usually compacted, have poor drainage and structure

and usually experience a shortage of organic matter. Further-

more, earthworms should not simply be placed on the soil surface

because they will be preyed on by birds or die from exposure to

the sun.

If earthworms are to be introduced into the soil, the following guide-

lines should be followed:

Dig a few trowel depth holes every square metre.

Add water as well as some natural compost.

Place about ten earthworms in each hole and fill it up with soil.

The earthworms will constantly need a food source in the form

of organic material, left as cover layer either from leaves or com-

post or as grass clippings.

Note that although advantageous, earthworms are not

the antidote for infertile soil and poor management.

CHARNÉ MYBURGH,

ARC-Grain Crops, Potchefstroom

1: A mature earthworm with the clitellum (belt-like

swelling) clearly visible.

FOCUS

Fertiliser

Special