What’s a Worm Worth?
Steve Wratten, Harpinder Sandhu, Sue Unsworth, Jacquie Bay
Earthworms, large or small, are all annelids belonging to the class Oligochaeta.
New Zealand has
native earthworms that belong to the family Megascolecidae. These are the earthworms that you will
find in native forests. Those that you commonly find in your garden, city parks, farmlands and orchards
are introduced species that belong to the family Lumbricidae.
Decomposers down-under
Earthworms are decomposers, feeding on dead and decaying plant
matter under the soil surface. Different species occupy niches in
different sections of the soil profile. Some species are well adapted to
live in the leaf litter of the forest floor while others occupy the rich
dark topsoil (just below the lead litter) and others the subsoil.
No matter what the niche, the earthworm is an essential component
of our ecosystems, with a public profile that does little to recognise its
true worth. Without decomposers, such as earthworms, bacteria and
fungi, the nutrient cycles would come to a standstill. Decomposers
play an essential role in unlocking the nutrients held in the tissue of
dead organisms. Through the work of the decomposers, cells and
tissues are broken down. The decomposer gains the energy it needs
to live from the dead tissues. At the same time important organic
matter and nutrients such as carbon, nitrogen and phosphorus are
released back into the environment., creating in the case of
earthworms rich soils that we rely on for horticultural and agricultural
productivity.
“The plough is one of the most ancient and most
valuable of man's inventions; but long before he
existed the land was in fact regularly ploughed, and
still continues to be thus ploughed by earth-worms. 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 (p 313)
The formation of vegetable mould, through the action of worms.
London: John Murray
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What’s a Worm Worth?
New Zealand spends millions of dollars each year on soil management practices. These are activities that
are designed to improve the productivity of soil such as the addition of fertilisers or compost, irrigation or
drainage, crop rotation or effluent application.
This investigation is best carried out between the months of September and December (Southern
Hemisphere), however useful results can be obtained in any season.
Aim:
a)
To investigate the effect of soil management practices on earthworm populations.
b)
To calculate the economic value of earthworms in terms of the topsoil they produce.
Materials:
Garden spade, plastic bags, newspaper, ruler, data sheet, electronic balance (2dp), calculator.
Method:
Make sure that all groups in your class agree on the method so that you can combine the results.
1.
Select two or more contrasting sites, where different soil management practices have been in
place.
Examples of contrasting sites could include:

A well maintained garden which has had a lot of organic matter (e.g. compost) added to it.

A fence line with no vegetation because it is regularly sprayed with weed killer.

A grassy area such as a garden lawn or city park (with permission in all cases!).

A farm paddock with a history of fertiliser use.
2.
Create a square on your spade:
Measure the width of the spade blade and record this in
the data sheet (W).
Measure this distance up from the bottom of the spade
blade.
Using a permanent marker pen, draw a line across the
top of W.
3.
At each of your sites, dig holes the same width as your
spade, and as deep as the horizontal red line on your
spade. Collect the sample and put it into a labelled bag.
4.
Spread the soil from each sample on to a plastic sheet or newspaper. Pull
the soil sample apart with your fingers and count the number of juvenile
and adult earthworms in each sample. See page 3 for instructions on how
to tell the difference between juvenile and adult earthworms.
5.
Using an electronic balance to 2 decimal places, measure the total mass
of earthworms each sample. This is called the earthworm biomass.
6.
Record your measurements in the data sheet on page 4 and then calculate the
value of the earthworms based on the total biomass present in your samples.
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Earthworm Identification
You will not be able to identify the individual species of earthworm that you find; however you will be
able to tell whether the earthworms are juveniles or adults.
The adult earthworm has a recognisable saddle or clitellum that is part of the reproductive system,
forming a sac or cocoon into which the eggs are deposited. The clitellum is visible in adult earthworms,
distinguishing them from the juveniles.
Clitellum
Adult earthworm showing clitellum
The Basis of the Calculations
The economic value of earthworms in soil formation will be calculated based on the assumption that:
 that one tonne of earthworms forms 1000kg of topsoil.
 the value of farmland includes the contribution to production
made by its topsoil.
 A tonne of topsoil can be purchased from garden centres in
Christchurch, New Zealand at a rate of NZ$40 per tonne.
Useful Facts:








1 hectare = 10,000 m2
1 tonne = 1000kg; 1kg = 1,000g
1m = 100cm
1 tonne of earthworms in a hectare creates 1000kg of topsoil/hectare/year
Topsoil is worth $40/tonne
Topsoil that you can purchase commercially comes from building sites where it is scraped off the
ground before construction starts. Often this scraped layer is not just the topsoil, but also contains
the less valuable lighter coloured subsoil.
The most valuable topsoil is dark in colour and contains a
high level of organic matter.
The price charged by commercial topsoil suppliers
(around $40 per tonne) is probably far less than the real
worth of good quality topsoil when we consider how long
it takes to produce and the value that this soil adds to the
productivity of the land.
Some assumptions we’ve made:
Earthworms are uniformly spread through the topsoil.
Earthworms will not be found deeper than a spade depth.
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Results:
Complete the results table for each of your samples.
To improve the reliability of your data, you should share the results of each group in your class, enabling
you to take the average of several samples for each site.
Soil Management Histories:
Describe (and if possible photograph) your sites. Try to find out as much as possible about the soil
management history. An example has been provided for you on page 6.
Site
Site 1:
Site 2:
Site 3:
Location
Aspect
Current
planting where
samples were
taken
Historical
planting
Soil
management
history
Information
sources
Raw Results – count and biomass of earthworms at each site
Site 1:
Site 2:
Count (No.)
Adult
Juv.
Biomass (g)
Combined
Adult & Juv
Site 3:
Count (No.)
Adult
Juv.
Sample 1 (Gp 1)
Sample 2 (Gp2)
Sample 3 (Gp3)
Sample 4 (Gp4)
Sample 5 (Gp 5)
Sample 6 (Gp 6)
Sample 7 (Gp 7)
Sample 8 (Gp 8)
Average
(Enter into the table
on page 5)
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Biomass (g)
Combined
Adult & Juv.
Count (No.)
Adult
Juv.
Biomass (g)
Combined
Adult & Juv.
Calculating the area of each sample dug by the spade
E= Number of earthworms per hectare
E=HxN
H = Number of soil samples that will fit into 1
hectare H = 10,000m2 ÷ A
A = Area of soil sample in m2
A = length x width of sample
A=WxW
M = Average total biomass of earthworms per
sample for each site (from page 4)
N = Average number of earthworms per sample
for each site (from page 4)
Calculations (Use 2 significant figures)
330,000 x 4.00g
= 1,300,000g / hectare
330,000 x 20
= 6,600,000 earthworms / hectare
10,000m2 ÷ 0.03m2
= 330,000 samples/ hectare
0.17m x 0.17m
= 0.03m2
4.00g
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Example calculation for 1 site
Site 1
Site 2
If your spade is 17cm (0.17m) wide, the area of a sample hole will be 0.17 x 0.17 = 0.03m 2 (2 significant figures)
Spade width (W) = Length of Sample (L)
Area of the surface of the sample = W x W = W2
Because we are assuming that earthworms are not found below approximately 1 spade length, we can assume that using
the surface area of the sample (rather than volume) we can estimate the number of earthworms per hectare under the
ME = Mass of earthworms per hectare in grams
ME = H x M
1,300,000g / hectare ÷ 1,000,000
= 1.3 tonnes / hectare
If there are 1,320,000g of worms
in 1 hectare, they will make
1,300,000 grams of topsoil / year.
Convert ME from grams to tonnes
1 tonne = 1,000,000 grams
ME (grams) ÷ 1,000,000 = ME (tonnes)
1 gram of living earthworm biomass makes 1
gram of topsoil per year
Therefore how many grams of topsoil do our
worms make / hectare?
If it costs $40 to buy 1 tonne of topsoil and our
earthworms made 1.32 tonnes per hectare –
how much money are the earthworms making
for the farmer?
1.3 tonnes / hectare x $40 = $52
per hectare.
The worms in 1 hectare of soil are
making $52 worth of topsoil each
year.
On a 100 hectare farm that is
$5,200 worth of topsoil / year.
Economic value of earthworms in
terms of topsoil production / hectare
$ = ME x 40 (1 tonne of topsoil costs $40)
W = Width of
Spade Used (cm)
Site 3
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Example Soil Management Histories:
Site
Site 1: Community Garden
Site 2: School Field, Auckland
Grammar School
Site 3: Fence edge
Location
Cathedral Place, Parnell,
Auckland
Mountain Road, Epsom,
Auckland
Cathedral Place, Parnell,
Auckland
Aspect
Land slopes towards the east at an Flat, open on all aspects.
angle of approx. 30 degrees.
No shadows.
Large oak trees and mixed natives
on the northerly border. Open
lawn on southerly border.
Current
planting where
samples were
taken
Mixed vegetable planting / fruit
trees.
Samples taken from bean /
tomato / spinach mixed beds.
Historical
planting
Mixed vegetables in rotating crops Perennial ryegrasses > 50 years.
for the past 4 years.
Previous planting - overgrown
scrub.
Perennial ryegrasses beside
concrete block fence.
Soil
management
history
Mulched every 4 months
Watered during the summer.
Drainage laid.
Watered during summer months.
Drainage.
Annual fertiliser.
Sprayed with weed killer every 6
months.
Information
sources
Community Garden Coordinator
School Grounds Manager
Grounds Manager
Fence is alongside car park with
narrow strip of lawn (1m wide)
between the car park and the
fence.
Perennial ryegrasses.
Perennial ryegrasses beside
Sample taken from outside edge of concrete block fence.
the field.
Sample taken from soil
immediately adjacent to the fence
(in the sprayed zone).
Conclusion
From the results, what conclusions (if any) can you draw about the effect of soil management practices on
earthworm populations and their value in your samples.
Discussion Questions:
1.
Considering the information you collected about soil management histories at each site, discuss
factors that may have contributed to the patterns seen in the data.
2.
What does the adult : juvenile ratio at each site suggest about the suitability of the soil at the time
of collection for sustaining earthworm populations. How could you use this ratio to track the
positive or negative impact of soil management over a period of time.
3.
Discuss the validity and reliability of your data.
4.
Discuss the value of the environmental service that worms provide in horticultural, agricultural and
forestry ecosystems, considering in your discussion soil management and productivity.
Acknowledgements:
This resource is derived from research on biodiversity at Lincoln University funded by the Foundation for Research, Science
and Technology (LINX0303) conducted by Professors Steve Wratten, Ross Cullen and Dr Harpinder Sandhu.
The resource was initially developed by Sue Unsworth and Steve Wratten for Science Outreach, Lincoln University and later
adapted by Jacquie Bay and Steve Wratten for the BioProtection Research Centre in collaboration with LENScience.
Photos and diagrams from istockphoto.com (used under licence), LENScience, Lincoln University, or the public domain.
For further information contact:
[email protected] | [email protected] | [email protected]
http://bioprotection.org.nz | http://www.lincoln.ac.nz | http://lens.auckland.ac.nz
Copyright © Lincoln University, 2011
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