1

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

2

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.

3

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.

4

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.

Raw Results – count and biomass of earthworms at each site

Site Site 1: Site 2: Site 3:

Location

Aspect

Current planting where samples were taken

Historical planting

Soil management history

Information sources

Site 1: Site 2: Site 3:

Count (No.) Biomass (g) Combined

Adult & Juv

Count (No.) Biomass (g) Combined

Adult & Juv.

Count (No.) Biomass (g) Combined

Adult & Juv. Adult Juv. Adult Juv. 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)

5

Calcu

lating th

e area o

f each

samp

le d

ug b

y the

spad

e

If you

r spad

e is 17

cm (0

.17

m) w

ide, th

e area of a sam

ple h

ole w

ill be 0

.17

x 0.1

7 = 0.0

3m

2 (2 significan

t figures)

Spad

e wid

th (W

) = Length

of Sam

ple (L)

Are

a of th

e surface o

f the sam

ple = W

x W = W

2

Becau

se we are assu

min

g that earth

wo

rms are n

ot fo

un

d b

elow

app

roxim

ately 1 spad

e length

, we can

assum

e that u

sing

the su

rface area of th

e samp

le (rather th

an vo

lum

e) we can

estimate th

e nu

mb

er of earth

wo

rms p

er hectare u

nd

er the

Calcu

lation

s (Use 2

significan

t figures)

Examp

le calculatio

n fo

r 1 site

Site 1

Site 2

Site 3

N = A

verage

nu

mb

er o

f earth

wo

rms p

er sam

ple

fo

r each

site (fro

m p

age 4)

20

M = A

verage

total b

iom

ass of e

arthw

orm

s pe

r sam

ple

for e

ach site

(from

page 4

) 4

.00

g

A = A

rea o

f soil sam

ple

in m

2

A = len

gth x w

idth

of sam

ple

A

= W x W

0.1

7m

x 0.17

m

= 0.0

3m

2

H = N

um

be

r of so

il samp

les th

at will fit in

to 1

h

ectare

H = 1

0,0

00

m2 ÷ A

10

,00

0m

2 ÷ 0.03

m2

= 33

0,0

00

samp

les/ hectare

E= Nu

mb

er o

f earth

wo

rms p

er he

ctare E = H

x N

33

0,0

00

x 20

= 6

,60

0,0

00

earth

wo

rms / h

ectare

ME = M

ass of e

arthw

orm

s per h

ectare in

grams

ME = H

x M

33

0,0

00

x 4.00

g = 1

,30

0,0

00

g / hectare

1 gram

of livin

g earth

wo

rm b

iom

ass make

s 1

gram o

f top

soil p

er ye

ar Th

ere

fore

ho

w m

any gram

s of to

pso

il do

ou

r w

orm

s make

/ he

ctare?

Co

nve

rt ME fro

m gram

s to to

nn

es

1 to

nn

e = 1,0

00

,00

0 gram

s M

E (grams) ÷ 1

,00

0,0

00

= ME

(ton

nes)

If there are 1

,32

0,0

00

g of w

orm

s in

1 h

ectare, they w

ill make

1,3

00

,00

0 gram

s of to

pso

il / year. 1

,30

0,0

00

g / hectare ÷

1,0

00

,00

0

= 1.3

ton

nes / h

ectare

If it costs $

40

to b

uy 1

ton

ne

of to

pso

il and

ou

r e

arthw

orm

s mad

e 1

.32

ton

ne

s pe

r he

ctare –

ho

w m

uch

mo

ne

y are th

e e

arthw

orm

s makin

g fo

r the

farme

r? Eco

no

mic valu

e o

f earth

wo

rms in

te

rms o

f top

soil p

rod

uctio

n / h

ectare

$

= ME x 4

0 (1

ton

ne o

f top

soil co

sts $4

0)

1.3

ton

nes / h

ectare x $4

0 = $

52

p

er hectare.

The w

orm

s in 1

hectare o

f soil are

makin

g $5

2 w

orth

of to

pso

il each

year. O

n a 1

00

hectare farm

that is

$5

,20

0 w

orth

of to

pso

il / year.

W = W

idth

of

Spad

e Used

(cm)

6

Example Soil Management Histories:

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.

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 angle of approx. 30 degrees. Large oak trees and mixed natives on the northerly border. Open lawn on southerly border.

Flat, open on all aspects. No shadows.

Fence is alongside car park with narrow strip of lawn (1m wide) between the car park and the fence.

Current planting where samples were taken

Mixed vegetable planting / fruit trees. Samples taken from bean / tomato / spinach mixed beds.

Perennial ryegrasses. Sample taken from outside edge of the field.

Perennial ryegrasses beside concrete block fence. Sample taken from soil immediately adjacent to the fence (in the sprayed zone).

Historical planting

Mixed vegetables in rotating crops for the past 4 years. Previous planting - overgrown scrub.

Perennial ryegrasses > 50 years. 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

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: steve.wratten@lincoln.ac.nz | sue.unsworth@lincoln.ac.nz | j.bay@auckland.ac.nz

http://bioprotection.org.nz | http://www.lincoln.ac.nz | http://lens.auckland.ac.nz

Copyright © Lincoln University, 2011