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Investigating
the Presence of Micro-organisms in the Different Soil
Layers
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PDF version
Introduction
...
Like all living things,
micro-organisms need to feed. The Petri dish contains a
jelly-like agar which holds food for micro-organisms. The
Petri dish is prepared under sterile conditions so that no
micro- organisms are present when you receive it.
Aim
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To find out if there
are any micro-organisms present in the different soil levels
of the samples taken from the deciduous forest.
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Letter |
Name
of Layer |
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A |
The
Leaf Litter |
|
B |
The
Humus Layer |
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C |
The
Humus mixed with Minerals Layer |
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D |
The
Mineral Layer mixed with very little Humus |
Method
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1. Set up the apparatus as shown in the
diagram below using a small sample of leaf litter. Take care
not to open the Petri dish too much and to use a sterile
spatula when you add your soil sample.
2. Carefully replace the Petri
dish lid and label your dish. Stick the label to the underside
of the Petri dish.
3. Secure the lid to the Petri
dish using Scotch tape.
4. Repeat points 1., 2. and 3.
with small amounts of the three other soil samples. Remember
to use a sterile spatula for each sample.
5. Store the four Petri dishes
upside down (with the labels facing up) at room temperature
for one week.
6. Your teacher will prepare a
control Petri dish for the class. This dish will not be
opened. It will be labelled and secured and stored with the
other Petri dishes.
7. After one week observe the
contents of the Petri dishes and record your observations. For
safety reasons DO NOT TAKE ANY OF THE LIDS OFF OF THE DISHES.
Results
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The best way to show your
results is by drawing what you see on each of the four Petri
dishes.
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Soil sample A
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Soil sample B
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Soil sample C
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Soil sample D
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N.B. Bacteria colonies are
mostly small, circular and look 'wet' or glisten. Fungal
colonies are mostly larger, irregular in shape and dull in
appearance.
The micro-organisms which grow inside the agar may well be
anaerobic. Those which grow on the surface are more likely to
be aerobic.
When you see a colony of either a bacterium remember that it
represents millions of individuals.
When you see a fungal colony remember that is has probably
grown from one spore.
Conclusion
Write a
conclusion to this experiment.
What has happened to the control Petri dish? Does this mean
your results are valid or invalid?
Are there any differences between the number of colonies of
micro-organisms which have grown in the four Petri dishes?
Explain any differences you observe.
When you look at soil you cannot see micro-organisms with the
naked eye. Why can you see the micro-organisms in the Petri
dishes?
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