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Investigating the Presence of Micro-organisms in the Different Soil Layers

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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.

Petri dish from top ©  Shirley Burchill Petri dish from side ©  Shirley Burchill


To find out if there are any micro-organisms present in the different soil levels of the samples taken from the deciduous forest.


Name of Layer


The Leaf Litter


The Humus Layer


The Humus mixed with Minerals Layer


The Mineral Layer mixed with very little Humus


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.

Introducing soil sample into Petri dish ©  Shirley Burchill

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.


The best way to show your results is by drawing what you see on each of the four Petri dishes.

Petri dish with nutrient agar ©  Shirley Burchill

Soil sample A

Petri dish with nutrient agar ©  Shirley Burchill

Soil sample B

Petri dish with nutrient agar ©  Shirley Burchill

Soil sample C

Petri dish with nutrient agar ©  Shirley Burchill

Soil sample D

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.


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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