Soil Temperature and how it affects the Growth of Yeast

Problem: Does a higher soil temperature cause an increased yeast population in an environment with a controlled common vegetation?

Hypotheses: A higher soil temperature causes an increased yeast population in an environment with a controlled common vegetation.

  Variables kept constant: Presence of jewelweed (our common vegetation)


                                       Amount of Soil Collected/ Number of Samples

                                       Petri film used for testing

                                       Test tubes used for dilutions

                                       Interval of time between samples taken within a day


Independent Variable: Soil temperature

Dependant Variable: Yeast



1)      Find two sites that have jewelweed and mark a 1m x 1m area.

2)      Select 4 times to collect samples, make sure there is an equal interval of time between all of the samples taken in a day.

3)      Take the soil temperature in four spots within each plot. Record the data. You must leave the thermometers in the same spots for the duration of the experiment.

4)      Collect 4 samples (only 8cm deep) from each site where the temperature was taken with a core sampler, remembering to rinse the core sampler after each sample. Place soil in Ziploc bags, and label them with their site number and use letters and times to label each sample accordingly.

5)      Place 1 cc of one sample into a culture tube containing 10 ml of sterile water. Cap the tube and shake vigorously.

6)      Using a serological pipette, remove 1ml of the soil/water mixture and place into a fresh culture tube.

7)      Add 9ml of fresh sterile water to the 2nd tube. Cap and shake vigorously.

8)      Repeat step 6 using the second, diluted tube, and then repeat step 7 with this 3rd tube.

9)      Continue step 8 with each additional tube until you have diluted the original soil/water mixture a minimum of 4 times. You should have a total of 5 culture tubes.

10)  Place 100μl samples from the 4th and 5th tubes onto their own separate, individual petri films. Allow to incubate at room temperature overnight.

11)  Examine each of the plates for individual bacteria colonies and choose the plate with the fewest colonies to make your estimates of the number of bacteria in the original 1cc soil sample.

12)  If there are not individual colonies but still a “lawn” at the ten to the negative fourth dilution, repeat steps 5-11, continuing to the 5th dilution, etc. until individual colonies are observed.

13)  Record the number of colonies observed. Choose the best dilution data, based upon the procedures located in step 11.

14)  Repeat entire protocol once every day for two days.


      Serological pipettes