Testing the soil with earthworms: how many variables?

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Hi. I figure out how to post. I have a question about science fair ideas. My son and I think the one on testing soil with earthworms is perfect for him. We have been advised not to use humans or animals because they have too many variables. Testing the soil with earthworms seems to only have one variable. What do you think?

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Thank you for visiting Science Buddies!

Testing earthworms and plants/soil is a great project! There is a link below that you and your son can view to find additional information on this experiment. This experiment would be ideal for a person with background knowledge on plants since plants have various factors that must be accounted for in order order to have a successful experiment, such as temperature. If your plant can grow in more mild temperatures, this may be a better adjustment to the experiment. Keep in mind the amount of time that allotted to complete this project, as plant growing can take up to several months.

You can visit this site for a Science Buddies project on earthworms, "Earthworm Castings — The Ideal Proportion in Soil for Young Garden Plants": https://www.sciencebuddies.org/science- ... ng#summary