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Earthworm Castings — The Ideal Proportion in Soil for Young Garden Plants

Time Required Very Long (1+ months)
Material Availability Readily available
Cost Low ($20 - $50)


Everybody knows that worms are good for the soil, but not everybody knows why. Here's a project that investigates just one of the ways earthworms improve the earth.


The purpose of this project is to find out what the ideal proportion of earthworm castings to soil is for young garden plants.

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Science Buddies Staff. "Earthworm Castings — The Ideal Proportion in Soil for Young Garden Plants" Science Buddies. Science Buddies, 25 Sep. 2014. Web. 26 Oct. 2014 <http://www.sciencebuddies.org/science-fair-projects/project_ideas/PlantBio_p002.shtml>

APA Style

Science Buddies Staff. (2014, September 25). Earthworm Castings — The Ideal Proportion in Soil for Young Garden Plants. Retrieved October 26, 2014 from http://www.sciencebuddies.org/science-fair-projects/project_ideas/PlantBio_p002.shtml

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Last edit date: 2014-09-25


Earthworms play an important role in soil fertility, and they are often used as a quick indicator of soil fertility. What do earthworms do to the soil and do they really help to improve plant growth?

Terms and Concepts

In order to properly conduct this experiment you should understand the following terms and concepts:
  • Earthworms, how they live and their benefits to soil fertility.
  • Soil productivity and soil nutrients
  • Worm castings, how they are produced and their composition


You can check out this resource to better understand earthworm biology and different types of earthworms: This resource provides information on the benefits of earthworms:

Materials and Equipment

  • Earthworm castings
  • Potting soil
  • Pots (enough to have 2 pots for each plant variety at each combination of castings to soil)
  • Plants (at least three varieties)

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Experimental Procedure

  • Prepare the pots with mixtures of earthworm castings and potting soil. You should plan to have at least two pots per plant variety for each mixture of soil and castings. Some possible mixtures of earthworm castings are 0%, 20%, 40%, 60%, 80% and 100%.
  • Select plant varieties to be used for the experiment. Remember that you will need at least two test of each plant with each mixture of castings, so don't select too many different varieties. We suggest using no more than three different plant types to keep the experiment manageable. (Some options: Tagetes, Lactuca sativa, and Viola tricolor)
  • Plant one plant per pot and make sure that you test each plant variety in each mixture of castings.
  • Place the pots in locations where they will get equal amount of light and make sure you provide them with equal amounts of water.
  • At the end of the experiment, photograph each of the plants examine them for growth.

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  • Instead of earthworm castings, you could try adding other additives to potting soil and investigate how the additives affect plant growth. Some additives you could try include bone meal, blood meal, compost, fish meal, kelp, livestock manure, and wood ash. You will want to do some research into the additives you pick so you can make a hypothesis about how they might affect plant growth.

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