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

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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. Would earthworm castings (or earthworm manure) help your plants prosper and flourish? If so, how much should you use?


Areas of Science
Time Required
Very Long (1+ months)
Material Availability
Readily available
Low ($20 - $50)
Sabine De Brabandere, PhD, Science Buddies


Find the ideal proportion of earthworm castings to soil for young garden plants.


Earthworms play an important role in soil fertility; they are actually often used as a quick indicator of soil fertility. While their burrowing improves soil structure, the worm castings (or worm manure) enrich the soil by converting dead plant or other organic material back into nutrients. Getting worms to feel at home in your soil is not as easy as it sounds. Worms have their preferences and store-bought earthworms might quickly migrate to other places when released in your garden. Would earthworm castings —also called vermicompost—be enough to improve the soil and help improve plant growth?

Worm manure and soil are mixed together with a small shovel
Figure 1. Enrich your soil with worm castings before potting the plant.

In addition to air, water and sunlight, young plants need nutrients from the soil—like nitrogen, phosphorus, and potassium—to grow and prosper. Some gardeners add animal manure or chemical fertilizers to improve the nutrient content of the soil. Adding earthworm castings is an environmentally friendly alternative that is safe for pets and children.

Using vermicompost has some important benefits. The worm's gut converts nutrients that available in organic matter they eat into a form that is readily taken up by plants. In addition, the nutrients in vermicompost are released slowly over time. Worm castings also improve the soil texture, soil aeration, and water retention.

You might wonder about the amount of worm castings you should add to your soil to get optimal plant growth. The answer will depend on the quality of your soil, the type of plants you would like to grow, and the particular worm castings used. Note that the nutrient content of worm castings depends on the food the worms consumed. Can you figure out a general guideline? Is it possible to mix too many castings into the soil?

In this project, you will grow plants in pots containing various amounts of earthworm castings mixed into the potting soil, and evaluate their growth and health. You will limit your study to one type of soil (store-bought potting soil), one type of castings (store-bought or from the same harvest of a worm farm) and 2–3 types of plants. Will your plants flourish in a particular mixture of soil and castings?

Terms and Concepts

In order to properly conduct this experiment, you should understand the following terms and concepts:



You can check out this resource to better understand earthworm biology and different types of earthworms:
  • Card, A. (2011, December). Earthworms. Colorado State University Extension. Colorado Master Gardener Program. Retrieved September 10, 2014.
This resource provides information on the benefits of earthworms:
  • United States Department of Agriculture. (n.d.). Wonders of Worms. Natural Resources Conservation Service. Retrieved September 10, 2014.
This resource leads to research studies on the benefits of earthworm castings:
  • Castings4Growth, LLC. (n.d.). Research. Retrieved November 2, 2015.

Materials and Equipment

Experimental Procedure

In this project, you will evaluate the capacity of the soil to support plant growth—also called soil productivity—of soil samples composed of different mixtures of potting soil and vermicompost. You should plan to have at least three pots per plant variety for each mixture of soil and castings. It is possible to limit the study to one particular plant type, but two or three types might give more convincing results. As an example, for two plant types (3 pots × 2 types = 6 pots) and three different soil mixtures (6 pots × 3 soil mixtures), you will need 18 pots. The size of your pots will depend on how long you want to run your test.

  1. Prepare the pots with mixtures of earthworm castings and potting soil. You need at least three pots per plant variety for each mixture of soil and castings.
    1. Select the mixtures you will test. Some possible mixtures of earthworm castings are 0%, 20%, 40%, 60%, 80% and 100%. We recommend 0%, 40% and 80%.
    2. Create your mixtures. As an example, to create 100 cups of a mixture of 40% castings, add 40 cups of earthworm castings to 60 cups of potting soil. Mix well.
    3. Fill and label your pots.
Eight plastic pots are filled with a mixture of worm manure and soil mixed at different ratios
Figure 2. Fill identical pots with the prepared soil mixtures. Add water-resistant labels.
  1. Select plant varieties to be used for the experiment. (Some options: Tagetes, Lactuca sativa, and Viola tricolor.) Buy three healthy looking young plants for each soil composition you will test.
  2. Plant one plant per pot.
  3. Place the pots in locations where they will get equal amount of light, ambient temperature and make sure you provide them with equal amounts of water.
  4. Start a log in your lab notebook, writing down the start of the test and taking regular notes (such as twice a week) on the plants' growth and health. Look for ways to gather quantitative data, like the number of buds, the height of the plant, etcetera. Following resource can help you:Measure plant growth.
  5. At the end of the experiment, photograph each of the plants. You can use these photographs to decorate your Display Board.
  6. Present your quantitative data in tables and graphs. These will help you detect trends.
    1. Below is an example table listing the number of flowers counted over the total testing period for one particular plant type. Note that you should also average your three test samples (your three pots with identical soil composition).

      Number of flowers produced over the total test period for Plant A
      Soil Mixture 0 % vermicompost 40 % vermicompost 80 % vermicompost
      Pot 1    
      Pot 2    
      Pot 3    
      Table 1. Example of a table gathering quantitative data.

    2. The data in Table 1 can easily be represented with a bar graph, showing the soil composition on the horizontal (x-axis) and the average number of flowers produced on the vertical (y-axis).
    3. For help with data analysis and setting up tables, see Data Analysis & Graphs.
  7. Looking at your results, what do you conclude? Did vermicompost mixed in with potting soil help plants grow? For a guide on how to summarize your results and write conclusions based on your data, see Conclusions.
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General citation information is provided here. Be sure to check the formatting, including capitalization, for the method you are using and update your citation, as needed.

MLA Style

De Brabandere, Sabine. "Earthworm Castings — The Ideal Proportion in Soil for Young Garden Plants." Science Buddies, 23 June 2020, https://www.sciencebuddies.org/science-fair-projects/project-ideas/PlantBio_p002/plant-biology/earthworm-castings-gardening?from=Home. Accessed 3 Dec. 2023.

APA Style

De Brabandere, S. (2020, June 23). Earthworm Castings — The Ideal Proportion in Soil for Young Garden Plants. Retrieved from https://www.sciencebuddies.org/science-fair-projects/project-ideas/PlantBio_p002/plant-biology/earthworm-castings-gardening?from=Home

Last edit date: 2020-06-23
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