Composting In Schools

Plant Growth Experiments

The instructions below outline a protocol for conducting plant growth experiments in the laboratory. You may want to make adaptations for use in a greenhouse or outdoors.

Purpose: To determine the effect of compost on plant germination and growth.



1. Design your own experiment. There are many possibilities--a few ideas are listed here, but the variations are endless:

  • Test various combinations of soil and compost on plant growth. For example, you might wish to dig a soil sample from your school yard and mix it with various amounts of finished compost for planting experiments. (Natural soil is better than prepackaged potting soils for experiments such as this because the potting mixes are formulated for optimal plant growth and already contain significant amounts of compost or humus.)
  • Another possibility is to mix your own potting soil by using vermiculite, sand, and compost. Creating several mixtures using the same percentages but different types of compost is a good way of comparing the influence of the various types of compost on plant growth. For example, you could compare compost at various levels of maturity, compost created using different mixtures of organic wastes, or vermicompost versus compost created in a thermophilic system.
  • If you are interested in investigating the effects of compost tea on plant growth, you could fill the pots with a sandy soil or potting medium such as vermiculite, then use compost extracts for watering.
  • 2. Whatever type of experiment you choose, make sure that you design your experiment to include replicates of the various treatments. For example, your design might look like the following:

    (% soil/% compost)
     # Flats
    (with 6 plants in each)
     # Plants
    100% compost 3 18 
     25/75 3 18
     50/50 3 18
     75/25 3 18 
     100% soil 3 18


    3. Plant your seeds, water them, and place them in a well-lit location. Many type of seeds will work, but radish or lettuce are often chosen because they grow quickly. Melon seeds are sensitive to fungal diseases, and thus they provide a sensitive indicator of whether fungi have been killed through heating or curing of the compost.

    4. Keep all the pots in the same setting to minimize any variation in temperature, lighting, pests, and other environmental factors. Even when the environmental conditions are kept as constant as possible, it is a good idea to randomize the grouping of plants rather than placing all the plants that are receiving the same treatment together in one group. This helps to further minimize the effect of any environmental differences.

    5. Record on a daily basis the number of seeds that have germinated, plant growth, and observations about plant health such as color, vigor, or damage due to pests and diseases. You can decide what measurements to use as indicators of plant growth; possibilities include plant height, number and size of leaves, and dry weight of the entire plant at the end of the experiment. (For dry weight, weigh the plant after drying in a 105C oven for 24 hours.)

    Analysis and Interpretation

    1. Graph germination rates and plant growth over time for the different treatments. Also, determine the mean number of seeds germinated and mean size or mass of the plants at the end of the experiment. Compare average germination rates, plant growth, and health for the different experimental treatments. Based on your experiments, what was the optimal potting mix for plant germination? For plant growth? For plant health?

    2. Some things may have gone wrong in your experiments. For example, you may have over-watered your plants, causing them all to die from fungal infection regardless of the treatment. Or you may have taken measurements only on plant height, and later decided that measuring the number of leaves and length of the main stem would have given better information. These types of problems are normal and can be used as a basis for redesigning the experiment. How might you change your experimental design if you were to carry out another set of growth experiments?

    3. You may not find any differences between the treatments. Or, you may discover that the plants grown without compost did best. If this is the case, it may be difficult to determine whether the compost had no effect, or you did something wrong. The tendency is to assume the compost really has an effect and to attribute insignificant or negative results to experimental mistakes. However, the interpretation of results should not be biased by your predictions or preconceived ideas about the way experiments will turn out. Often unexpected results lead to important insights and questions. Maybe your compost is of poor quality, or maybe the plant species you chose grows well in poor soils. Explore all the possibilities for explaining your results with an open mind, through discussions and new experiments.

    4. The conclusions and recommendations that you are able to make based on your results will depend on how and where you carried out your experiments. For example, if you used potted plants in a classroom or greenhouse, it may be difficult to extrapolate from your results to what would happen if the same plants were grown outdoors in a garden. However, your results may give you some ideas about what would happen, allowing you to make predictions or hypotheses. You could then use these predictions to design a new experiment on plant growth in a garden setting.


    Science &

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    Cornell Waste Management Institute ©1996
    Department of Crop and Soil Sciences
    Bradfield Hall, Cornell University
    Ithaca, NY 14853