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Be a soil scientist


Investigate whether adding homemade compost to soil helps your plants grow bigger and stronger.

  • Estimated time: 30 minutes
  • Location: Outdoors & Indoors
  • School term: All year round
  • Level of experience: No experience needed
  • Subject(s): Maths, Science

Learning objectives

  • State three ways to achieve a fair test
  • Observe and record the plant growth
  • Learn to interpret data


Purchase identical plants from a garden centre, DIY store or supermarket.


  • Three identical containerised plants (size, shape and health)
  • Three larger pots (than the container plants)
  • Well-rotted compost from your compost bin
  • Garden soil
  • Multi-purpose peat-free compost
  • Watering can or clear plastic bottle for watering containers
  • Sieve, trowels & gloves
  • Equipment to record growth (paper, pencils or ICT)

Step by step

  1. Discuss the reasons for doing this experiment and ask the group to predict the results.
  2. Collect some homemade compost and sieve it if required. Next, collect some soil from the garden. Try to choose an area which has not had any compost added recently.
  3. Present the group with the equipment and ask them how they would conduct a ‘fair test’.
  4. Pot on plants into the larger containers, one containing only soil and another with a mixture of 75% homemade compost and 25% soil which have been mixed together. To make the test fair, set up a control plant that is planted up into multi-purpose compost. Remember to label each one with the compost mix you have used.
  5. Water the plants using the same method and amount of water.
  6. Place both pots in the same environment (inside or outside depending on the type of plant).
  7. Record the growth over a few weeks by counting leaves or flowers, or measuring the height. Review the results compared with the initial predictions.

Hints & tips

  • In winter, try this activity with houseplants instead
  • Potatoes work well in spring
  • Pansies and marigolds are good value for money
  • Mark lines on the plastic bottle to ensure the plants get the same amount of water
  • Use this activity as part of the Earth Matters class growing topic
  • Try resources that will help to extend the topic of soils, such as How much soil? and Soil texture test