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Grade 7 Beginner About 15 min Version 1 Awaiting review Premium

What makes things rot? Decomposition in sealed jars

When plants and animals die, or we throw away food scraps, decomposers break them down. Most decomposers are bacteria and fungi (moulds) too small to see one by one. They feed on the dead material and return its nutrients to the soil, where plants can use them again. Decomposers are living things, so they need water, warmth and oxygen. Materials made by living things, like peel, leaves and paper, are biodegradable; most plastics are not, because no decomposer can feed on them.

What you will learn

  • Compare how fast different materials decompose.
  • Find out which conditions decomposers need.
  • Explain why plastic waste is a problem.

Before you start

  • Know that microorganisms are living things.

Materials

  • Fruit peel, dead leaves, paper, a piece of plastic bag, garden soil, water

Equipment

  • Clear jars with lids, tape, labels, top-pan balance

Procedure

  1. Put fruit peel on moist soil in a sealed jar at 25 °C. Record how much has decomposed after 7, 14 and 28 days.
  2. Set up jars with a leaf, paper and plastic in the same conditions and record after 28 days.
  3. Set up fruit peel in dry soil, and fruit peel in a fridge at 5 °C, and record after 28 days.

How this simulation is simplified

  • Each material loses a fixed fraction of what is left each day: peel 8%, leaf 3%, paper 1.5%, plastic none, in warm moist soil.
  • Dry soil cuts the rate to a tenth, waterlogged soil to 40%, and the fridge to a quarter.