الأسبوع 25 · الوحدة الرابعة، المفهوم 2.4 «التربة والتغير البيئي» (أ): أهمية التربة وتكوّنها · 3 دروس
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Decomposers, nature’s cleaners
Science P6: Term 2, Unit 4, pp. 60–65
GoalDescribe how bacteria, fungi and earthworms break down dead matter and return nutrients to soil.
New words
decomposer · a living thing that breaks down dead plants and animalsnutrient cycle · the way nutrients move from soil to living things and backfungi · living things such as mushrooms and moulds
Explanation
When plants and animals die, they do not just pile up. Decomposers feed on them. Bacteria, fungi and earthworms are decomposers. They break dead leaves and bodies into very small pieces and return carbon, nitrogen and other nutrients to their surroundings. What is left makes humus. Plants take up the nutrients, animals eat the plants, and when they die the cycle starts again. That is why decomposers are called nature’s cleaners. Without them, dead material would build up and the soil would run out of nutrients. Earthworms also mix the soil and make small tunnels that let air and water in. You can see decomposers at work if you bury a leaf in soil for two weeks and dig it up again. It will be thinner and torn.
dead leaf or animal→decomposers break it down→humus and nutrients in the soil→plants take up nutrients
Worked examples
A fallen leaf lies on the ground. After weeks a fungus grows on it and the leaf becomes soft and brown. The fungus is a decomposer.
Without decomposers, the dead plants in a forest would stay for ever. Nutrients would not return to the soil.
Habiba asks whether earthworms are harmful. No. They are helpful decomposers that make soil better.
Class activity
Bury a leaf. With a grown-up, bury one fresh leaf in a pot of garden soil (use gloves). Keep the soil damp and out of strong sun. After two weeks, dig it up gently and compare it with a fresh leaf. Draw both. Wash your hands. Never taste or sniff the soil.
