What it shows
Natural materials such as peels, paper and cotton are broken down by soil organisms within weeks, while plastic, foil and glass stay unchanged. Burying samples and digging them up shows which waste is biodegradable and which will stay in a dump for centuries.
The science
Bacteria, fungi and small animals in soil feed on materials made by living things, such as cellulose in paper and cotton and the sugars in fruit peel, and break them into simpler substances. Plastics are long chains of carbon made in factories that these organisms have no way to digest, and metal and glass are not food at all. Moisture, warmth and air speed up decay, which is why the monsoon is a good time for this test.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Eight items: an apple peel, a leaf, a piece of newspaper, a cotton rag, a plastic bag piece, a bottle cap, a piece of foil, a polystyrene chip
- A patch of garden soil or a large pot
- Eight ice-cream sticks as labels
- A ruler
- Gloves
- A camera or a sketchbook
Safety
For this project: Wear gloves when digging and wash hands afterwards.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Measure and draw each item, and weigh it if a kitchen scale is available.
Dig eight small holes about 10 centimetres deep in a row.
Bury one item in each hole and push a labelled stick in beside it.
Water the row lightly every two or three days if there is no rain.
After two weeks dig up one of each, sketch it, and bury it again carefully.
After six weeks dig everything up and lay the items on a sheet of paper in the same order.
Compare each with your first drawing and note what changed.
Make a chart with two columns: rots and does not rot.
What you should see
The peel and leaf are almost gone, the paper and cotton are soft and torn, and the plastic, cap, foil and polystyrene look just as they did on day one.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What does biodegradable mean?
It can be broken down by living things in the soil.
2Why did the peel disappear?
Bacteria, fungi and worms ate it.
3Why did the plastic stay?
Soil organisms cannot digest plastic.
4What should we do with the things that do not rot?
Use less of them, reuse them and send clean ones for recycling.
Take it further
Bury a "biodegradable" carry bag and check whether it really breaks down in six weeks.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Science projects
Easy Science Projects for Class 1 to 5
Simple, safe projects young children can make with a little help
Science projects
Science Projects for Class 6 to 8
Working models matched to middle-school science topics