What it shows
Fallen leaves are broken down by fungi, bacteria and soil animals. Burying equal weights of different leaves in mesh bags and weighing them later shows which kinds decompose faster.
The science
Decomposers such as bacteria and fungi, helped by soil animals like mites and worms, break down dead leaves and return minerals to the soil. Leaves with tough, woody or waxy tissue and with little nitrogen decompose more slowly than soft, nitrogen-rich ones. Moisture and warmth also speed up decay. The mesh lets small decomposers in and keeps the leaf pieces together so they can be weighed.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Dry fallen leaves of four kinds, for example neem, mango, banyan and teak, or any that you can find
- Four to eight small mesh bags (onion bags or net cloth) about 10 cm by 10 cm
- A scale reading to 0.1 g
- A garden trowel and gloves
- Waterproof labels or plastic tags and string
- A garden bed or large pot of soil
- A notebook
Safety
For this project: Wear gloves and wash hands after handling soil and leaves. An adult must supervise the digging if you use tools.
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.
Dry the leaves indoors for two days, then weigh 5 g of each kind and write each mass down.
Put each 5 g sample into its own mesh bag, and tie a label on it with the leaf name and starting weight.
Make two bags per leaf kind so you can dig up one at the halfway point and one at the end.
Bury all bags 5 cm deep in the same moist garden bed, spread apart, and mark the spot.
Water the bed lightly in dry weather so the moisture stays about the same everywhere.
After 4 weeks dig up one bag of each kind, brush off the soil gently and dry the contents indoors for two days.
Weigh the dry leaf remains and calculate the percentage lost from the starting mass for each.
Dig up the second set after 8 weeks, repeat the weighing and draw a bar graph of percentage loss for each leaf.
What you should see
Soft thin leaves lose a large share of their mass, while thick leathery or waxy leaves lose much less. The bar graph at 4 and 8 weeks shows the order.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Which living things break down dead leaves?
Fungi and bacteria, helped by small soil animals.
2Why does the mesh bag have holes?
To let small decomposers in while keeping the leaf pieces together.
3Why dry the leaves before weighing?
So water in the leaves does not affect the mass.
4What does decomposition give back to the soil?
Minerals and humus that plants can use.
Take it further
Bury another set in a dry sandy soil and compare the loss to the moist garden bed.
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
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