What it shows
Spreading soil from a garden bed, a compost heap and bare ground on white trays and counting the earthworms, beetles, millipedes, ants and springtails shows that living soil is full of animals and that richer soil has more of them.
The science
Soil is an ecosystem. Dead leaves and roots are eaten by earthworms, millipedes, woodlice and tiny springtails, which break them into smaller bits; fungi and bacteria then turn these into humus and minerals that plants can use. Beetles, spiders and centipedes hunt the smaller animals. Soil with lots of dead plant material supports more of these decomposers, so compost and leaf-covered beds teem with life while bare, trampled ground has few.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- 3 white trays or lids (or a white cloth)
- A trowel or an old spoon
- A hand lens
- Gloves
- A notebook, a ruler and a camera or pencil for drawings
Safety
For this project: Wear gloves and wash hands afterwards. Do not dig where there may be snakes or scorpions; an adult should check the spot. Watch for centipedes, which bite. Put every animal back where it came from.
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.
Step 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.
Choose three spots: a garden bed under leaves, a compost or manure heap, and bare trampled ground.
At each spot dig up a trowel of soil about 10 cm deep and tip it on a white tray.
Spread the soil gently with the old spoon and wait a minute for the animals to move.
Count and draw each type of creature with the hand lens: earthworm, millipede, centipede, beetle, ant, spider, springtail, grub.
Measure the longest earthworm with a ruler and note how many legs each kind of animal has.
Return all the animals and soil to their spot within 15 minutes.
Make a table of counts for the three spots and a bar chart.
Write why the compost heap had the most creatures and what each one does in the soil.
What you should see
The compost or leaf-covered soil shows 10 to 30 animals of several kinds; the bare ground shows only a few ants. The chart shows that soil with dead plant matter supports the most life.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01What do earthworms do for the soil?
They eat dead leaves, mix the soil and make tunnels that let in air and water.
02How is a millipede different from a centipede?
A millipede has two pairs of legs per segment and eats dead leaves; a centipede has one pair per segment and hunts.
03Why did bare ground have few animals?
It has little dead plant matter for food and is dry and hard.
04What is a decomposer?
A living thing that breaks down dead material and returns nutrients to the soil.
Take it further
Repeat after the first monsoon rain and compare the counts with the dry season.
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.
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