What it shows
Composting worms eat vegetable waste and pass it out as dark, crumbly castings that are a rich plant food. A tub with drainage, bedding and the right moisture turns a family's peels into compost and shows how much kitchen waste never needs to reach a dump.
The science
Composting worms live in the top layer of decaying matter, not deep soil. They swallow soft, partly rotted waste along with the microbes on it and their gut breaks it into fine, nutrient-rich castings. Air holes keep the tub from going smelly and anaerobic, the bedding soaks up extra moisture, and the lid keeps it dark and damp. The liquid that drains out is rich in dissolved nutrients and can be diluted and used on plants.
Materials
Cost: under Rs 500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A plastic tub with a lid, about 20 litres, with holes drilled in the sides and base
- Shredded newspaper and dry leaves as bedding
- About 100 composting worms (Eisenia fetida) from a nursery or a farmer
- Kitchen peels without citrus, onion, oil or dairy
- A tray under the tub to catch liquid
- A spray bottle
- A kitchen scale
Safety
For this project: An adult should drill the holes. Wash hands after handling the tub, and keep it shaded; worms die above about 35 degrees.
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
9 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.
Fill the tub one third with damp shredded newspaper and leaves, squeezed so it is moist but not dripping.
Place the worms on top and leave the lid off for ten minutes so they burrow away from the light.
Weigh 200 grams of chopped peels and bury them in one corner under the bedding.
Cover with the lid and keep the tub in a shaded, cool place.
Every three or four days add another weighed portion in a different corner, and spray if the bedding feels dry.
Keep a log of the total weight of waste added each week.
After six weeks push the contents to one side and add fresh bedding on the other; the worms move across in a week.
Harvest the dark castings, sieve them, weigh them and spread a thin layer on a potted plant.
Dilute the drained liquid ten times with water and use it on a second plant; compare growth with an untreated plant after a month.
What you should see
A tub that takes in 2 to 4 kilograms of peels over six weeks with no bad smell and yields a kilogram or more of dark castings; plants given castings show greener leaves than the control.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why not add oil, dairy or meat?
They rot slowly, smell and attract flies and rats.
2Why are air holes needed?
Without oxygen the waste ferments and the tub smells; worms need air too.
3What is vermiwash?
The nutrient-rich liquid that drains from the tub, used diluted as a plant feed.
4Why keep the tub out of the sun?
Heat above about 35 degrees kills composting worms.
Take it further
Count the worms at the start and after eight weeks to estimate how fast they multiply.
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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