What it shows
Microbes in a compost heap give off heat as they break down waste. A balanced mix of green and brown material, kept moist and aerated, warms up more than green waste alone.
The science
Bacteria and fungi break down dead plant matter and respire, and this releases heat. Microbes need carbon (from brown material) as well as nitrogen (from green material), air and moisture. A mixture of about two parts brown to one part green keeps the balance right. Green waste alone is too wet and low in carbon, packs down, runs short of air and can turn sour and smelly. In small buckets the temperature rises only a few degrees above the air, so a thermometer needs to be read the same way each day.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Two 10 litre buckets with holes drilled in the sides and the bottom
- Green waste: vegetable and fruit peels, tea leaves (no meat, dairy or cooked food)
- Brown waste: dry leaves, paper torn small, coir
- A thermometer reading to 0.5 degrees Celsius (non-mercury)
- A garden trowel and gloves
- A spray bottle for water
- A notebook
Safety
For this project: An adult must drill the holes. Wear gloves and wash hands after handling compost, and wear a mask when turning dry, dusty material. Keep the buckets away from food. Do not add meat, dairy or pet waste.
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.
Fill bucket A with only chopped green waste and bucket B with a mix of two parts brown to one part green, chopped small and mixed.
Moisten each until it feels like a squeezed-out sponge, with no dripping water.
Place both buckets in the same shady spot and cover them with a cloth to keep out flies.
Push the thermometer 10 cm into the centre of each bucket at the same time each day and wait one minute before reading.
Record the temperature of each bucket and of the surrounding air in a table.
Turn the contents with a trowel once every week and add a little water if the material is dry.
Note the smell and look of each bucket, such as earthy and crumbly or sour and slimy.
Draw a line graph of temperature against day for the two buckets and the air, and write what it shows.
What you should see
The mixed bucket usually runs a few degrees warmer than the air for the first week or two and smells earthy, while the green-only bucket warms less and may smell sour. The graph shows both curves above or near the air line.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What makes compost warm?
Microbes respire as they break down the waste and release heat.
2What are green and brown materials?
Green ones give nitrogen, such as peels, and brown ones give carbon, such as dry leaves.
3Why add air by turning?
Microbes that need oxygen work best and the heap does not turn sour.
4Why include the air temperature?
To see how much warmer the compost is than its surroundings.
Take it further
Repeat with different ratios of brown to green, such as 1:1, 2:1 and 3:1, and find which gives the greatest rise.
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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