A Compost Jar Experiment Kids Can Run in 3 Weeks

Food does not rot by itself. Something eats it. That is the whole idea behind this compost jar experiment kids can run on a kitchen windowsill, and the quickest way to prove it is to build two jars instead of one. Give one jar air and keep the other sealed. In ten days the ventilated jar smells faintly of woodland floor and has white fungal threads lacing through the leaf layer. The sealed jar smells like a bin on a hot day. Same food, same soil, same room. The only difference is oxygen, and that difference is the lesson.

Key Takeaways

• You will layer soil, dry leaves and fruit scraps in two clear jars, ventilate one and seal the other, and track both for three weeks.
• The concept is aerobic decomposition: microbes respire, taking in oxygen and releasing carbon dioxide, water and heat. Cut off the air and different microbes take over, producing the sour compounds we call “rotten”.
• Best for ages 6 to 13. Six-year-olds draw and describe; eleven-year-olds measure the drop in layer height and graph it.
• The usual failure is too wet and too green. A soggy jar goes anaerobic no matter how many holes you punch.
• A jar never gets hot. Say so up front, because a child who has read that compost piles reach 60 °C will be looking for it.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Clear jars or bottles	2, about 1 litre each	Kitchen	Two 1.5 L clear plastic bottles with the tops cut off
Garden soil or finished compost	2 cups total	Garden, or a bag of topsoil	Potting mix works, but sterile seed compost has fewer microbes and will run slower
Dry brown material	3 cups, loosely packed	Dry leaves, shredded uncoated cardboard, plain shredded paper	Torn egg carton (not the glossy kind)
Fresh green material	1 cup, chopped to 1 cm pieces	Apple cores, carrot peel, lettuce, coffee grounds	Grass clippings, tea leaves from a bag
Spray bottle of water	100 mL	Cupboard	A teaspoon, used sparingly
Paper towel or muslin + elastic band	1 square	Kitchen	A square of old T-shirt
Permanent marker and ruler	1 each	Drawer	—
Nail or skewer, for the holes	1	Toolbox	An adult does this part

Allergens and hygiene: coffee grounds and wheat-based paper appear in most versions, and opening the jar releases mould spores, which matters for a child with asthma. See the parent table.

Time: prep 20 min, then 2 minutes every second day for 3 weeks Cost: everything is probably in your kitchen Mess level: medium, and the layering is best done outside or over newspaper

Do the Experiment

Step 1 — Punch the air holes

An adult puts six to eight holes, each 2–3 mm wide, around the upper third of jar A, plus three in its lid. Jar B gets no holes at all. In plastic bottles, a hot skewer makes cleaner holes than a nail. Label the jars A (air) and B (sealed).

Step 2 — Build the layers in the same order in both jars

Start with 2 cm of soil, then 3 cm of loosely packed browns, then 1 cm of chopped greens. Repeat until each jar is three quarters full, finishing with soil on top. Press lightly, never pack. The ratio that matters is roughly three parts browns to one part greens by volume, which the EPA describes as about 30:1 carbon to nitrogen by element. Get this wrong in the green direction and you will smell it by day four.

Step 3 — Wet it to wrung-out-sponge damp

Spray as you go, a few pumps per layer. The target, in the EPA’s phrase, is the consistency of a wrung-out sponge: a squeezed handful holds together and releases at most a drop. Most people overshoot. Water pooling at the bottom means you have already created the anaerobic condition you meant to confine to jar B.

Step 4 — Mark the starting line and cover

Draw a line on the glass at the top of the material in each jar and write the date. Cover jar A with paper towel held by an elastic band so air moves but fruit flies cannot get in. Seal jar B with its lid, screwed down. Put both in the same place, out of direct sun, somewhere between 15 and 25 °C.

Step 5 — Observe every second day, and loosen jar B’s cap

Record four things per jar: material height against your ruler, condensation, colour change, and smell. Smell by wafting with a hand, never with your nose at the opening. Important: unscrew jar B’s cap for two seconds each visit, then retighten. Microbes produce carbon dioxide, and a tightly sealed jar of fermenting food can build real pressure. Burping it keeps it anaerobic without turning it into a projectile.

Step 6 — Shake jar A once a week, and compare at three weeks

On days 7 and 14, roll jar A gently on its side to mix and re-aerate it. Leave jar B undisturbed. At day 21, tip both onto newspaper and look for recognisable pieces, white fungal threads, dark crumbly material and any small animals.

What you should see: By day 3, condensation inside jar A and the first softening at the edges of the apple pieces. By day 7, white threads of fungal mycelium through the brown layer in jar A, and a definite sour smell when you waft jar B. By day 14, jar A’s material has dropped 1–3 cm and the greens are unrecognisable. Jar B has also dropped, often more, but has gone slimy and grey-brown and smells of vinegar and sulphur. At three weeks neither jar contains finished compost, and that is normal: the EPA puts a well-maintained outdoor pile at three to five months, and a year if left alone.

What’s Actually Going On

Decomposition is eating. Bacteria, actinomycetes, fungi and a few small animals break the long polymers in food and leaves into smaller molecules they can absorb, then run those molecules through respiration for energy. Penn State Extension’s soil summary puts one teaspoon of agricultural soil at a hundred million to a billion bacteria, hundreds of nematodes and up to a hundred small soil insects: the two cups of garden soil you added were not a growing medium, they were the inoculum. Aerobic respiration takes in oxygen and gives out carbon dioxide and water, which is why the inside of jar A fogs up and why its contents lose volume and mass. No matter is destroyed. Carbon leaves as gas, water leaves as vapour or stays as moisture, and what remains is the hard-to-digest fraction plus the bodies of the microbes themselves. That residue is compost.

The nitrogen ratio is the part that makes or breaks a jar. Microbes need carbon for energy and nitrogen to build proteins, in roughly thirty parts carbon to one part nitrogen. Dry leaves and paper are almost all carbon; fruit, vegetables and grass are comparatively nitrogen-rich. Too much nitrogen and the population explodes, consumes the available oxygen faster than it can diffuse in, and the community shifts to organisms that respire without it. Those anaerobes produce organic acids, ammonia and hydrogen sulphide. Rotten food does not smell bad because it is decomposing; it smells bad because it is decomposing without oxygen. That single sentence is the most useful thing a child can take away from these three weeks, and it explains sealed lunchboxes, waterlogged lawns and landfill gas all at once.

Now correct the expectation about heat. A managed outdoor pile reaches 131 to 160 °F, roughly 55 to 71 °C, because millions of microbes generate metabolic heat faster than a large insulated mass can shed it. A one-litre jar has a huge surface area relative to its volume and no insulation, so the heat leaks away as fast as it is made. Your jar will sit at room temperature. It is not broken. The thermophilic phase is a consequence of scale, not a requirement for decomposition, and saying so honestly is better than letting a kid conclude the experiment failed.

How to Explain It to Your Kid

Ages 5–8: Tiny creatures are eating your apple

Tell them the jar is full of living things too small to see, and they are hungry. The dry leaves are like bread and the apple is like the filling. The little creatures need to breathe, just like we do, which is why one jar has windows and one does not.

Then give them the job of jar-keeper. Each visit, they draw what they see and choose one word for the smell. By the second week they will notice on their own that the no-windows jar smells worse, and that is the moment to ask why instead of telling them.

Ages 9–12: Count the layers shrinking

Introduce decomposers and aerobic versus anaerobic. Have them measure the material height in millimetres every second day and plot both jars on one graph, days on the x-axis. The shape is the interesting part: fast drop in the first week, then flattening as the easy sugars run out and only tough cellulose and lignin remain.

Then give them the ratio. Three scoops of brown to one scoop of green, because the microbes need about thirty times more carbon than nitrogen. Ask them to predict what an all-apple jar would do. Build it, if you have a third jar. It will go slimy in four days and it will make the point better than any explanation.

Ages 13+: Mass balance and what the gas was

Weigh both jars at the start and at day 21 on a kitchen scale, lids on, and work out the loss. Where did the mass go? Some left as water vapour, some as carbon dioxide from respiration. This is conservation of mass in a system whose outputs are invisible, a genuinely hard idea and worth the time.

The extension that makes it an investigation: three jars at brown-to-green ratios of 5:1, 3:1 and 1:1, moisture and temperature identical. Measure volume loss and rate your own smell score from 0 to 5 at each observation. That is a dose-response curve for an ecological variable, and it holds up to science-fair questions.

The question to ask: “If we had put the apple in the freezer instead of the jar, it would still be an apple in three weeks. Nothing was taken away from it. So what exactly does the jar have that the freezer doesn’t?”

Why Teaching This Early Pays Off

Decomposition is where a lot of children’s biology quietly goes wrong. Many treat rotting as a property of the object, something food does when it gets old, rather than the activity of organisms with needs. That misconception survives school because it is useful enough for everyday life, and it blocks later topics: the carbon cycle, soil fertility, landfill methane, why a sealed compostable bag in a bin does not compost. Two jars side by side are a cheap and durable counterexample.

Chi and Wylie’s ICAP framework argues that gains rise as learners move from passive watching to active doing, to constructive explaining, to interactive dialogue. A compost jar suits the top of that ladder, because nothing much happens in any given minute, so there is nothing to do but notice, record and argue about why. The National Research Council’s K-12 framework names analysing data and constructing explanations among the core practices, and a three-week height graph with a smell score trains both.

Be honest about scope. Three weeks in a jar does not make a child care about waste, and no study claims a single activity shifts behaviour. It gives them a mechanism, and mechanisms are what make later facts stick.

A Compost Jar Experiment Kids Can Run Safely: What Parents Should Know

Best age	6 to 13. Below 6 the three-week horizon is too long to hold, though a four-year-old can happily help build it and look at it
Supervision	An adult makes the holes and handles jar B’s cap. Children can do everything else with washed hands
Real hazards	Mould spores are released when the jar is opened, which can trigger symptoms in a child with asthma or a mould allergy — open jars outdoors and skip this one if that applies. Pressure build-up in a tightly sealed jar of fermenting food, which is why you burp jar B every visit. Soil bacteria, so hands get washed with soap afterwards
Don’t	No meat, fish, dairy, grease or cooked leftovers with oil. These attract pests and support pathogens that fruit and leaf waste does not. Do not use a glass jar for the sealed control if you can use plastic. Never taste anything, and do not plant seeds in the three-week product, which is not finished compost

When It Doesn’t Work

• Both jars stink by day four. Too many greens, too much water, or both. Tip out, add a double handful of dry shredded paper and leaves, remix and restart the clock. Soggy is the commonest error.
• Nothing happens for two weeks. Usually sterile potting compost instead of real soil, so there was no inoculum. A handful of garden soil fixes it. Cold rooms also stall it: below about 10 °C activity drops sharply.
• Fruit flies. A gap at the paper-towel band, or green material exposed on the surface. Always finish with soil on top and check the band is snug. The flies are harmless but they end the experiment for most households.
• Dried out and dusty. Too many holes, a dry room, or too little water at the start. Mist until the squeeze test gives one drop, then re-cover. A dry jar does not fail dramatically, it just quietly stops.
• Nothing visibly different between the jars. Check that jar B really is sealed and that you did not accidentally punch holes in both. If the smells are similar, the moisture in jar A is probably too high, so both have gone anaerobic. Rebuild jar A drier.

Take It Further

• Chop size as the variable. Three jars, identical except that the apple goes in as a whole core, as 1 cm pieces, and grated. Measure volume loss weekly. Surface area controls the rate, and this makes that abstract claim measurable in a fortnight.
• The compostable-packaging test. Add a square of certified compostable bag or a corn-starch spoon fragment to a fourth jar and photograph it weekly against a ruler. Three weeks in a cool jar will barely mark it, which is the honest result and a good bridge to burying five things for sixty days.
• Fungi versus bacteria. Build one jar heavy on wood chips and dry leaves and one heavy on soft green waste. Fungal mycelium dominates the woody jar and is visible as white threads; the soft jar goes bacterial and slumps faster. Record which organisms you can actually see, and compare notes with a mould garden in a sealed bag.

What to Watch For Over the Next 3 Months

• Week 4: They apply the oxygen explanation somewhere you did not prompt: a forgotten lunchbox, a bin bag, a pond that smells in summer. That transfer is the signal.
• Month 2: The vocabulary turns up in the right wrong place, such as asking whether the fridge “kills” decomposers or just slows them down. It slows them, and that is a better conversation than the one you planned.
• Month 3: The question worth encouraging is about scale: what happens to the whole city’s food waste, and why a landfill behaves like jar B. If they want to start an outdoor bin, the jar has done its job. Our round-up of citizen-science environmental monitoring projects for kids is a reasonable next step.

Frequently Asked Questions

Why is there no heat in our jar?

Scale. A big insulated pile holds the metabolic heat its microbes make and can reach 55 to 71 °C. A one-litre jar loses it as fast as it is made. Decomposition is still happening; the thermophilic phase simply needs mass.

Is the material safe to put on plants after three weeks?

No. It is partly decomposed, which means it can still tie up nitrogen and may contain viable seeds and plant pathogens. Add it to an outdoor bin to finish, or bury it in an unplanted corner of the garden.

My child has asthma. Should we skip this?

Open the jars outdoors, have the child stand back during opening, and consider having an adult do all the opening. If the asthma is mould-triggered or poorly controlled, pick a different project. This is not worth a flare-up.

How small should the food pieces be?

Around one centimetre. Smaller pieces decompose faster because microbes work on surfaces, but a purée turns to sludge and blocks air movement. One centimetre is the compromise that reliably works in a jar.

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About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

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Sources

1. U.S. Environmental Protection Agency. “Composting at Home.” https://www.epa.gov/recycle/composting-home
2. U.S. Environmental Protection Agency. “Types of Composting and Understanding the Process.” https://www.epa.gov/sustainable-management-food/types-composting-and-understanding-process
3. Penn State Extension. “Soil Quality Information.” https://extension.psu.edu/soil-quality-information
4. Chi, M. T. H., & Wylie, R. (2014). “The ICAP Framework: Linking Cognitive Engagement to Active Learning Outcomes.” Educational Psychologist, 49(4), 219-243. https://doi.org/10.1080/00461520.2014.965823
5. National Research Council. (2012). A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas. National Academies Press. https://doi.org/10.17226/13165
6. White, R., & Gunstone, R. (1992). Probing Understanding. Falmer Press. (Source of the Predict-Observe-Explain routine used in the age sections.)