Science and Technology K–6 · Year 4

Decomposers at work: what rots in a sealed bag?

Science understanding: Biological sciences

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

Decomposers such as fungi and bacteria break down dead plant and animal material and return nutrients to the soil, while made materials such as plastic are not broken down.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

mould spores
asthma and allergy triggers
soil bacteria

Controls

bags stay sealed from the start until disposal; no one opens them
sealed bags double-bagged and put in the rubbish unopened
wash hands
learners with asthma or mould allergy observe without handling the bags

Note

Risk assessment before the lesson using Primary RiskAssess or the school's own template.

What you need

5 heavy-duty zip bags
5 items of similar mass (about 10 g): a slice of bread, an apple slice, a lettuce leaf, a piece of newspaper, a piece of plastic bag
600 g damp garden soil (100 g under each item and about 20 g more to cover it)
kitchen balance reading to 1 g
camera or drawing sheet for weekly records
a warm, dark cupboard

How to do it

Weigh each item and record the mass, so all five start at about 10 g.
Put 100 g of damp soil in each bag, lay the item on the soil, cover it with about 20 g more soil and seal the bag. The bags stay sealed from now until they are thrown away.
Photograph or draw each bag through the plastic every week for three weeks and record colour, mould, softness and smell (through the bag only).
Each week score each item from 0 (no change) to 3 (mostly broken down) for mould cover and for softening, judged through the bag.
Rank the items from most to least decomposed using the weekly scores and photos.
At the end, an adult places the sealed bags, unopened, in a second bag and puts them in the rubbish.

What you should see

Bread and apple grow visible mould and soften; lettuce collapses and darkens; newspaper softens and starts to break at the edges; the plastic is unchanged. The weekly photos and scores together rank the items from most to least decomposed. The bags are never opened, because opening a bag of mould growth releases spores. The mould and softening are decomposers feeding; the plastic shows that not every material is food for them.

What changes

What you change: the material buried
What you measure: visible change over three weeks, scored each week through the sealed bag
What you keep the same: soil mass and dampness
temperature and darkness
time
bag type

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Things rot because they get old.
Plastic rots too, only slower (no measurable change in weeks; it fragments over decades, it is not decomposed).
Mould is dirt, not a living thing.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science and Technology K-6 Syllabus (2017), current, taught until 2026; code read from the syllabus document on 22 September 2026ST2-4LW-S ST2-7MW-T ST2-1WS-S
Science and Technology K-6 Syllabus (2024), implemented from 2027; NESA's timeline is 2026 plan and prepare and 2027 start teaching, and schools may choose to implement it during 2026; code read from the outcomes page on 22 September 2026ST2-DAT-01
Australian Curriculum v9AC9S4U01AC9S4U04AC9S4I03AC9S4I04

Sources

The pages the author read to write this activity.

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