Grade by Grade
E. Earth and Space Systems · Soils in the Environment

Composting: Turning Scraps into Soil

E2.6 E1.2 A1.2 A1.5 A2.1About 45 minutes
Learning goals
  • I can describe the process of composting.
  • I can name the four things compost needs: greens, browns, water and air.
  • I can sort what can and cannot go into a backyard compost.
  • I can explain benefits of composting for soil and the environment.
  • I can follow and fix a simple code that sorts compost.

Words to know

compost
Dark, crumbly material made when food scraps, leaves and other plant materials decompose. It is added to soil.
composting
The process of letting food scraps and plant materials rot into compost.
greens
Wet compost materials, like fruit and vegetable scraps, coffee grounds and fresh grass clippings.
browns
Dry compost materials, like dry leaves, straw, shredded paper and cardboard.
landfill
A place where garbage is buried in the ground.
vermicomposting
Composting with worms in a bin. Usually done with a kind of worm called red wigglers.

From banana peel to garden gold

What happens to your banana peel after you eat the banana? In many Ontario homes it goes into a green bin or a compost bin. A few months later, that peel can become dark, crumbly, earthy-smelling compost that helps vegetables grow. Gardeners call it "black gold"!

Today you'll learn how composting works and why it is so good for soil.

A wooden compost bin with banana peels, apple cores, carrot tops and leaves on top, dark compost below and earthworms inside.
Fresh scraps and leaves go on top. Below, decomposers like earthworms have already turned older scraps into dark, crumbly compost. AI-generated image

Remember from Grade 1

In Grade 1 you learned to sort waste: food scraps go in the green bin or compost, not in the garbage. Now you will find out what happens to them and how to make compost yourself.

How composting works

Composting is decomposing on purpose! The same decomposers you met in soil do the work:

  • Bacteria and fungi break down the scraps. They are so busy that the middle of a big compost pile gets warm, even in fall.
  • Earthworms, sowbugs and millipedes chew the scraps into smaller pieces.

Decomposers need four things to do their job well:

  1. Greens: wet, fresh things like fruit and vegetable scraps, coffee grounds and fresh grass clippings.
  2. Browns: dry things like dry leaves, straw, shredded paper and cardboard.
  3. Water: the pile should be damp, like a wrung-out sponge. Not soaking wet, not dry.
  4. Air: decomposers need air. Turning (mixing) the pile with a garden fork lets air in.

A good mix has more browns than greens. If your compost smells bad, it usually needs more browns and more air.

In a few months to a year, the scraps turn into finished compost. You can tell it is ready when it is dark, crumbly, smells earthy, and you can't recognize the scraps anymore.

a little soilair holes in the sidesgreens (food scraps)browns (dry leaves)compost bin
A compost bin with layers of browns (dry leaves) and greens (food scraps), and a little soil on top to add decomposers. Air holes in the sides let air in.
Yes, into a backyard compostNo, not in a backyard compost
fruit and vegetable scrapsmeat, fish and bones
coffee groundsmilk, cheese and other dairy
eggshells (crushed)oily or greasy food
dry leaves and grass clippingspet waste
shredded paper and plain cardboardplastic, glass and metal
What goes into a backyard compost?

Why not meat or dairy? In a backyard bin, they can smell bad and attract animals like raccoons. Many towns and cities in Ontario have a green bin program that can take some of these foods, because their big composting plants get very hot. Every town has its own rules, so check yours.

Benefits of composting

For soil:

  • Adds humus, so soil gets darker and more crumbly.
  • Adds nutrients (plant food), so gardens need less fertilizer.
  • Helps soil hold water (sandy soil) and helps it drain (clay soil).
  • Feeds soil living things like worms.

For the environment:

  • Less garbage goes to the landfill, so landfills fill up more slowly.
  • Food scraps buried in a landfill rot without air and make a gas that helps warm the planet. Composting them is better.
  • Composting is a way of reducing waste: nothing is wasted, it becomes new soil, just like in nature.
for each item in my scrap bucket:
if item is a fruit or vegetable scrap:
put it in the compost
else if item is meat, fish, bones or dairy:
put it in the green bin (if your town takes it)
else if item is plastic, glass or metal:
put it in the recycling or garbage
else:
ask a grown-up

A compost sorting code. It repeats (loops) for every item, and uses "if" to make a decision.

Worked example

Let's follow and debug the code

Mia's bucket has: an apple core, a chicken bone, a yogurt cup (plastic). Follow the code. Then fix Mia's bug: she wrote "if item is a bone: put it in the compost".

  1. Apple core: it is a fruit scrap, so the first "if" is true: compost.
  2. Chicken bone: not a fruit or vegetable. Next check: is it meat, fish, bones or dairy? Yes: green bin (if your town takes it).
  3. Yogurt cup: not a scrap, not meat or dairy. Is it plastic? Yes: recycling or garbage.
  4. The bug: Mia's line puts a bone in the backyard compost. That is wrong, because bones attract animals and break down slowly.
  5. The fix: change it to "if item is a bone: put it in the green bin (if your town takes it)", or just use the line for meat, fish, bones or dairy.

Apple core → compost. Chicken bone → green bin. Yogurt cup → recycling or garbage. Debugging means finding and fixing mistakes in code.

Worked example

Let's do a fair test: Does compost need water?

Ravi makes two compost bottles the same way, with the same scraps, leaves and soil. He sprays Bottle A with water to keep it damp. He keeps Bottle B dry. What will happen?

  1. Prediction: "Bottle A will decompose faster, because decomposers need water."
  2. He keeps everything the same except the water.
  3. Every week he measures how tall the pile is, because the pile shrinks as things decompose.
  4. Start: both 20 cm. After 4 weeks: Bottle A is 12 cm. Bottle B is 18 cm.
  5. Bottle A shrank 20 − 12 = 8 cm. Bottle B shrank 20 − 18 = 2 cm.

The damp compost decomposed faster. Decomposers need water to do their job.

Ravi's compost bottles after 4 weeks05101520start: 20 cm12 cmBottle A: kept dampshrank 8 cm18 cmBottle B: kept dryshrank 2 cm
Ravi's fair test. The damp bottle shrank 8 cm, but the dry bottle shrank only 2 cm. Decomposers need water!
Just for fun

What did the banana peel say in the compost bin?

"I am feeling a little rotten today... and that is a good thing!"

Hands-on activity

Compost in a bottle

You need: a clear 2-litre plastic bottle, scissors (for a grown-up only), soil from outside, fruit and vegetable scraps chopped small (a grown-up chops), dry leaves or shredded newspaper, a spray bottle of water, tape and a marker.

  1. A grown-up cuts the top off the bottle and pokes several small air holes in the sides.
  2. Add layers: soil, then browns (leaves or paper), then greens (scraps). Repeat until the bottle is almost full. Finish with a layer of soil.
  3. Spray with water until it is damp like a wrung-out sponge. Tape the top back on loosely.
  4. Mark the height of the pile with tape and write the date.
  5. Keep it out of direct sun. Every week, gently mix it with a long stick, spray if dry, and measure the height. Record your observations: smell, colour, what you can still recognize.
  6. After 4 to 8 weeks, what changed? Write a short report with a drawing and your measurements.

Safety: No meat, dairy or oil. Wear gloves when you handle the soil, scraps and compost, and wash your hands with soap every time you mix or check it. If it smells really bad, add more browns and mix it.

Your turn

1

What is compost?

2

Name the four things compost needs.

3

Is a dry maple leaf a green or a brown?

4

Is a carrot peel a green or a brown?

5

Name two things that should not go into a backyard compost.

6

Give two benefits of composting.

7

How can you tell compost is finished?

More practice: warm-up

1

Green or brown: coffee grounds?

2

Green or brown: shredded newspaper?

3

Should a compost pile be soaking wet, damp or dry?

4

Who does the work of turning scraps into compost?

5

Where does garbage go if it is not composted or recycled?

More practice: on your own

1

Why do people turn (mix) a compost pile?

2

A compost bin smells bad and is very wet. What should you do?

3

Why does the middle of a big compost pile get warm?

4

In the code, what happens to a banana peel? What happens to a glass jar?

5

How does compost help a sandy garden soil?

Challenge

1

Ravi's dry bottle shrank only 2 cm in 4 weeks, but his damp bottle shrank 8 cm. How many times more did the damp one shrink? What does this show?

2

Explain how composting is like what happens on a forest floor.

3

Write one more "else if" line for the compost code that sorts paper and cardboard.

Check your understanding

Quiz

1.

Which is a brown for compost?

2.

Compost needs greens, browns, water and...

3.

Which should not go into a backyard compost?

4.

How does compost help soil?

5.

Composting helps the environment because...

6.

Finished compost is...

7.

In code, finding and fixing a mistake is called...

For parents and teachers

Big idea (E2.6): Composting is controlled decomposition by bacteria, fungi and invertebrates, needing a mix of greens (nitrogen-rich) and browns (carbon-rich), moisture and oxygen. Benefits include adding organic matter and nutrients to soil, improving water-holding and drainage, supporting soil life, and diverting organic waste from landfills (where it produces methane). This connects to E1.2 (ways humans improve soil).

Coding (A2.1): The sorting code uses a loop (for each item) and conditionals (if, else if, else). Have your child trace items through it and debug a mistake; then add a new rule.

How to help: Make a compost bottle or start a backyard bin together. Weekly measuring and observation notes are real data (A1.2); a short report with a drawing communicates findings (A1.5). Check your municipality's green bin rules: they vary across Ontario.

Common difficulties:

  • Thinking compost is garbage. It is a resource, the same thing nature does on a forest floor.
  • Too wet and smelly. Add browns and air.
  • Mixing up greens and browns by colour. Greens are wet and fresh; browns are dry. (A brown banana peel is still a green!)

Safety: An adult cuts the bottle and chops scraps. No meat, dairy or oils. Gloves for handling soil, scraps and compost, and hand-washing with soap after every check (compost and soil can carry germs and mould spores). Keep compost bottles away from very young children and pets.