Compost Cake

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Students examine the compost pile and add materials to learn about decomposition and the organisms involved with the process. 


Students will be able to...

  • know that systems are made up of subsystems that have inputs and outputs
  • know that kids’ choices and actions make a difference
  • explain the make-up of compost and it's usefulness in the nutrient cycle



  • Debrief
  • Writing a compost "recipe"

Age group: 7 and up


Venue/s: garden compost bins


Materials: Green waste, brown waste,  watering can, soil or compost, shovels or pitchforks. 


Time: 30 minutes to 60 minutes


Set up: 30 minutes with kids or 5 minutes if prepped by Garden Educator

The Lesson Plan


Decomposition is occurring in nature all the time as part of the nutrient cycle.  In the garden, we can examine the process more closely.  We can see the materials that go into the compost pile, discuss the organisms that break down materials, and see the end result – new soil (or humus)!


Part 1: Examining Compost in the Bins

  1. Examine the compost bins in the garden.  Have kids dig in with pitchforks to turn over Bins 1 and 2.  Explain that the materials in Bin 2 used to look like the materials in Bin 1. 
  2. Looking closely, what can you see in Bin 1?  (Garden trimmings, veggies, leaves, etc). 
  3. Look closely in Bin 2.  Have students touch and smell the compost in Bin 2.  What does it smell like?  Can they see bits of what is in Bin 1 – or is it in too small of pieces?  
  4. What observations can you make about the material that has changed from Bin 1 to Bin 2?  How did it change? 
  5. Introduce (or re-introduce) the FBI - fungus, bacteria, and invertebrates.  Can choose to teach the Decomposer song to the kids.
  6. Have them look for signs of life in the compost in Bin 2 (they should see worms and macroinvertebrates).  The worms and macroinvertebrates break down matter, but it’s the organisms that we can’t see – the fungi and bacteria - that are doing most of the work in breaking down the plant matter into soil. They break the plants into smaller and simpler molecules that are then available to growing plants as nutrients.   
  7. Take the temperature of the compost pile.  As the FBI eat their way through the plant materials, they give off heat.  The more heat there is, the faster the decomposition process goes. 
  8. Hold a handful of soil out to the kids and tell them that there are more organisms in a handful of soil than there are people on Earth (more than 7 billion). 
  9. Why do we compost?  Any idea what will we use the compost in Bin 2 for?  Composting is a great way to re-use materials that may otherwise be headed for the garbage can (and landfill) and is part of an amazing cycle!  Compost is full of nutrients and is used to enrich the soil so plants can grow and be healthy.  New vegetables grow that we can eat.  The trimmings and stems can be put in the compost pile.  The compost that we create can help other plants. 
  10. Healthy soil = healthy plants
  11. Review the whole process again to ensure understanding. The IslandWood Food System Journal page (p35) or the Common Soil Organisms (p18) may be useful.

Part 2:  Add a Layer to the Compost Cake

  1. Prepare the materials.  Collect materials with kids or use materials prepared by the Garden Educator.  If collecting your own, you’ll need a wheelbarrow full of both brown and green plant matter.  The easiest brown material to get is dried leaves from the trails or outside the gates of the garden.  Green materials can include garden trimmings and kitchen waste.  If you make a request to the kitchen the day ahead, they may be able to save vegetable/fruit scraps to be picked up by your group.  Get a watering can full of water and some large shovels and/or pitchforks from the Slug Shed.   
  2. Put kids into four groups – the Browns, the Greens, the FBI (put 3 kids in this group – one F, one B and one I) and the Water Girl/Boy.  Have the Brown kids add a 4 to 6 inch layer of Browns to Bin 1 first.  Then have the Green kids add 4 to 6 inches of Greens.  This should be followed by several scoops of garden soil or Earth Flow compost by the FBI group (from Bins 4 or 5) and a good watering by the Water person.  Repeat the process at least twice, ideally 3 times so the kids completely get it and the F, B, and I kids all have a turn to add to the pile. 
  3. Air is also important to the pile to encourage the right kind of bacteria.  Did we add air?  How can we be sure there’s plenty of air in our pile?  (there are screens on the sides to increase airflow and we can turn the pile periodically). 
  4. Talk about what not to put into the pile – meat, cheese, eggs or any other animal matter (this will draw rodents to our compost pile and won’t completely break down since this compost doesn’t reach a high temperature).  Don’t put in compostable spoons or forks as it doesn’t get hot enough to break them down.  Don’t put in: noxious weeds, greasy foods, toxic materials, plastic.  You may want to look through the Earth Flow compost (Bins 4 and 5) to find materials that didn’t break down and ask how they got in the compost?
  5. Ask the kids if they could compost at home or at school?  Kids can write the “recipe” for compost in their journals. 
  6. Thank them for their great help in the garden.  The compost that they made will be ready in about 3 to 6 months and will be used in the garden to nourish the vegetables that we grow for other kids that visit. 
  7. Have kids wash their hands after handling the compost. 

Notes on making compost: 

  • The brown materials add carbon to the pile and green materials add nitrogen.
  • The smaller the pieces of brown and green materials that you add, the quicker the process takes.  Could have kids chop or rip leaves and veggie scraps into smaller bits. 
  • A pile is ideally about 3 ft x 3 ft x 3 ft. or slightly larger.  This helps to generate heat so that the materials break down more quickly but still allows airflow into the pile.  

Debrief Questions

  • What was the input and output of the composting process?  What types of materials decompose? (plant materials)  Why is this cycle important?    
  • Is it a good idea to compost your food waste?  Why or why not?
  • If you want to build a compost pile at home or at school, how would you do it?  What if you don’t have a very big outdoor space, can you still compost?
  • How do the organisms get into the pile?