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Decomposing Jack-O-Lantern - A Fun Fall Science Lab




Let's face it kids LOVE gross things, and gross things can teach us a lot! What better way to teach about decomposition, and how it is nature's way of recycling nutrients than watching a pumpkin rot?!




For younger students there is a cute book called "Pumpkin Jack" that will work great with this unit.

What you will need:

  • A small pumpkin (pumpkin pie pumpkins are a good size!)

  • A tank/terrarium, or a container with a lid (Animal Cracker jars work great!)

  • Soil

  • Spray bottle of water

  • Saran Wrap (If your tank doesn't have a lid)

  • Pumpkin Carving tools

Set Up:

You will need to carve your pumpkin (this allows for faster decomposing as the air is able to get inside of the pumpkin too) students may enjoy watching or participating in this! You can leave some seeds in the pumpkin and watch the full cycle. You should have sprouts starting in Spring. Once your pumpkin is carved, find some soil outdoors to put in the bottom of your tank, you will want 4-6" of soil. Next you will want to make sure that your soil is moist. If there is less than 45% moisture, the bacteria can slow down and may become dormant. If there is more than 65%, water will either force air out of the biomass or restrict the flow of air through the biomass, both potentially suffocating the aerobic bacteria. Anaerobic bacteria will then take over. Since its difficult to measure moisture, as a rule of thumb, the biomass should be about as moist as a wrung-out kitchen sponge. We added a hydrometer to measure the humidity of our enclosure. If your soil is dry when you find it, you can mist it with a spray bottle.


Optional: If you are wanting to add additional clean up crew, you can order some isopods, and springtails be sure to add some leaf litter to your tank too (this will provide food for them and allow the tank to start cycling. Make sure your soil is nice and moist but not soaked. Most microorganisms need oxygen to produce energy, grow quickly and consume more materials, adding these guys will help speed up the cycle.


When you are ready to add the pumpkin, make sure you nestle it into the soil some, this will allow the isopods to start munching and the moist soil will help kick things off!

Here's what happens:

When a picked pumpkin is left out exposed to air it begins to decompose. Microorganisms like bacteria and fungi, along with enzymes, start breaking down the pumpkin’s organic matter.


With some time, the students will start to notice some visible changes as the microorganisms and enzymes start breaking down the complex carbohydrates and proteins of the pumpkin he will begin to start looking mushy. As he continues to decompose, they might start noticing color changes, and fuzzy mold. Their drawings should be getting interesting at this point!


If they get close enough, they may notice a funky smell, this is from the gases he releases as he is breaking down. (I suggest using a covered tank or terrarium. We covered ours with saran wrap to help not only speed the decaying process up, but also help hide the smell some.)


Insects play a very important role in decomposition, they are the "clean up" crew. They munch on the organic matter and in return their waste adds to the breakdown and provides more nutrients to be recycled back into the soil. (Our tank is a bio-active terrarium that I happened to have already cycled with plants, Isopods, and Springtails. Isopods help by breaking down the organic matter while Springtails help by controlling the mold and fungal growth, together these guys willl produce nutrient filled waste that will really enrich the soil.


If you have a class that really wants to get hands on you can do some cultures to see if you can grow mold/bacteria to observe! I ordered this set (it comes with everything you need) and we will be swabbing Fred when he gets funky.




We are incorporating the Scientific Method with this project, and we started a class discussion by asking the question, "What will happen to Fred as time passes?" We made some hypotheses together and wrote them down on an anchor chart to hang in the classroom with the date we started.


Each week we will observe him, and draw our findings on our observation sheet (below) so we can see the process of decomposition. It will be neat to see the changes.


Here are some questions you can ask and talk about:


  • Do you think an uncarved pumpkin would decompose faster or slower? Why? (If you have another tank or container, and you can recreate the conditions, adding a whole pumpkin would be a fantastic way to show the control in an experiment! You can talk about the particle size of the Carbon/Nitrogen materials and how it affects the rate of organic material breakdown. Microorganisms are able to digest more, generate more heat and multiple faster with smaller pieces of material. So the smaller the particle size then the greater the surface area that is exposed to the microorganisms which also then provides for more organisms. If we cut the pumpkin into pieces it would decompose at a much faster rate.


  • What role do the microorganisms play in decomposing? Microorganisms such as bacteria, fungi and Actinomycetes account for most of the decomposition that takes place in a healthy aerobic composting effort. They are considered as being chemical decomposers because they change the chemistry of organic materials. The larger decomposers or macroorganisms, include isopods, springtails, mites, centipedes, sow bugs, snails, millipedes, spiders, slugs, beetles, ants, flies, nematodes, flatworms, rotifers and worms. They are considered physical decomposers because they grind, bite, suck, tear and chew materials into smaller pieces.


  • Do you think it would decompose faster if the temperature was warm or cold? Why? Warmer outside (ambient) temperatures in late Spring, Summer and early Autumn (Fall) stimulate the bacteria and speed of decomposition. Low Winter temperatures will slow or stop the composting process. The temperature in the classroom will be considered towards the warmer side, if it is cooler outside where you live, you can ask them if they think it would decompose faster inside or outside. (Great source for more composting info)

A Great video on the Carbon and Nitrogen Cycles:



If you are doing the Pumpkin Jack decomposing experiment, these vocabulary words and facts are great additions to the classroom! Not only do they look cute hanging around Jack, they are great for the students to reference key words about what is going on with Jack!







I printed ours on neon orange and green paper and laminated them to hang around the enclosure! I will update photos here when I get them hung up!





Some of our fun games, perfect for social emotional learning:


Social Skills UNO grab it here:


Emotional Charades grab it here:







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