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Learn to recycle, reduce, reuse and revere with Environmental Resources from Gale

Living Landfill: What effect do the microorganisms in soil have on the decomposition process?

Purpose/Hypothesis

The purpose of this experiment is to determine what happens to common household items that are discarded and placed in a landfill. In nature, physical, chemical, and biological factors act upon our waste and work together in the process of decomposition. This experiment will determine what action organisms in the soil have on garbage. Before you begin, make an educated guess about the outcome of this experiment based on your knowledge of composting and decomposition. This educated guess, or prediction, is your hypothesis. A hypothesis should explain these things:

  • the topic of the experiment
  • the variable you will change
  • the variable you will measure
  • what you expect to happen

A hypothesis should be brief, specific, and measurable. It must be something you can test through observation. Your experiment will prove or disprove whether your hypothesis is correct. Here is one possible hypothesis for this experiment: "Household garbage covered with soil will decay faster than garbage not covered with soil."

In this case, the variable you will change is the presence or absence of soil, and the variable you will measure is the differences in condition between the garbage in the two bags after two to three months. If the garbage in the bag with soil has decayed more than the garbage in the bag without soil, you will know your hypothesis is correct.

Level of Difficulty

Easy/Moderate, because of the time involved.

Materials Needed

  • Two 1-gallon plastic bags with holes. Each bag should have approximately 20 randomly placed holes. The holes should be about 0.5 inch (1.25 centimeters) in diameter. A hole puncher or pencil can accomplish this task.
  • 2 twist ties to seal bags
  • 5 pairs of household garbage items (for example, 2 food containers, 2 glass bottles, 2 pieces of leftover food or bones, 2 small sticks or leaves, and 2 metal cans)
  • permanent marker
  • 3 to 5 cups of soil
  • plastic gloves

Approximate Budget

$5 for the materials that cannot be found in your household or at school.

Timetable

Three to four months for decomposition to take place.

Step-by-Step Instructions

  1. Prepare a sketch and written description of the materials being placed into each bag.
  2. Prepare the control experiment. The control for this experiment will remove as many variables as possible from the test in order to see the results from a single variable. In one bag place one of each item and sprinkle a little water over them. Do not add soil to the control bag. Seal the bag with a twist tie.
  3. Prepare the test bag. In the other bag, place one of each item. Add to the bag 3 to 5 cups of soil to cover the garbage. Sprinkle the mixture with water and seal the bag with a twist tie.
  4. Label each bag ("control" or "test") and place both of them outside in a shady spot.
  5. Open the bags every 2 to 3 weeks, sprinkle more water over the contents, and reseal the bags.
  6. After 3 months, open the bags and pour out the contents of each onto separate pieces of newspaper. Remember to wear gloves. Record what changes have occurred to each item. Compare the differences in breakdown between the control and test bags.

Summary of Results

When analyzing the contents of each bag, sketch the objects and write a brief description of their conditions (use a chart like this one). Look for any activity of organisms like worms or insects. If anything is smelly, slimy, or has a black stain due to bacterial action, record it in the result chart (see sample chart). Note the difference in decay between the organic waste (food) and the inorganic waste (containers).

Change the Variables

You can vary this experiment by changing the variables. For example, you can place one bag in a chilly basement or the freezer and the other bag in a warm spot outside to determine the effect of temperature. You could also add water to one bag, but not to the other, to determine the effect of water. To determine the effect of pH on decomposition, you could add an acidic material like vinegar to one bag, and add water to the other bag.

Source: Experiment Central. U·X·L, 2000.

 

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