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How Composting Works

Making a Compost Column

You can make compost columns to study the composting process on a small scale for home science experiments and science fair projects. Compost columns can be made easily with the following materials:
  • Three clear, plastic, 2-liter soda bottles with caps - cleaned, with the labels removed
  • Window screen - 2-inch by 2-inch (5-cm by 5-cm) square piece (hardware stores may have scrap pieces this size)
  • Piece of wire - 6 inches (15 cm) long; picture frame wire or electrical wire works well (hardware stores may have scrap pieces available)
  • Small nail, skewer or dissecting needle
  • Clear, plastic packing tape
  • Coffee filter - basket type
  • Pair of pantyhose
  • Scissors
  • Newspaper
  • Soil
  • Trash to be composted

Assembling the compost column from pieces of 2-liter soda bottles (top left): Window screen is wrapped around Bottle 3 (top right); air holes are poked into Bottles 2 and 3 with a needle (bottom left, bottom right).

To assemble the compost column, do the following:

  1. Use the scissors to cut the three bottles as shown. Cut Bottle 1 in half and trim the top portion just below the curve. Cut the top and bottom of Bottle 2 along the curved portions. Cut the bottom off Bottle 3 along the curved portion.
  2. Take Bottle 3, wrap the window screen over the mouth of the bottle and fasten it below the neck with the wire, as shown here.
  3. With the nail, skewer or needle, poke many air holes in the sides of Bottle 2 and 3, as shown.
  4. Place Bottle 3 upside down in the bottom half that was cut from Bottle 1, as shown, and fasten it with tape. You may want to use a few pieces of tape that you can remove periodically to drain the water that will collect in the bottom half.
  5. Place Bottle 2 inside Bottle 3, as shown below, and fasten it with tape.
  6. Take the top piece from Bottle 1, with its cap, and use it to cover the open end of Bottle 2. This will be the cap of the finished compost column, as shown below.

Stacking the pieces of the compost column (tape has been omitted for clarity)

To load the compost column, do the following:

  1. Place the coffee filter snugly in the bottom of Bottle 2 so that it sits just above the opening. The coffee filter acts as a pre-filter to keep the screen from becoming clogged with soil particles.
  2. On a sheet of newspaper, mix the soil and trash to be composted. You can experiment with the amounts of soil and trash used.
  3. Fill the compost column with the soil/trash mixture.
  4. Cap the compost column. Fasten the cap with tape.
  5. Add water to the column through the top-bottle cap. You can measure the amount of water if you wish, and see how much works best. Water will percolate down through the soil/trash mixture and collect in the bottom. You will have to drain the bottom periodically.
  6. Cut one of the legs in the pantyhose, turn it upside down and cover the compost column with it. The pantyhose cover is effective in keeping fruit flies and gnats from moving in and out of the column.

Coffee filter used as a pre-filter

You will have to continually add water and drain the bottom, either daily or every other day. You can turn the compost if you want to, but you can also leave it as is -- the air holes in the side should provide sufficient aeration.

Here are some experiments that you can do with your column:

  • Weigh the column daily and graph the change in weight as the compost develops.
  • Monitor and graph the amount of water used by the column daily. Do this by subtracting the volume of water collected at the bottom from the volume of water you"ve added to the top.
  • Measure the temperature of the column with a soil thermometer daily and graph it.
  • Take notes about the appearance of the trash in the column daily. How long does it take to decompose?
  • Collect the drainage water and look at it under a microscope to see the microscopic organisms that live in the compost.