Composting: Using organic material to grow plants
This experiment will examine the principle of composting, the process of converting complex organic matter into the basic nutrients needed by living organisms. This experiment will utilize organic waste (household and yard waste) as nutrients for plants. It will allow you to investigate which waste products can be composted and best utilized by plants. Before you begin, make an educated guess about the outcome of the 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: "Yard waste will break down faster than household waste and will provide more nutrients for plants."
In this case, the variable you will change is the type of waste used to make compost, either yard waste or household waste, and the variable you will measure is the amount of decomposition of the waste and the growth of the plants. You expect the yard waste to break down faster and produce taller plants. As a control experiment, you will grow one plant without any waste to judge the growth without compost. If the plant with yard waste compost grows taller than either of the other two plants, and the yard waste has decomposed more than the household waste, your hypothesis will be supported.
Level of Difficulty
Moderate, because of the time involved.
- Three 2-gallon (7.5-liter) potting containers (terra cotta, ceramic, or plastic) with one or more holes in the bottom for drainage
- 3 pounds (1.3 kilograms) topsoil
- 3 to 5 pounds (1.3 to 2.3 kilograms) sand
- 3 to 5 pounds. (1.3 to 2.3 kilograms) organic waste (Use two types: household—table scraps, rotten vegetables, coffee grounds, etc.—and yard waste—leaves, twigs, grass clippings, weeds, etc.)
- 3 small identical living plants (annual flowers or vegetable plants), such as sunflowers, beans, or tomatoes
- 3 stakes for markers (Popsicle sticks will work)
- plastic or rubber gloves
$5 (use topsoil from your yard if available).
Two to four months.
How to Experiment Safely
Wear gloves when handling waste and mixing soil.
- Mix the topsoil and sand together to create the soil base.
- Prepare the control experiment. Fill pot #1 with the soil base, leaving 2 inches (5 centimeters) at the top of the pot. Place one plant into the soil, covering all the roots. Water generously.
- Prepare pot #2. Add to the soil base the household waste you collected (scraps, rotten vegetables, etc.). Mix the soil thoroughly. Place a plant into the soil, covering all the roots. Water generously.
- Prepare pot #3. Follow the directions for pot #2 but substitute the yard waste (grass clippings, leaves, etc.) instead of household waste.
- Put markers in the pots identifying them as "control," "household," or "yard." Place the pots in a sunny location and monitor the growth of the plants. If possible, take photographs of them at the beginning of the experiment. Water the plants when the soil feels dry. Do not allow them to dry out completely.
- Graph the weekly growth of the plants, recording the plant height, number of leaves, and root development, if visible.
- After 2 to 4 months record the final heights and differences in the plant growth between each pot. Empty the pots and evaluate the amount of composting that occurred in the soil. Look for recognizable waste materials, record results.
Summary of Results
During the experiment you will be recording the plant growth in the three pots. Ideally, the pot that is composting fastest will provide the most nutrients for its plant. It is essential to measure the height of each plant. You may also want to record which plant flowered first, how often it bloomed, and whether it produced fruit.
Source: Experiment Central. U·X·L, 2000.