The Biochemistry of Compost Bins

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1 CHALLENGE LAB DESIGN YOUR OWN The Biochemistry of Compost Bins BACKGROUND All material that was once alive will decay. Decomposition is a natural and healthy process that returns the building blocks of life to the environment, where they are reused. Decomposition occurs everywhere but typically needs water and above-freezing temperatures so that chemical reactions responsible for the process can take place. Composting speeds up the process by providing ideal conditions for decay. Compost heaps are piles of organic, or carbon-based, waste material (such as food leftovers) mixed together with water at warm temperatures and left to decompose. Composting can take place on a large scale, at farms or through city-sponsored programs, and on a small scale, such as in someone's backyard or on a kitchen countertop. When a compost pile is finished decomposing, the result is a rich material called humus. Within a compost pile, microscopic organisms called bacteria break down the organic molecules. The bacteria use chemical compounds of the compost for their energy needs and for building new molecules needed for cell function. The chemical reactions that take place in the bacteria first break down molecules into simpler products. Then more chemical reactions build the more complex molecules that the bacteria need to survive. Most of these bacteria also need moisture and oxygen to survive, so compost heaps are usually open to the air. (In some cases compost is not exposed to air, but that can cause bad smells.) Compost piles can be in containers or lie freely on the ground. What goes into the mixture is up to the person doing the composting. The contents can be layered, completely mixed, or can take the form of sludge. Once the compost pile or bin has been made it can take as little as a few weeks or as many as several months to form humus, depending on the conditions. In this lab, you will design an experiment to test the effect of an environmental condition on two small compost bins over time. PROBLEM How does the environment affect the composting process? MATERIALS large jars with lids dried leaves grass clippings Holt McDougal Biology 1 Chemistry of Life

2 small sticks shredded food waste soil plastic forks thermometer ph paper PROCESS SKILLS Collecting Data Designing Experiments Analyzing Data Graphing Data PROCEDURE 1. Fill two jars with compost. Each jar should have plant material, including yard waste and kitchen scraps, and soil. 2. Look at all the materials that are available to use. Decide how to test an environmental condition, such as light source, temperature, or air circulation, on the rate of decomposition in your compost. Or, you may choose to examine how the contents of the compost affect the rate of decomposition. Have your teacher approve your choice. 3. Label one container the control compost and the other one the experimental compost. Identify the constants in your experiment and write them below. 4. Form a hypothesis about how your chosen variable will affect the compost and write it below. 5. Build your compost bins. You can add organic material in layers, along with water. Remember, you want all conditions to remain the same in each container except the one you are testing. For example, if you are testing how mixing affects decomposition, you would build your control and experimental containers with the exact same contents the only difference between them would be that the experimental compost is mixed and the control is not. 6. Set the jars in the appropriate areas when you are finished constructing them. Wash your hands when you are finished. Holt McDougal Biology 2 Chemistry of Life

3 7. At the same time once a week, record in the table below the temperature and ph of the center of both compost piles. Record the appearance of your compost: Is it crumbly, slimy, or slick? What color is it? How has it changed in appearance since the previous observation period? 8. Wash your hands when you are finished. Continue collecting data for 2 months or until the decomposition is done. When the compost looks very similar to dirt or mud, the process is likely complete. TABLE 1. COMPOST CONTROL OBSERVATIONS Day/Time ph Temperature ( C) Compost Appearance Holt McDougal Biology 3 Chemistry of Life

4 TABLE 2. COMPOST EXPERIMENTAL SET-UP OBSERVATIONS Day/Time ph Temperature Compost Appearance ANALYZE AND CONCLUDE 1. Identify What are the independent and dependent variables in your experiment? 2. Analyze What temperature or ph changes did you observe in your compost bins? What do you think may have caused the changes? Holt McDougal Biology 4 Chemistry of Life

5 3. Analyze Draw a graph of each compost container's temperature and ph changes over time. Determine the type of graph to use to best represent your data. 4. Apply Explain why you think gardeners add humus to their gardens. 5. Conclude Did one of your compost piles finish decomposing faster than the other? Based on the data and observations, what can you conclude about how your variable affects the rate of decomposition? 6. Infer How do you think a larger compost pile may be different from your sample? Holt McDougal Biology 5 Chemistry of Life