The Heat's On!

Preparation and points to note

It may be unrealistic for each group to have its own compost heap; if this is the case, establish a class heap and get different groups to take temperature readings at different times of the day.

As this task involves monitoring heap activity over a minimum of 2 weeks, the students need to remember to take the readings, do the observations, and keep a record. Challenge them to be fully responsible for this process; in this way, they will be both demonstrating and developing the key competency managing self. They may find that taking the temperature in the centre of the heap is easier if they securely tape the thermometer to a metre ruler, broom handle, or similar.

All students should take care when handling compost; this is particularly important for any who are known to be sensitive to or allergic to mould, fungi, or bacteria.

There have been cases when, in hot, dry conditions, compost heaps have become so hot that they have caught fire (and haystacks that were stacked when the hay was wet have ignited spontaneously). While the likelihood of this happening is extremely low, make sure that heaps are not right next to buildings. The students will, of course, need the co-operation of grounds staff for this project.

The Internet is a useful resource when researching compost heaps and decomposition.

Points of entry: Mathematics

In this practical activity, temperature and time are both variables. Time is an independent variable: it goes marching on regardless of heap temperature (or any other factor, for that matter). Temperature is a dependent variable: it depends on which day we are talking about (in this case, which day in the life cycle of the heap).

This activity requires students to use a thermometer to measure temperature in the middle of the compost heap. Have the students discuss and problem-solve how to get the thermometer into the centre of the heap and take an accurate reading, bearing in mind that the reading will drop rapidly as soon as the thermometer is removed from the heap. (They may not realise this because spirit-type thermometers used to take body temperature hold the maximum until shaken down.) A digital thermometer with a probe will get around this problem. If one is not available, speed and teamwork will provide the best remedy. Ask: How long will you need to leave the thermometer in the heap? Will poking around in the heap change its characteristics (and therefore infl uence the outcome of the experiment)? The instructions call for once-a-day readings; would it be best to take more?

The temperature data needs to be graphed. Challenge the students to come up with a graph that effectively tells “the story of the heap”. It will need to be a time-series graph of some kind: time or date (the independent variable) on the horizontal axis and temperature (the dependent variable) on the vertical axis. Note that the data points can be joined, unlike those in the graph for Location, Location … Ask your students if they can suggest why. (In the earlier activity, the temperature will fluctuate quite dramatically between data points; joining them with a line would obscure this fact. In this case, the temperature at the centre of the heap will change only slightly between readings; joining the data points will emphasise the continuity.)

Although the students are only asked to include the temperature data on their graphs, you could challenge them to find a way of incorporating the appearance data on the same graph. This will reinforce the difference between quantitative data (data that can be counted or measured, such as temperature) and qualitative data (data that can only be described or categorised, often subjectively). The best solution may simply be to annotate in handwriting those points where a visible change was noted.

Use this as an opportunity to compare and contrast change that is linear (constant, always at the same rate) and non-linear (speeds up, slows down, or fl uctuates over time). For example, ageing is linear: we grow 1 day older every day (never more, never less) but compost heap temperature is not linear – it increases, stabilises, then decreases.

Points of entry: Science

Make connections between your students’ prior experience of temperature and decomposition (things decaying, going mouldy, going bad) and the experiment with the grass clippings. Get them to think about what they already know about the role of temperature in decomposition. Ask: Do things spoil faster on the bench or in the fridge? What sort of temperatures do bacteria like? Do living things heat up their environment? They are also likely to have had experiences that suggest a link between moisture and decomposition (for example, mould growing in damp or humid conditions or places). These experiences can usefully be explored

Encourage the students to do their own research into what goes on in a compost heap and to relate this to the grass clippings experiment. For example, What does your graph suggest about the life cycle of the bacteria in the heap or when the grass clippings will be fully composted? If the local landfi ll has a composting operation, it may be possible to get the manager to come and talk about it. The students could ask what the optimal conditions for composting are and how these are achieved.

The smell given off by the compost heap comes from the gases produced when organic matter decays. Link this to what students already know of gases such as methane or nitrogen. Ask: What properties do they have? When organic matter is buried in a landfill, large quantities of these gases are produced and gradually make their way to the surface. What are the implications of this?

If circumstances allow for groups to have their own compost heaps, consider a competition, for example, to generate the hottest composting temperature. Location, use of black polythene as a cover, adding other materials, aeration, amount of moisture, and turning are some of the variables worth exploring.

Answers

1. Practical activity

2. a. The temperature should be rising. (Temperatures can reach 70º or more if the moisture content is right; the bacteria that cause the rotting need moisture.)

b. Theories will differ, but the temperature increase is caused by bacteria. The tarpaulin will help keep the pile hot, but even if uncovered, a large heap of clippings will heat up due to the heat  released by the bacteria as they break the grass down.

c. At first, the clippings look green and smell sweet. Before long, they become a smelly brown  sludge. (Grass needs to be layered with other material to make good compost.) If the heap is too dry, the bacteria can’t function properly and the grass may not reach the sludge stage.

3. a. Results and graphs will vary.

b. The graph should show the temperature peaking and then beginning a long, slow decline. The increase in temperature is associated with the darkening of the clippings.

c. A bigger heap will tend to generate more heat, so you could add more grass clippings or other  material that breaks down easily. A dark or black tarpaulin will attract more heat from the sun.