monitor fermentation in a compost heap

Is mown grass in the compost heap really dead? When you cut the grass you remove its source of vital materials and the cut leaf ‘dies’. But when you feel a pile of cut grass you may notice how warm it gets.

What we did

We cut the lawn and put the cut grass in a heap. We placed two temperature sensors on the pile of grass – one was deep in the middle, the other rested near the surface.

Our results

This graph shows the changing temperatures of a pile of cut grass. One trace came from deep in the middle, the other came from near the surface.

You will find some of the questions easier if you put these results into your data logging software. Click here to get the results.

Looking at the results

  • Which trace do you think is which?
  • What are the highest temperatures recorded by each probe?
  • Why is one highest temperature more than the other?
  • Measure the time interval between the small peaks on the time axis. Does this help you to understand why the temperatures are fluctuating?
  • Why do the temperatures eventually drop to the same level?
  • We were told off because we forgot to do a control experiment! What exactly should we have done?
  • Use your software to subtract one graph from the other. Does this tell us anything?

What you can do

Wonder if all grass heaps behave in this way? Would a pile of rotting vegetables behave in this way? See if your sample shows a similar temperature peak and fall. Do include a control experiment this time.

Use your sensors to find how hot a pile of grass can get. (Get advice, there may be a fire risk!)

This page was written up by Roger Frost. Idea and results by Laurence Rogers, Leicester University. Questions suggested by teachers in Leeds.

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