Re: How can I tell in which environment organic things will decompose fastest?

I am sorry that I have taken so long to answer this question. Hopefully other young scientists who are carrying out these type of experiments will read this and make use of it. When food decomposes, both water and heat are released and not absorbed. You should try to keep things simple by varying either moisture or temperature. You could also vary the size of your compost pile or the size of the food scraps being broken down.

There are a number of properties that can be studied when looking at decomposition rates of different materials in different environments.

Temperature

Higher rates of decomposition generate more heat and therefore you can use temperature as an indicator of rate. Using a thermometer is the easiest way or (if your school has them), set up a few temperature probes at different places within the compost medium. Temperature may vary considerably in the compost medium so you must take readings in several places.

Moisture

Moisture can be measured by taking a known mass of the compost and roasting it in an oven on low heat until there is no further change in mass. In order to use changes in mass to measure moisture content, you must prevent other processes from occuring that would affect the mass of the compost sample. This includes respiration and burning, both of which result in a loss of carbon in the form of carbon dioxide and water. This can be achieved at a temperature of about 105 degrees C. Having taken your initial mass, you measure the mass of the sample after 24 and 36 hours until no further change is noted. This will occur when all the water has evaporated.

%moisture content = 100X(change in mass/mass of initial sample)

Respiration rate

It is difficult to measure the oxygen content of a soil. In fact, researchers find it more useful to measure the rate at which oxygen can diffuse through the medium to replace the O₂ that is being used up. What you can do to study the level of decomposition is to measure the rate of respiration. If there is still a lot of undecomposed organic material left, then the rate of respiration will be high (all other things being optimum). When the decomposition process is complete, respiration still takes place, but a lower rate. You can measure rates of respiration by enclosing your compost material in a container with a jar of sodium hydroxide. The sodium hydroxide can then be titrated with hydrochloric acid to find how much carbon dioxide has been produced. The sodium hydroxide aborbs the carbon dioxide. If you measure the amount of hydrochloric acid required to neutralise your both your untreated sodium hydroxide and your treated stuff with the compost, then you can use the following equation.

# mg of carbon dioxide generated = Difference in HCl used between untreated and treated (in ml) x [HCl] x 12

This stuff is more suitable for grade11/12 but a teacher might be able to help you do this. Alternatively, you can buy compost maturity test kits for around 20 dollars US which will do the same thing.

The best book I have seen for these type of experiments is called "Composting in the Classroom" by M. Trautmann and M. Krasny. You can find a more detailed treatment on measuring respiration rates in their book.

Richard Kingsley