Decomposition rates using different ratios of Green and Browns.
Carbon and nitrogen are the most important of the many elements required for microbial decomposition of organic
matter to produce compost. The C: N ratio (Carbon: Nitrogen Ratio) is often referred to in composting books and websites, although less emphasis is placed on calculating the precise ratio of a mix than was once the case. Home composters are now advised
to use equal amount of Greens and Browns rather than a calculated mix to provide an “ideal” C:N ratio of 25-30:1.
Actinomycetes, aerobic bacteria and fungi, are the micro-organisms responsible for most of the
decomposition during composting. The most important of these are the aerobic bacteria which use carbon as an energy source and nitrogen to make the protein necessary for growth and reproduction. They obtain energy by oxidizing organic material,
especially the carbon fraction. This oxidation process heats the composting material. If proper conditions are present, the material will show a rise in temperature within two or three days.
In this project, we are using Rotbots, to show how the ratio of Greens (nitrogen rich) and Browns (carbon rich) affect the microbial activity in, and the rate of decomposition of, the organic material. In addition we will be using different
containers and recording the temperature using a soil or compost thermometer to see if the ratio affects the heatint produced. This exercise was developed from a Science Project: Compost Ingredients available at:
The original projected used plastic bags to contain the composting
material but 2l plastic bottles (as used to make Rotbots) are used in our first version as an alternative container. Later we will use larger plastic bottles to increase the volume of material and finally plastic bags.
and Materials (per group)
- 5 x2 litre plastic bottles with the top cut off (save the top to seal the bottle if fleece is not available). A drainage hole should be drilled in the bottom.
- Fleece to cover the
Rotbot or the saved top that ca be inverted to use as a cover and retainer for the straw.,
- 5 “smoothie” drinking straws (paper is more environmentally friendly than plastic) pushed into the centre of the material. This avoids compressing
the material by inserting a thermometer into it.
- Digital soil or compost thermometer that will fit inside the straw
- Greens and Browns to compost (Each group should use a different Green material.
- Compost (1 table spoonful per bag)
as an activator
- Scissors to cut the compostable material into small pieces
- Marker pen or self-adhesive labels to label the bags or bottles
- Measuring jug
- 5 plastic containers to catch the waste liquid
- Measuring cylinder
- Tape to hold the straw in place and seal the bags or to fix the bottle top in place
When choosing the materials to use in this exercise it
is suggested that, if this is forming part of a class experiment, Greens and Browns with a range of C:N ratios are used by each group to see if materials with different C:N ratios will react differently.
Mixed Fruit Waste
In the table above the midpoint has been used where a range is given in the carryoncomposting.com C: N table
The organic materials both the Green and Browns should be cut into small pieces preferably 1”-1. In theory the smaller the organic material the quicker it will compost as a larger surface area is exposed to the microbes.
As most home composting is based on volume rather than weight of material i.e. a 5-litre caddy full of waste rather than the weight of waste the caddy it is recommend that the materials used are measured by volume rather than weighed. The volume of the I suggest
adding the Greens and Browns separately to the bottle and measuring by eye and then removing the contents and mixing them.
Students should have the choice of different Green material e.g. lettuce, other salad leaves, grass,
comfrey, banana skin.
Each group should set up 5 bottles using different quantities of the Green material (A: 100%, B:75%, C:50g, D:25% E: 0%). The same type Brown material should be added to bring material up to the volume
required (100%) in each bottle e.g. shredded paper or small pieces of corrugated cardboard.
Each composting container should be labelled with its contents, the start date
and the students name
A tablespoon of compost may be added to each bag/bottle to introduce composting microorganisms together with a tablespoon of water to provide moisture.
to providing a means to insert thermometers without compressing the contents the straw provides to air centre of the compost in the bottle.
The bottles should be kept in a well-ventilated area during the experiment but not
in direct sunlight. It is best that they are kept in a warm room as the ambient temperature will affect the rate of decomposition.
The temperature of the organic material should be recorded daily for the next two weeks inserting
the thermometer down the straw to the same position in the composting material each day. The experiment should be continued until there has been no temperature change for two days.
Photographs should be taken every two or
three days or when ever the is an obvious change in any of the bottles.