Most organic compounds can be composted and the resulting material used to great benefit in your garden. Organic compounds refers to anything that comes from animals and plants and usually has a high percentage of carbon and hydrogen, plus trace amounts of other elements such as sulfur, oxygen etc.
The aim of a compost pile is to make an environment ideal for the micro-organisms that are the main decomposers of the organic material. In addition ants, worms and snails also play a part. All organic waste will decompose over time but in a compost pile you are providing the ideal conditions for these organisms to thrive, thus accelerating the process. Organic waste will contain compounds such as proteins, sugars, carbohydrates, starches, cellulose etc which are broken down at different rates. Carbohydrates are easily broken down to their constituent sugars while plant remains containing cellulose take longer.
Getting the carbon to nitrogen ratio right
For optimal conditions that encourage microbial growth it is important that the ratio of carbon to nitrogen in the compost pile is correct. The ideal ratio is around thirty parts carbon to one part nitrogen. Carbon is the energy source for the growth of the microbes, nitrogen is needed in smaller amounts for growth as it is a crucial element in enzymes, proteins and DNA. However, if you supply too much nitrogen it will be turned into ammonia (NH3) which will make your compost pile smell! Too little nitrogen however will prevent the microbes growing at the fastest rate and slows down the decomposition.
So how do you ensure the ratio is correct? The ratio of 'green' to 'brown' compost materials is the best way to do this. Green materials are the fruit and vegetables scraps from the kitchen, coffee grounds, grass clippings etc which will be high in nitrogen. They also contain the bacteria that you need to begin the decomposition and to supply energy in the form of heat. Brown materials are high in carbon and are the woody materials such as paper, cardboard, dried autumn leaves and sawdust. You need to make the ratio of these materials 30:1 for the best conditions inside your compost pile. If you have too many autumn leaves make a seperate pile for composting into leaf mold. Don't make the mistake of only adding green waste to your bin or you'll end up with a rotting mess full of fruit flies with a horrible smell!
Composting is an aerobic process
Aerobic means 'with oxygen'. The microbes in the compost pile use oxygen when they process the material, using the carbon for both energy and as building material for their cells. They also use the nitrogen to make proteins and DNA but need much more carbon which is why you need only a small proportion of nitrogen in your bin. Other trace materials need by the organisms to grow include phosphorus, sulphur and a variety of metals such as iron, copper, and calcium in trace amounts. In order to encourage aerobic decomposition, oxygen needs to be supplied. This can be accomplished by agitating the pile with a fork or spade regularly. In aerobic respiration a lot of energy in the form of heat is produced.
The chemical equation for aerobic respiration follows:
If the supply of oxygen is inadequate anaerobic respiration (without oxygen) will result in the production of methane and volatile organic acids, ammonia and other compounds. Sulfur containing compounds such as hydrogen sulfide are also released. Contrary to popular belief methane has no smell, it's the other compounds that are also produced by the methanogens (the bacteria active in anaerobic decompostion) that give a badly aerated compost heap it's smell.
The chemical equation for anaerobic respiration follows:
Why does a compost pile become hot?
The microbes present in the green, moist materials added to the compost pile respire aerobically using up oxygen and releasing carbon dioxide. They also release a great deal of enegy in the form of heat. A compost pile can reach temperatures as high as 150F in a few days given the right conditions. Oxygen is needed to sustain this temperature which is why it is important to agitate the pile regularly or the aerobic microbes will die off and anearobic microbes take their place.
The pH of the compost pile
The pH level in the compost needs to be between about 5 and 8. In a new compost pile the digestion of the organic matter often produces some organic acids which lower the pH. This is not a bad thing however as this encourages fungi to grow which then digest the cellulose in plant materials. In a properly aerated pile the organic acids are themselves broken down. However if you don't aerate the pile adequately the pH can drop below about 5 because the organic acids begin to accumulate and this starts to restrict the activity of the aerobic microbes.
Conclusion
Chemistry in everyday life is fun to find out about! Now you know what is going on in your compost pile, why it gets hot and why it smells if you don't get it right! If you're eager to get started on your own compost pile see How To Make A Compost Heap In Your Garden
