Carbon to Nitrogen Ratio and Composting
Essentially, composting comes down to balancing two components: Nitrogen and Carbon.
The Carbon to Nitrogen ratio is a proportion of the mass of carbon, to the mass of nitrogen in a material. For example, a C:N of 20:1 means there are twenty units of carbon for each unit of nitrogen in a given material. Soil microorganisms have a C:N ratio approximately 8:1. For them to be able to thrive and reproduce, they must acquire enough carbon and nitrogen from the environment, in which they live, to maintain that ratio of carbon and nitrogen in their bodies.
The microorganisms burn carbon, in the form of sugars or carbohydrates, as a source of energy. Not all of the carbon, the composting microorganism consume, remains in its body; a certain amount is lost as carbon dioxide during respiration process. To acquire the carbon and nitrogen, the microorganisms need to stay alive, it requires a diet with a C:N ratio near 24:1; with you might say in non-scientific terms, 16 parts of the carbon used for dinner and eight parts for structural maintenance purposes. It is this C:N ratio (24:1) that rules the compost.
If we can get the mix between carbon and nitrogen correct in our compost heap, we will be creating the perfect environment for the microbes to thrive. The good composter aims to get a ratio of about 25-30:1 in his heap this provides for all the carbon needs of the microbes with a little extra keeping the heap balanced but leaning toward the carbon side. If we go below the 24:1 ratio we will lose Nitrogen. The nitrogen will gas-off and become a part of the atmosphere. This being the case it is always best to be a little heavy with the carbon as the nitrogen is the rarer commodity that we do no want to lose.
You’ll often hear keen gardeners refer to this as the ‘browns’ (Carbon) and the ‘greens’ (Nitrogen), respectively. Brown because this is the predominant colour of the materials in this group and Green also because also it is the predominant colour but as well this group tends to be fresh and moist. The main thing to remember, when trying to make compost, is getting the mix balanced between the main two ingredients. This means having the right amount of the wet green materials (nitrogen rich) and dry brown (carbon rich) materials. So for every bucket of fresh, moist kitchen scraps you add, you need approximately the same volume of dry brown leaves or chopped up twigs. Many gardeners have a big pile of dried leaves, straw, or shredded paper… the brown, high carbon, slow composting materials, next to their kitchen waste compost bin, ready for each time they arrive with their bucket of nitrogen-rich food scraps.
Once the kitchen waste has been added to the bin the brown materials can be hand sprinkled, as required on top. This can be a very effective way of filling your compost bin and getting a good result. This might be classed as a form of cold composting but in actual fact it might get reasonably warm if you are adding to it regularly, however, without being turned it won’t get really hot and the top layer might be a bit smelly and attract flies and rodents.
Almost anything can be added into this mix - newspaper, melon rinds, leaves, manure, lawn clippings, simply anything from an organic source. The more varied the raw materials in your compost pile the wider the range of nutrients your compost will contain to feed your plants with. You can compost a single material, which would mean you would only be putting the nutrients into the ground that that material can provide. Many commercial types of compost are made in this way. It should be remembered, however, that a diverse input will create a diverse output and this is what we are looking for in our garden beds. Compost made from a range of different waste materials are sure to have a wider range of nutrients to feed your plants. Should you have an abundance of any waste product with nothing to mix it with, you should still get busy composting it, it will still have good qualities and will definitely be of beneficial to the soil!
This type of composting is often done best in a commercially available compost bins or a well-built compost box with a good lid. They keep things tidy and can be kept close to your kitchen door in the permaculture Zone 1, for ease of access. They need a secure lid that reduces the problem with flies and vermin. Generally, the bin is filled slowly which is the main reason the compost does not get hot. By the time the bin is full the compost should start becoming available to harvest at the hatch designed into the bottom of the unit. When you take from the bottom, the compost falls into space you have made clearing room for more fresh material at the top.