Composting ratios are confusing and in this post we’ll simplify them and show you which ratio you should be using. Once you understand the ratio and the importance of browns and greens you will have no trouble making fast superior compost.
Why Are Home Composting Ratios So Confusing?
The main reason for confusion is that there are two types of ratios and gardeners tend to mix them up. There is a “brown to green” ratio, and a “carbon to nitrogen” ratio. These are not the same thing and the values of the two are quite different. When someone says, “the brown to green ratio is 30:1”, they are mixing up the two ratios. A brown to green ratio of 1:1 is NOT the same as a carbon to nitrogen ratio of 1:1.
The mathematical concept of a ratio is also a bit confusing, but it needn’t be. A ratio of 30:1 (read as thirty to one) means that there are 30 parts of the first item and 1 part of the second item. If the ratio of apples to oranges is 30:1, there are 30 apples for each orange.
A ratio is not an amount. So in my example you could have a total of 31 fruits (30 apples + 1 orange), or you could have 310 fruits (300 apples + 10 oranges), or even 30 bags of apples + 1 bag of oranges. Ratios are great when you are making compost piles. You can make a small pile or a big pile, as long as you keep the ratio the same.
Why is the Ratio Important?
No matter what you pile up, or put into a compost bin, nature takes over and composting will happen. You DO NOT need the right ratio to make compost.
However, if you do use the right ratio, composting happens much quicker and at a higher temperature. If you want to make fast compost, kill weed seeds and pathogens using heat, it is a good idea to use the right ratio.
What is the Right Ratio?
Compost happens because microbes (mostly bacteria and fungi) eat and digest the organic matter in the compost pile. It is as simple as that. Microbes like to have a certain diet and their main food requirements are carbon (sugars, starches) and nitrogen (proteins). Microbes eat more and make compost faster when the ratio of carbon to nitrogen (C:N) is 25:1. That is the perfect food for microbes.
During the process, carbon is lost to the air as CO2 and so the amount of carbon drops. To compensate for this, the best C:N ratio to start a pile is 30:1. Over time carbon is lost and the ratio drops to 25:1 and by the end of the composting process it is closer to 20:1. So the best starting ratio for C:N is 30:1.
The C:N Ratio for Compostable Material
Each item that goes into a compost pile will have a different C:N ratio, as outlined in the list below. Fall leaves have a value between 30 and 80, while fresh grass clippings have a value of 15 to 25. Since fall leaves have more carbon than nitrogen (a higher ratio), they can be mixed with fresh grass clippings (lower ratio) to make a pile that has a 30:1 value.
It is important to understand that the 30:1 ratio is based on dry weight, not volume. To make this perfect compost pile you need to weight the materials and gardeners don’t usually do that.
Another complication is that gardeners tend to mix different things together; manure, fall leaves, weeds, kitchen scraps, paper products, etc. To make the perfect 30:1 ratio you need to find the C:N ratio of each item and weight it. Then you need to correct for moisture and calculate how much of each item you need. That is just too complicated and gardeners will do that.
Gardeners have simplified the process by using the green to brown ratio instead.
What are Browns and Greens
A “brown” is any material that has a C:N ratio above 30:1. A “green” is any material with a C:N ratio at or below 30:1.
Stated another way, browns have too much carbon for making an ideal compost pile and greens have too much nitrogen. If you mix browns and greens together you can achieve the perfect composting ratio of 30:1.
A benefit of using browns and greens is that they are measured by volume and you don’t have to correct for moisture. You can just use shovels, buckets or wheel barrow loads to measure them. This is so much easier for gardeners but there are some limitations.
Using browns and greens is not as accurate as using C:N ratios, but even if it is not perfect it will still make compost.
The second limitation is that color is not a great way to identify material. For example, manure has a brown color, but since it has a lot of nitrogen relative to carbon, it’s considered a “green”. For the same reason coffee grounds are a green. Fresh cut grass is a green due to the high nitrogen level, but as it dries it loses nitrogen and becomes a “brown”. The same goes for leaves. The chart below will simplify all of this for you.
The statement “greens are high in nitrogen” is not correct. Even the greens have much more carbon than nitrogen. Chicken manure, for example, has a C:N ratio of about 10:1 which is ten times more carbon than nitrogen.
The Best Brown to Green Ratio
There is one other complication with the brown to green method. There is no established “right” ratio. Some people use a brown to green ratio of 1:1. Others use a value of 10:1, and still others recommend a value between these extremes and use something like 4:1 or 3:1.
Which ratio should you use? It really depends on the browns and greens that you are using. Most gardeners make compost in fall because that is when they have a lot of material; fall leaves and yard waste. Since yard waste has a C:N ratio of 30:1 it is not a very strong green. In this case use a ratio of 1:2.
If your green is manure, which has a lot of nitrogen, or kitchen waste, the brown to green ratio should be more like 3:1.
As a general rule a brown to green ratio of 1:2 works for most gardens. Start there, monitor the pile, and adjust it as needed (see below). Over time you will develop a ratio that is perfect for the type of material you have.
note: the above table gives values for the C:N ratio, but names it C/N instead of C:N. They mean the same thing. You will also note that the value is a single number and not a ratio. This is standard practice. A C/N of 15 for alfalfa pellets is saying “the C:N ratio is 15:1”. The “1” is implied. The C/N value is a dry weight value. The % carbon and % nitrogen are wet weight values.
Easy Home Composting
The brown to green method described above with a 1:2 ratio is easy to use and works. But …., there is always a but. You do have to monitor the pile. If you added too much brown you need to mix in some more green material, and vice versa. How do you know if this is a problem? Follow these simple rules.
- If it smells like ammonia, you have too much nitrogen – add more browns.
- If the pile is not heating up, you have too much carbon – add more greens.
To troubleshoot other problems with your compost pile, check out Compost Science for Gardeners for a detailed list.
Q? Do you need a compost activator?
No. Every single piece of material you add to the compost pile is covered with microbes – you don’t have to add any more. Some commercial activators also include some nitrogen, but if you make the pile correctly, you already have enough nitrogen in the pile. If you really feel the need to add more, add some inexpensive fertilizer with a high first number in the NPK ratio. Fertilizers are much cheaper than compost activators.
Q? Do you have to turn a compost pile?
No. Composting happens quicker if you do turn the pile which mixes ingredients and adds extra oxygen, but if you are OK with slow composting, you do not need to turn it.
Q? Does a compost pile smell?
Not if you build it properly and get the ratio correct. It will smell if it does not get enough air or if it is too wet. As mentioned above, too much green will make it smell like ammonia. Done correctly, a compost pile has virtually no odor.
Q? How can you tell when compost is finished?
Finished compost has cooled down and is nice and black. You should not see most of the input ingredients, but a few will still be there because they either don’t compost or compost slowly. Nut shells, pine cones and twigs compost very slowly. Egg shells and compostable plastic does not compost at all.
Q? Is composting bad for the environment?
Composting produces CO2 and as you know that adds to our global warming problem, but it is much more complex than that. When you compare composting to other options, composting on your property is the most sustainable way for dealing with waste organic matter.
Q? Why bother composting?
The average household produces more than 200 pounds of kitchen waste every year and that does not include yard waste. The best way to deal with all this organic matter is to compost it on your property. It is good for the environment, it is the best way to improve your soil and it grows better plants. It is key to a better garden.