The common advice for making compost is that you should use the correct ratio of browns and greens. Why is this important? What is the correct ratio? Are dried green grass clippings, a brown or a green? Good questions that will help you understand how to compost.
Browns and Greens – What are They?
In the simplest form, the terms are quite descriptive. Browns are any plant material that is brown, and includes fall leaves, dried grass, wood products, paper and straw. Greens are – you guessed it- green. It includes fresh grass clippings, freshly picked weeds, plant clippings and most kitchen scraps.
Calling composting ingredients brown or green is useful because it is simple for people to understand. However the terms are not always correct. It would be better to use the terms high nitrogen ingredient, and low nitrogen ingredient. The greens contain higher levels of nitrogen. For example, fresh green plant material contains high levels of nitrogen.
As the greens age they lose nitrogen and turn brown at the same time. Green leaves have high levels of nitrogen, but as they go brown in fall, the nitrogen levels drop. Wood products and straw have low levels of nitrogen.
So is manure a brown or a green? Based on color it is a brown, but based on nitrogen levels it is a green. As far as composting goes, it’s a green.
Other ingredients are also confusing. Alfalfa hay is ‘brown’ in color, but is considered to be a green since it contains a lot of nitrogen.
The bottom line is that the brown and green rule does not always work. Browns can be green, greens can be brown—it’s getting confusing! Stick with me, there is a simple solution.
How to Compost – the C:N Ratio
Recipes for making compost usually tell you to combine the browns and greens in the correct ratios. The recommendations usually go something like this:
The ideal C:N ratio is 30 parts brown to 1 part green.
Or
Use 6 inches browns to 2 inches of greens
Both of these recipes are simple to understand and simple to follow. Both are wrong.
The first one, “The ideal C:N ratio is 30 parts brown to 1 part green”, is just wrong. The author does not understand the term C:N ratio. It is not a ratio of browns to greens. It is a ratio of carbon to nitrogen. The carbon to nitrogen ratio in the compost pile should be 30:1, not the ratio of browns to greens.
The second composting recipe could be correct, but it is probably not. It all depends on which browns and greens you use.
Every ingredient has it’s own C:N ratio. For example horse manure is about 25:1. Fall leaves have a ratio of 30-80:1, depending on age of leaves and type of leaves. Reference 1 has a good list of C:N ratios for common composting ingredients.
The composting recipe of 6” green to 2” brown will only be correct if you use the right combination of ingredients – and that is not likely.
Why is the C:N Ratio Important?
Why is this ratio so important for composting? As discussed in Compost – What is Compost?, composting is a process whereby microbes degrade the organic matter added to the compost pile. These microbes have basic requirements for food, just like you and me. It turns out they grow best when there is a ratio of carbon to nitrogen of about 30:1.
The right amount of carbon and nitrogen makes the microbes happy, and they grow fast. Fast growing microbes means that the composting process happens quickly and the pile heats up to desirable temperatures. So for fast compost it is important to feed the microbes the right ratio of carbon:nitrogen.
What Happens if the C:N Ratio is Wrong?
The microbes will not be as happy, and they won’t decompose the organic material as fast. Composting takes longer and takes place at lower temperatures. However, it does happen.
You do not need the right C:N ratio to make compost. You only need the right ratio if you want to make it quickly!
The Reality of Composting
For the average home owner, it is very difficult to have the right ratio of material. Firstly, how can you figure out if your fall leaves have a ratio of 30:1 or 80:1? You can’t. Secondly, you rarely have the right ingredients available.
Most greens are available in summer. Most browns are available in fall. Some sources recommend holding the browns until you get enough greens and then using them. That is certainly an option, but it is a lot more work, and it needs more space. Who has extra space in their gardens these days?
The reality is that if you simply add your ingredients to the compost pile when you get them, and you turn the pile, you will make compost. It might be a slower process, but that is OK.
Don’t make composting complicated.
How to Make Compost Faster
Most home gardeners have more browns than greens, unless they bring in some manure or have a cow in the back yard. Most of the stuff you collect in fall and spring are browns. Based on the above discussions, too much brown results in a slow composting process. There is a simple solution to this problem.
The problem with too much browns is that the there is not enough nitrogen for all of the carbon. A very simple solution is to add some nitrogen fertilizer to the compost pile. Adding a handful of Urea to a pile of leaves will speed up the process. If you want to go 100% organic, use some fresh chicken manure or you can pee on the compost pile. Both are good organic sources of nitrogen.
If your compost pile starts to stink, you added too much nitrogen. It only takes a bit of Urea.
I stopped worrying about green and brown ratios a long time ago. I still ended up with compost.
References:
1) How to Compost – Carbon to Nitrogen Ratios: http://www.planetnatural.com/composting-101/c-n-ratio/
2) Photo Source: Peter & Ute Grahlmann
Last summer we experimented with growing black beans and pinto beans. We have alot of the dried out pods left from last summer. Are they considered green or brown?
Pods are probably a brown. The beans are a green.
You can grab a large batch of coffee grounds from coffee shops in fall to add nitrogen to go with the leaves. That’s what I do.
Pleasure to have stumbled across your websites. “Don’t make composting complicated..” – what a pleaseure to find someone who gives truly practical advice! Thank you !! and very impressed that you answer every comment. I appreciate your scientific approach.
2 compost questions-
1/I live in a region where there are lot of walnut trees. I have seen advice to not put the black pods of walnuts into compost – is this a nitrogen “thing”?
2/ can I compost pistachio, other nut shells, avacado seeds… or will they not break down ?
thank you!
1) Walnuts should be fine in compost. The concern is about Juglone
https://www.gardenmyths.com/walnuts-juglone-allelopathy/
2) hard shells from seeds won’t break down quickly. But there is no problem putting them in compost, or directly on the garden as mulch.
Good read! I am too lazy to maintain a compost pile. Compost in place. I put stuff right in the gardens, not against any plants, and cover it with mulch or just dirt. .
No protein nor citrus, but coffee grounds, tea bags, any bread or sugar (sugar’s nitrogen) small paper things, bittings from veggies, leftover salad, etc. It just disappears.
Since I can get a lot of leaves right now (mid-fall in Canada), can I shred them and build a leaf mould pile now; and, then steal leaves as needed for my compost piles through the winter, and then later through spring and summer?
That is what I do.
Thanks for your great article. I can’t seem to find a simple answer to a question I have, and I was wondering if you can help. My lawn is treated by a lawn service company, and they apply weed killers and fertilizers on a regular schedule. Can I use my grass clippings in my compost piles? Thanks!
That depends on the herbicide they use. Some will still be in the compost when it is done and can harm plants when applied. Fertilizer is not a problem. I just leave the clippings on the lawn to feed the lawn.
Is 6 month old, dry horse manure brown or green????s
6 months is not very old – green.
I’m trying to compost oak leaves and sawdust. I add what greens I can…weeds, scraps, and even add alfalfa. My 3 piles, started 2 years ago, are still mostly oak leaves & sawdust. I mow most of them first, and I turn and/or stir them at least twice a month. The sawdust is pine wood pellets, which I use for cat litter loaded with cat urine & droppings. My piles are just that-8 foot circles of 4 foot welded wire mesh, sitting on the ground, piled about half full. I do not cover. They are in quite shady spots. Despite looking like its still just chopped leaves & sawdust, things seem to grow well in it. Any suggestions to turn this leafy, saw-dusty wet mess into dirt faster?
It is mostly about the carbon:nitrogen ratio. Sawdust is almost 100% carbon. You need to add a lot of nitrogen to get it to decompose.
Fallen oak leaves are very nigh in carbon, and they are tough for microbes to decompose. Visualize chewing on leather.
You need more nitrogen. Urea fertilizer, urine, blood meal etc. The alternative is don’t compost it – use it as a thin – 1″ mulch as is. Nature will compost it right in the garden.
I was doing my usual fall harvest of leaves. I collect the leaves using my rear-bagger lawn tractor, dump them on my lawn, then finely mulch them with my lawn mower, collect them again with my lawn tractor bagger and finally, dump them in my compost bin.
While doing this, I was again reminded that original volume of un-mulched leaves was reduced dramatically to much less than 1/4.
I remembered reading your piece on C/N rations and I thought that even though the proportions of un-mulched leaves to mulched was dramatically less, the C portion had to be the same. So if I want to keep my composting C/N rations proportionately acceptable, I’ll have to reduce the “volume” of mulched leaves significantly.
Does this make sense?
Yes. You can add a much smaller amount of mulched leaves than unmulched.
All of the discussions about C:N ratios are weight based, size of leaf pieces don’t matter. However, everything we do in the garden with composting is volume based. Gardeners have no easy way to convert one to the other.
Great article! Question: I’m impossibly short on carbon-rich browns. Other than shredded newspaper, I have no easy access to browns. I think I know the answer to this question, but I can’t find confirmation anywhere. If I let my green material thoroughly dry out before I add it to compost pile, does it then count as brown? Thank you in advance for your help.
sort of. Dry grass does have less nitrogen than green grass.
If you don’t have browns – maybe you should stop composting. Just spread the greens over the soil and they will decompose.
If Mel spreads the greens over the soil to decompose, will this deplete any nutrients from the soil & have an adverse effect growing in the soil in that area?
Mel could also add cardboard toilet roll cores & cardboard kitchen roll cores, surprising how many of these we use in a year and they all add up.
No. it is how I do all my composting now.
In fact a compost pile probably loses more nutrients.
Thats interesting-‘The Cut & Drop’ method off composting I think its called, Robert do recommend sprinkling blood & bone over the greens when they are spread over the surface of the soil (or over the browns for that matter?
No. blood meal would speed up decomposition by adding nitrogen. Bone meal adds phosphate and calcium, and soil rarely needs those added.
Robert why wouldn’t anyone want to speed up the decomposition blood meal? Or do you want the cut& drop method to act as a mulch & not break down quickly?
What is the rush? It will slowly decompose over several years.
One reason not to buy it, is that it is not needed – which helps save the environment.
Thanks Robert, I wasn’t thinking along the same lines.
“What’s the rush”, just seems to be ingrained in me (especial with our gutless sandy soils in WA), that one wants the plants to benefit from the nutrients in the compost as soon as possible.