The Carbon : Nitrogen Ratio in Composting

This page builds on the basis information relating to Greens and Browns given earlier.

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 it than was the case, as home composters are now encouraged to refer to Greens and Browns and attempt to obtain a balance of these rather than achieve a specific C:N ratio.

However, I have included this section as it adds to the understanding of the composting process. We are used to the fact that some materials decompose more quickly than others decompose but may not have made to connection with their carbon content. Those with the simplest molecules, e.g. simple sugars such as Glucose,  breakdown fastest while those with complex carbon molecules, such as lignin, take considerably longer and require greater enzyme activity.  Carbon makes up about 50 percent of the mass of the cell of microorganisms, provides an energy source while Nitrogen is a key component of proteins, and is essential for microbial growth. The C: N ratio gives the amount of carbon in a material compared to the amount of nitrogen, which is expressed as 1.  The higher the C: N ratio the more carbon is present e.g., pig manure has a C: N ratio of 6:1, which means that there are six units of carbon present for every unit of nitrogen

While the ideal C: N ratio is important a precise calculation of the ratio is not actually necessary. Which is just as well as there are differing opinions as to the best C: N ratio.  Generally, we accept the best ratio as being either 25:1 to 30:1.   That is 25 or 30 parts carbon to each part nitrogen by weight.  Other sources recommend a range of between 20:1- 40:1.  At the lower C: N ratios there will be excess nitrogen provided which can result in the production of ammonia. This has the unpleasant odour that many composters will experience at some time in their composting career.  At C: N ratios of over 30:1 there will not be enough  nitrogen to support sufficient microbial growth and the  microbe population will be too low to produce sufficient heat with the result that  the decomposition process will be very  slow.

To confuse the matters further some commercial compost producers operate at a ratio of 25:1- 30:1 as this results in the materials decomposing as more quickly.

 Indeed, the ratio in a single bin will vary at different times if we just add material as it becomes available. The formula used for the calculation of ratio is complicated and is given at the end of this section for reference and completeness but I do not expect many home composters to use it. I have also included details of the Klickitat County C: N Calculator. This is  simple to use and may be of interested to schools, colleges, community groups and those who wish to take a more scientific approach to their composting. This easy to use calculator  and is designed for home composting rather than professional or agricultural applications and I would recommend that you try it.   

 C: N tables may be based on dry or wet weights of materials so they may not be directly comparable. In addition, the tables are usually based on the total carbon present in the material but not all of this carbon will be available during the relatively short composting process. Straw and woody materials, have a high percentage of carbon ( Straw 75:1, wood and twigs 700:1 ) but it is locked up in complex molecules which take much longer to break down.  The amount of Lignin present in the materials is another key factor as lignin, present in woody materials, is decomposed slowly. This means that the more lignin in the material the slower it will compost compared with other ingredients with a similar C: N ratio. However, woody material while taking a longer time to complete the composting process, encourages fungal activity that results in compost that is excellent as a soil conditioner. The threads of the fungal mycelium can often be seen in wood-based composts.   Actinomyces species, and other fungi, produce a distinctive smell and break up the lignin making the carbon in the material available for the compost.  While slow to compost shredded woody materials, also make a good bulking agent when composting fruit and catering waste

 

Composting size of material

In addition to the C: N ratio the rate of decomposition will be dependent on the surface area of the materials. The smaller the size of the material added to the bin the more quickly it will be decomposed. This is why we some composters shred materials and others cut it into 0.5 -1.5 inch lengths. This is used in some “rapid composting” techniques to produce compost in 3-4 weeks but it requires daily turning to aerate the mixture. Aeration is the key to ensuring small particles compost aerobically as while these have a larger surface area exposed to the action of microorganisms, the small particles hold water and can become compacted more easily resulting in reduced airflow and the possibility of formation of conditions favouring anaerobic organisms. In the usual cold home composting methods the different sizes of the materials used help, create air spaces and aids aerobic decomposition

Greens, Browns, Moisture content, Oxygen and the C : N ratio

The general guide followed by many composters today is to use equal amounts of “Greens” and “Browns” and excellent compost can be made just by having a basic understanding as to whether the materials being added to the bin are Greens and Browns. This knowledge need to be supplemented with an understanding of how to correct the balance if the bins turn out to be too wet or too dry. 

To recap on the earlier page Greens are those materials that are rich in nitrogen, decompose quickly and provide protein to the microorganisms involved in the composting process. Nitrogen is a crucial component of the proteins, nucleic acids, amino acids, enzymes and co-enzymes necessary for cell growth and function

Browns are carbon rich materials that are slower decomposing and the carbon is the basic building block provides a source of energy and growth it makes up about half of the mass of microbial cells.

This page provides a little more detail on the Carbon: Nitrogen ratio and lists a range of Greens and Browns in order of their Carbon content.  I must stress that compost will still be produced if the C: N ratios of the bins are outside the recommended range. However, it is not just a question of getting the right C: N Ratio.  All living organisms require moisture and the moisture level in the compost heap or bin should be between 40-60%. If below 35 to 40%, the rate of decomposition will slow and below 30%, to all intents and purposes the decomposition process will stop. The bin will then offer a warm and dry housing opportunity to rats and other creatures.  If the materials are too dry, the problem can be easily rectified by adding more Greens, compost/comfrey tea or water.  

If waste food and plenty of Greens are added to the pile too much moisture is more likely to be the problem. If the mixture is, too wet air may not penetrate the wet acidic (see Compost pH)  contents with  the main microbial activity to be by anaerobic organisms, resulting in the wet and smelly bin that puts so many composters off composting.

 55% to 60% is the usual recommended upper limit for moisture content.  If it is too wet, extra carbon rich materials can be added to deal with this problem. Turning the contents as the additional Browns are added will also make more oxygen available encouraging aerobic decomposition.  In the worse cases, the bin can be emptied and fresh Browns mixed into the wet mass and the bin refilled.  Browns with a high C: N ratios such as wood chip or straw are very useful in such cases.

Too much  circulating air , or prolonged hot sunlight can make the pile or bin  too dry for fungi, bacteria and other decomposers to work effectively. Inadequate moisture levels are  the more  common problems found with home composting.

During the summer, it may be necessary to add water to the bin, even if the C: N ratio is within the recommended range, to compensate for that lost due to the heat from the sun and evaporation. This is particularly likely to happen with some of the smaller plastic bins.

The C: N ratio and moisture are not the only factor to consider. Compost organisms need oxygen, and larger particles help maintain an aerobic environment by increasing airflow in the compost.  Woodchip and wood pellets are excellent at increasing the available air in the compost bin (and make excellent bulking agents when composting cooked food waste) scrumpled paper and kitchen roll tubes provide air pockets, Jerusalem Artichoke and other hollow stalks also provide a useful means of providing air pockets. If woodchips are used, there are advantages in watering the material before adding it to the bin, as they do not absorb moisture as readily as paper or leaves. Many composters will add shredded paper or leaves to the woodchip to assist moisture retention in the compost bin.

 

 

Carbon Decomposition in Composting

The table below gives some examples of materials and their carbon content relevant to their ease of decomposition during composting in descending order i.e. fastest first.

 

Ease of composting

Organic Material

Carbon Compound

Normally present in

 QuickSmile

 

 

 

 SlowBlink

Fruit and vegetables

Carbohydrates

Sugars and starches

Dairy, fish, legumes, meat, nuts.

Proteins

Amino acids in plant and animal tissue

Cooking fat, soya bean oil

Fats & oils

Plants and animals

Leaves, stems, straw, wood, paper

Cellulose

Leaves

Animal structural components

Chitin

Crab  shells

Complex compounds in wood

Lignin

Wood

Table based on that given in Washington State Composter’s Educators Guide

C : N Ratio for compostable materials

            C: N Ratio for range materials in ascending order of carbon content.

Material

C:N

Source

Fish scraps

2.6-5:1

7

Hen Manure (laying hens)

3-10:1

7

Pig manure

6:1

5

Chicken Manure (laying hens)

6:1

8

Chicken manure (aged)

7:1

3

Humus

10:1

2

Poultry manure

10:1

5,6,8

Hair/fur

10:1

5

Vegetable waste

11:1

8

Alfalfa

12:1

4

Sheep Manure

13-20:1

7

Vegetable trimmings/scraps

12-25:1,11-13:1

1, 2,7

Table scraps

15:1

1

Kitchen Waste

15:1

6

Food Waste

15:1

8

Grass (loose)

15:1

8

Food Scraps

17:1

3

Grass Clippings

17-20:1, 9-25:1

1, 2, 3, 7

Seaweed

19:1

3

Coffee Grounds

14:1, 20-25:1

8,1, 3

Manure (Cow)

20:1

1, 5

Rotted manure

20:1

1

Fresh weeds

20:1

3

Clover

23:1

4

Vegetable Scraps

25:1

3

Horse Manure

25:1,  20-50:1

5,6,7

Ashes, wood

25:1

4,5

Green leaves (oak)

26:1

1

Fruit Waste

25-40 :1  25-49:1

1, 3,7

General Garden Waste

30-40:1

3, 5

Fruit Waste

32:1

8

Nut Shells

35:1

3

Fresh Leaves

37:1

8

Mushroom Compost

40:1

5

Leaves,  loose/dry or  compacted wet

47:`1

8

Newspaper

50-200:1, 54:1

1,8

Corn Cobs

56-123:1

7

Corn Stalks

60:1,60-73:1

1, 7

Manure (Horse) and straw

60:1

1

Leaves

35-85:1

2

Leaves

40:1

6

Peat moss

58:1

2

Pine needles

60-100:1

1

Tissue Paper

70:1

5

Straw/Hay

75-100:1

1, 3,5

Wheat straw

100-150:1

7

Paper towel

110:1

5

Office paper

129:1

8

Bark (softwood)

131-1285:1

7

Sawdust (weathered) 3 years

142:1

2

Shredded Newspaper

170:1

5

Paper

170:1

6

Wood chip (soft wood)

226

8

Sawdust (weathered 2 months)

325- 625:1

1, 2

Cardboard (shredded)

350:1

4, 5

Cardboard

378:1

8

Cardboard (corrugated)

563

7

Twigs (small)

500:1

5

Sawdust (fresh)

500-600:1

1, 3

Wood chips & twigs

700:1

3

 

  1. University of Florida http.//Sarasota.ifas.ufl.edu/compost-info/tutorial/index. ShtmSources
  2. http://whatcom.wsu.edu/ag/compost/fundamentals/needs_carbon_nitrogen.htm
  3. http://www.homecompostingmadeeasy.com/carbonnitrogenratio.html
  4. https://www.planetnatural.com/composting-101/c-n-ratio/
  5. http://www.compostjunkie.com/composting-carbon-nitrogen-ratio.html
  6. Garden Organic Book of Compost,  New Holland publishers
  7. Washington State Compost Educator’s Guide
  8. Klickita County Calculator( see below)

C : N Calculator Klickita County Washington

The Klickita Compost Mix calculator uses average bulk weights per cubic foot of a range organic materials and  their moisture content to calculate the total C:N  ratio of up 4 materials in a mix. The results are presented  in a form that is easy  to use and remember.

This calculator will help  design a compost recipe for many common household organic wastes and it could be a valuable teaching tool for school composters.

It will determine the Total C:N Ratio for a mix of materials and the volumes can be adjusted to bring the Ratio within the desired range.

The calculator has built in average values for bulk density, percent moisture, percent nitrogen and C:N Ratio, as well as the values required to compute availability of carbon. 

The results are presented in a simple, one part this - two parts of that, format. 

1. Choose up to 4 materials from the list provided.
2. Enter the amount of waste in CuFt.
3. Press Tab or click outside the CuFt field.
4. Each time a change is made to the Material or the CuFt, the form recalculates enabling you to adjust the quanties and mix to produce a C:N ratio within the desired range.
5. The recipe is displayed below the table in plain language.

As grass composting often gives problems I tried a quick test of the calculator to help in composting grass (CN Ratio 15:1) using cardboard (C:N Ratio 378:1) as the carbon source. It was the work of seconds to calculate the amount of grass and cardboard necessary to produce a  ratio of 27:1.  

For a total C:N Ratio of 27:1 mix
2 part(s) Grass (loose) 
0.5 part(s) Cardboard 

The calculator and further details are available at:

http://www.klickitatcounty.org/SolidWaste/ContentROne.asp?fContentIdSelected=993887739&fCategoryIdSelected=965105457

 

The raw data and formula for this calculator are taken from the Natural Resource, Agriculture, and Engineering Service (NRAES) On-Farm Composting Handbook and from the Cornell University Compost Science and Engineering web site. 

Alternatively you might like to  try the Maths using the calculation at:

http://compost.css.cornell.edu/calc/cn_ratio.html. The Cornell site is well worth a visit it contains a wealth of composting information and useful links and has the strap line:

Everything you ever wanted to know about composting, but were afraid to ask?

Not quite, but we do hope we've assembled some useful information.

 

They have! 

Rod

 

 

Simple C:N Calculator