Possibly there could be two reasons.First,C:N ratio of the materials-both wheat straw and branch wood are wide (75:1 or more).Materials with C:N ratio  in the range of 30:1 or lower will decompose fast.Wide C:N materials decompose slow and they need N supplementation or mixing with some high N containing material like vegetable waste ,cattle or chicken manure.The pot appears to be of small size.There should be sufficient material to build up sufficient temperature(55-60o C) in the heap/pot.The minimum size recommended is 1 m wide and 1.5 m height in heap method.The length can vary.So large sized pots need to be used with provision for turning /aeration.Moisture content of the material can be maintained around 60%.

Fournier , it is an intelligent question  . Composting , primarily is meant for reducing C: N ratio somehere 20-30: 1, it could be on the lower side as well. But the question is why compost maturity takes place   at C:N ratio of 20/30:1 , and in some cases at 10:1. The entire process of composting is facilitated by group of predominant microbes , either comes with introduction of bioinoculants  or specific group of microbes develop during the process of composting . The speed of composting in terms of changes  in C:N ratio is predominantly governed by C:N ratio of that predominant microbial community, temperature at which decomposition takes place and the composition( Including C:N ratio ) of the raw material  . It is  the  sink strength  ( Nutrient pool) of dominant microbial pool in the undergoing decomposing material , that plays an important role in dictating the  C:N ratio of final compost , in addition to temperature at which decomposition takes place coupled with  C:N ratio of the substrate.  

Well, I like those kind of questions too!!

The first compost that was made from green waste was a regular case with initial C/N ratio of 30-20 that has ended with final C/N of 10. The second and the third compost with very high initial C/N ratio (as my colleagues Rao and Srivastava have noticed before), will stop thermophile phase and remain on mesophile one due to nitrogen difficiency. Microorganisms need nitrogen to degrade carbone and once nitrogen is difficient their population will decrease and the temperature will drop.

Now you can re-activate the process be adding urea or calcium nitrate (or any nitrogen fertilizer) even I'm personally against this practice. Otherwise you can add chicken manure or any organic manure rich in nitrogen.

You can read my paper attached to have an idea on the role of initial C/N ratio in the compost optimization.

Conference Paper Dynamic composting optimization through CN ratio variation a...

 Dr.Khalid,your paper well answers the question.

Drs. Rao, Sarivastava and Azeem replied well. I understand time taken for compost maturity and the final C:N ratio mainly depends on the material to be composted along with the volume (pit size) of the material to be composted.

The properties and chemical composition of the start-up materials, composting conditions e.i. temperature, humidity of the materials, aeration, microorganisms innoculation are crtical in the composting processes. Some materials have readily decomposable organic materials with relatively low C:N ratio and the other have resistant organic compounds like lignin as in the case of wood debris with lots of aromatic components. 

Temperature is detrimental for decomposing microorganism gruops. Above 45C termophilic bacteria are active and their respective decomposition products can be different than the other bacteria groups.

As the comopsition organic compounds in soil regarded lignin and other resistant components are to increase upon the addition of plant residues. This also suggests resistant components can only be decayed by time therefore relative amounts of resistant components are to increase in the shorter composting events. This infact result in higher C:N ratio due to insufficient N for further decomposition of the material.  

Depend on the substrate, if it still has nitrogen, the degradation continue until to have C / N = 10. Otherwise, if the C / N is greater than 10 is because the concentration of nitrogen is low and stop degradation.

Fournier, especially, important for the composting process is the ratio of Carbon (C) and Nitrogen (N) in the composition material. C: N ratio ranges from 10: 1 at the grass up to 500: 1 for wood and sawdust.  Ideal ratio of C: N to obtain a qualitatively enriched compost should be between 25-35 : 1, at a lower value may be significant loss of Nitrogen during composting at the higher - inevitably slows down the decomposition process. The optimum ratio of C: N for the materials in the composition of the compost is achieved only complex proportional mix of all of the materials used.

C: N ratio for different types of organic materials

Material                                                 C: N ratio

Grass, types                                        10-20 : 1

Green manure                                    15-25 : 1

Waste crops (vegetables,fruit)             15 : 1

Garden waste                                       20 : 1

Manure                                                 20-30 : 1

Kitchen waste                                       25-30 : 1

Bark  of trees                                        35 : 1

Foliage                                                  40-50 : 1

The needles, pine, spruce                     50: 1

Straw                                                    50-125 : 1

Sawdust                                                 500 : 1

It should also be noted, that in the green mass of the plant contains a lot of carbohydrates as an energy source. They are easy to ferment, so that energy is released from the form of heat. Therefore, the temperature rises to 70 degrees.

I hope this information is helpful.

Regards

Relative temperature of compost (70°C). Composting is a dynamic microbial processes, flowing through the activity of microbial communities of different groups. The composting process is attended by over 2,000 species bacteria and about 50 species of fungi. At the beginning of the process composting, anaerobic thermophilic bacteria predominate, for the subsequent stages of their number falls, and increased population actinomycetes, because their growth rate is much smaller, than bacteria and fungi. Fungi are important in the degradation of cellulose, hemicellulose and lignin. Therefore, the state of composted mass must be adjusted so as to optimize the activity of these groups of microorganisms. One of the main factors regulating the development of fungi, is the temperature, as fungi are killed, if it rises above 55°C

Regards

The question is very good and of practical nature. The question has been applied in a very well manner by different contributors highlighting all the corners. As the researchers have pointed out that end product C: N ratio will depend upon the substrate C: N ratio and the factors necessary for proceeding of composting process which are; temperature of the material, aeration, moisture and microbial population. Stirring of material is highly required to keep up the aerobic conditions. If the process stops and you want to prepare narrow C: N compost, you can reactivate the process by applying N source, stirring well and keeping moisture content appropriate. 

The most interesting part of whole composting process is,  perhaps the guiding principles by which C:N ratio of composting microbes ( thermophillic or mesophillic in nature ) , whethe or not govern the C:N ratio of final  mature compost . Do you feel , it could have any effect on C:N ratio of mature compost ? Drs Nazir , Zenon , Ghafoor and other colleagues .

Composting process is influenced by the type of composting materials, climatic factors (temperature and moisture), pH and C/N ratio, which affect the function of microorganisms involved at different stages of composting. Different microorganisms are involved in the process of composting

Appropriate composting processes undergo four stages, including mesophilic, thermophilic, cooling and finally ending with compost maturation. The duration of each stage depends on the initial composition of the mixture, its water content, aeration and quantity and composition of microbial populations. The final maturing phase is characterized by even lower temperature below 25°C and reduced oxygen uptake rates of aerobic microorganisms. During this stage, degradation of the more refractory organic compounds continues and soil meso and macro-fauna enters. The final compost is attributed by a lower C:N ratio of 15-20 and a higher pH value compared to the starting feed mixture. It can contain considerable amount of plant available NO3- while NO4+ content is generally decreasing. What is most important is that the organic matter has been stabilized and contained fairly resistant C compounds. Finally, it is necessary to consider the stages involved and the factors affecting the whole composting process. The quality of any compost should be evaluated based on the composting process and the original materials used for composting. The reference listed below may be helpful for additional information.

Smith J.L.; Collins, H.P.; Bailey V.L. 2010. The effect of young biochar on soil respiration, Soil Biology and Biochemistry 42, 2345-2347

the whole degradation process is being carried out by microbes. microbes need carbon and nitrogen for their multiplication,therefore nitrogen amendment of crop residue (raw straw has C:N ratio between 80-100:1)is necessary so that resident or inoculated microflora can multiply by producing extracellular hydrolytic enzyme. most of the time final C:N ratio  of compost depend upon initial C:N ratio of the substrate.interestingly C:N ratio of bacterial cell is also approximately 10:1.

Composting is mostly undertaken with with three key feedstock. The substrate, amendment material and the bulking material. This depending on the major material being used as substrate, the C/N ratio of the final product will vary. Substrates that are easily degradable and have high fatty content would reasonably set a low C/N ratio at end point. However, standardising the use of C/N as an end point indicator is important.

Straw and wood do contain low concentrations of N, but other essential elements may also be present at low (limiting?) concentrations.

Dr.Paul,you have raised an important point.Along with C, nutrient elements N,P and S are important  constituents  present in organic matter,  predominantly  in organic  form.We always interpret the conditions for decomposition of organic matter in terms of only C:N ratios.We may rarely come cross discussion on ideal C:P or C:S ratios for decomposition of organic matter.However when we discuss the release of P and S through subsequent mineralization process ,we interpret in terms of C:P and C:S ratios. However a few papers may be available to show the effect of addition of soluble  P accelerating  the decomposition process.But on the effect of sulphur addition we may not find papers. Apart from ratios,  concentrations of  individual nutrient elements are  also important especially in mineralization  process.

c/N ratio is indicator of bio conversion process

do check papers on bio conversion using specific bacterial inoculum

i fully agree with the above authors answers

Composting/ decomposition process is mediated by presence of bacteria' likes mesophiles/ thermophiles. Badly these microbes need N as their substrate, when availability of N decreases then composition precess declines..

Yes, I agree to Dr. Shantappa. However, other factors; moisture, aerobic conditions and temperature (internal) should remain appropriate for the process to proceed.

Such things happen due to operation of predominant microbial species whose C:N ratio works in tandem with C:N ratio of composting material.

 Dear colleagues,we know a lot about composting as revealed in our answers.Still some fine-tuning is needed in understanding and practicing composting.The staring point is substrate for composting like cereal straws,legume straws ,woody materials ,green wastes etc.When sufficient moisture and aeration are there,the bacteria, fungi actinomycetes,protozoa etc are attracted to substrate by inoculation,dung application or  through air and water. Then,how to match the requirements of microbes with the available substrate is a problem..Majorly they may need carbon. and nitrogen but they also need P and S for their growth and multiplication.For decomposing woody materials or wide C:N ratio materials we need to add high N containing materials like urea,poultry manure,legume straw etc to bring down the ratio near to 25-30 :1 C:N.The organic material composition i.e cellulose,hemicellulose and lignin content also matter as they differ in their decomposition rate especially the lignin may resist decomposition and remain in the end and form key part of humus formation.The relative abundance of microorganisms in different phases of composting i.e mesophilic , thermophilic ,second mesophilic and cool stages is important. .While fungi and acid forming bacteria play active role in initial mesophilic stage ,the bacillus spp. and actinomycetes may dominate in thermophilic stage.Again after thermophilic stage in the second mesophilic stage the fungi may reappear.The organisms have their own food preferences.As mentioned by me earlier the depth of heap/pit ,the turning of the material help attaining high temperature 60-65 C ,better aeration for decomposition and homogenization of the materials.In the compost prepared C:N ratio of 15:1 is quite good for application to field.Even if manures (containing no pathogen load), composts,legume straws etc of 25-30:1 C:N ratio are applied to soil they will decompose in soil and release nutrients directly in soil.

 Dr. Annangi Subba Rao ·has correctly said that OM in soil will decompose with time to release nutrients. My experience and research indicates that physical presence of OM in soils is even more important to affect soil health than it being considering as a source of nutrients. I usually call OM as blood of soil.

Dr.Ghafoor one way you are correct .As blood is circulated in our body ,we have to continuously replenish or  add organic matter to soil to maintain the soil as living body and perform its functions.

Thanks Dr. Rao for complementary remarks.

I second Dr. Rao's input concerning [lignin]. In a litter-bag study I am currently running, [lignin] seems to be a strong driver of deltaC/N over time. DeltaC/N can be a good indicator for the decay rate. If your decay rate is very low because of a high lignin conc., C/N may seem to stabilize.

Other environmental factors like O2 availability could further impact microbial structure (e.g favoring bacteria over fungi) and therefore C vs N need, for instance with fungi having a low N need compared to bacteria.

From: https://en.wikipedia.org/wiki/Compost

The most efficient composting occurs with an optimal Carbon:Nitrogen ratio of about 10:1 to 20:1 [1]. Rapid composting is favored by having a C/N ratio of ~30 or less. Theoretical analysis is confirmed by field tests that above 30 the substrate is Nitrogen starved, below 15 it is likely to outgas a portion of Nitrogen as ammonia [2].

Nearly all plant and animal materials have both Carbon and Nitrogen, but amounts vary widely, with characteristics noted above (dry/wet, brown/green)[3]. Fresh grass clippings have an average ratio of about 15:1 and dry autumn leaves about 50:1 depending on species. Mixing equal parts by volume approximates the ideal C:N range. Few individual situations will provide the ideal mix of materials at any point. Observation of amounts, and consideration of different materials [4] as a pile is built over time, can quickly achieve a workable technique for the individual situation.

1.   Radovich, T; Hue, N; Pant, A (2011). "Chapter 1: Compost Quality". In Radovich, T; Arancon, N. Tea Time in the Tropics - a handbook for compost tea production and use (PDF). College of Tropical Agriculture and Human Resources, University of Hawaii. pp. 8–16.

2.   Haug, Roger. "The Practical Handbook of Compost Engineering". CRC Press,. Retrieved 26 October 2015.

3.   Klickitat County WA, USA Compost Mix Calculator

4.   Jump up to: a b "The Effect of Lignin on Biodegradability - Cornell Composting". cornell.edu.

I hope, that this  information will be useful.

Dear colleagues we had good discussion on composting.If the organic materials have C:N ratio of 30-35:1 in the beginning(original material or made up material by mixing with N rich sources) they decompose very well and we may get manure/compost with ideal C:N ratio of 10:1.But in many cases we may end up  with manure/composts having C:N ratios in the range of 15-20:1.Even in manure/compost quality standards we are prescribing a minimum value of 15:1.Not only C:N ratio of organic material but other conditions for composting are important as many farmers ' composts may have C:N  ratios in the range of 20-25:I(Some portion of partially composted material will go with the well composted material as repeated mixing and aeration are not provided .Weed seeds may survive and germinate in case of manure ).If the material is pathogen- free, the compost can be applied to field without any problem and further decomposition can occur in the field.We are very well aware that green manures with C:N ratio of 30-35 will decompose in  the field and provide nutrients to crops in 15 days.

Dr.Ghafoor: "My experience and research indicates that physical presence of OM in soils is even more important to affect soil health than it being considering as a source of nutrients. I usually call OM as blood of soil."

Dr.Ghafoor has in mind the “conservative” part of the soil organic matter – humus. This part is better not to compare with the "blood", but of the soil "skeleton".  Blood is a very dynamic system. Only the red blood cells regenerate with a fantastic rate - 250 billion per day. And live only 130 days. This is more compatible with the “labile” organic matter, which is the main source of nutrients and energy!!!

A humification ratio (the proportion of incoming carbon entrenched in the soil) is only 0.1 (10%) [Orlov et al., 1996].  But 90% of "fresh" organic material used for nutrition of soil microorganisms and plants. It is known, that the production of agricultural plants is ensured by 60% due to soil Nitrogen in the process of net-mineralization (Kudeyarov VN, 1989).

Thanks Dr. Ghafoor for a systemic approach to the problem of soil organic matter. 

All the best

Zenon , nice supplement . that is the perhaps the reason , we often call to restore organic matter in active pool ( Labile fraction )  and reserve it in passive pool ( Non -labile fraction ) . But you raised very pertinently ,  unless there is minimum level of organic matter resides in the soil , even response of bio-inoculation also goes into jeopardy .  

So, Can I ask what is the ideal C:N ratio of a mature compost? what does a lower C:N value signify?

It can be ranged from 10 to 15 but the idea is 12. The final C/N ratio depends on the initial C/N ratio, the optimum initial C/N ratio is 30 according to many literature, and it ends between the interval given above if composting was managed properly.

The ideal ratio of mature compost should be lesser than 20: 1, if it s 10: 1, then it is very good but all depends upon the organic material used for composting.

Research has shown that chemical composition of solid organic waste and its biodegradable fraction is heterogeneous. That means the organic materials used for composting may affect the quality and C/N ratio of mature compost. For instance, some organic waste may contain long chain polymers (lignocellulosic materials) that can contribute recalcitrant carbon, and thus influence the C/N ratio.

 Thank you for the prompt response. It would be great to back up the statement with a reference.I haven't come across a document that states the ideal C:N ratio for compost. Can you help me with standard guidelines for compost quality, if it exists. I have heard of BIS standards, however I am still unable to get a hold on their doc. If anyone has, kindly share.

Thanks.

Dear Aparna, you may find a range of docs that indicate different C/N ratios for compost quality, which is related with N immobilization. As mentioned above for a a mature compost the C value needs to be below 30.  This doc may be useful for additional information.

 Ms.Aparna,You can visit the website of National Centre of Organic Farming,Ghaziabad.In one of their publications,Training manuals ,you can find standards for both biofertilizers and manures.The title of the document is 'Biofertilizers and organic fertilizers in Fertilizer (control)Order,1985 pages1-68.Only a few organic manures have standards.

ncof.dacnet.nic.in/Training_manuals/Training_manuals_in_English/BF_and_OF_in_FCO.pdf

Nice discussion , still so alive..tropic is so very interesting . I agree with you Dr Getachew in response to query by Ms Aparna, but how the C:N ratio of different microbial communities  play any possible role in  guiding the rate  of mineralization-immobilization process to observe some kind of equilibrium in rate in reaction while composting 

Thank you everyone for all your responses! Indeed a pleasure interact with the active scientific community! 

The compost process does not 'stop' but stabilizes defined at a rate the farmer can make adjustments to changes in soil conditions. When not stabilized the changes can occur (nitrogen tie up or excessive release etc.) so fast no corrections can be made and the crop suffers. A block of wood is stabilized but grind to a sawdust and it is very unstable- the organic structure being the same. Also the rate is dependent to the optimum 'available' Carbon / available Nitrogen. If the available carbon is in excess the pH drops due to organic acids build up and process slows down. If the available nitrogen (ammonia form) is in excess the ammonia takes the pH up above optimum (perhaps toxic) and the process slows down. Therefore the compost has become stable with just excess carbon OR excess nitrogen even tho there is still plenty of available carbon left based on most test procedures that measure stability of compost. A more true measure of stability is to prepare a sample to optimal conditions then measure CO2 produced. I use such a procedure if that is of interest. 

Which is the most decisive factor in deciding the C:N ration of mature compost . Infact compost maturity could be defined in terms of number of properties like C:N ratio, CEC, mineralizable -N, humic acid: fulvic acid ratio ....

Compost maturity takes place at C:N ratio of 20/30:1 and some cases at 10:1 because most Microbes function best if the ratio of carbon to nitrogen (C:N ratio) is less than 25:1. But in case where the C:N ratios of some crop residues is species dependent (e.g Sweet clover  with C:N ratio of 12:1) the compost maturity takes place at 10:1 ratio.

Discus all changeable on the organic raw material during composting process .

A C/N ratio of 20–30 was generally considered to be optimal for Anaerobic digestion and composting But some AD studies have used C/N ratios as low as 10–20 and had good results, which was likely due to the biodegradability of the carbon ..I propose to use not only C/N to evolute Humification but to use HI Humification Index, PD Polymerization Degree and SI Sorption Index capacity were tested at the beginning and in the end of the composting process

As I understand,in composting process one may end up with C:N ratio of around 15 or more.It will be difficult to attain the C:N ratio of 10-12 under normal composting conditions and duration followed.This observation is based on our experience with aerobic composting.Amount of humus formed and extent of humification may differ in composts prepared with different startng/initial materials under different conditions(moisture,temperature,oxygen and micribial ) during composting and duration of composting.Based on the past experienc with literature support one has to define the materials, conditions of composting and duration before initiating the study. You can decide the parameters to measure with focus on more humification and better compost with excellent criteria.