To know the C:N ratio, soil carbon you may determined by either Dry Combustion Method or Walkley and Black Rapid Titration Method or Edens Method: for Nitrogen you may determined by modified Kjeldahl method.

For plant carbon and N analysis first you have to go for dry ashing or wet digestion technique followed by above methods for estimation.

A carbon-to-nitrogen ratio (C/N ratio ) is a ratio of the mass of carbon to the mass of nitrogen in a substance. It can, amongst other things, be used in analysing sediments and compost. A useful application for C/N ratios is as a proxy for paleoclimate research, having different uses whether the sediment cores are terrestrial-based or marine-based. Carbon-to-nitrogen ratios are an indicator for nitrogen limitation of plants and other organisms and can identify whether molecules found in the sediment under study come from land-based or algal plants. Further, they can distinguish between different land-based plants, depending on the type of photosynthesis they undergo. Therefore, the C/N ratio serves as a tool for understanding the sources of sedimentary organic matter, which can lead to information about the ecology, climate, and ocean circulation at different times in Earth’s history.

C/N ratios in the range 4-10:1 are usually from marine sources, whereas higher ratios are likely to come from a terrestrial source. Vascular plants from terrestrial sources tend to have C/N ratios greater than 20.The lack of cellulose, which has a chemical formula of (C6H10O5)n, and greater amount of proteins in algae versus vascular plants causes this significant difference in the C/N ratio. 

When composting, microbial activity utilizes a C/N ratio of 30-35:1 and a higher ratio will result in slower composting rates.However, this assumes that carbon is completely consumed, which is often not the case. Thus, for practical agricultural purposes, a compost should have an initial C/N ratio of 20-30:1

Example of devices that can be used to measure this ratio are the CHN analyzer and the continuous-flow isotope ratio mass spectrometer (CF-IRMS).  However, for more practical applications, desired C/N ratios can be achieved by blending common used substrates of known C/N content, which are readily available and easy to use.

The TKN method will analyze for organic N and ammonium N, but not nitrate N, Combustion method uses a combustion furnace such as the Leco 1000 CHN analyzer, 

UC studies show that C:N ratios of 30-35 were optimum, If unavailable C is small, the C:N ratio can be reduced by bacteria to 10, Composting material with a higher C:N ratio would not be harmful to the soil,because the remaining carbon is so slowly available, N immobilization would be minimal

https://www.researchgate.net/post/Whats_the_procedure_to_determine_calculate_CN_ratio_in_soil2?_tpcectx=profile_questions

N content changes considerably over time, seasons and years, in both plants and soil. Total N in soils in not necessarily physiological relevant for plants (trees). A snap shot N sampling is therefore likely to be biased.

C estimates in forest are either very laborious (sampling and weighing roots plus branches) or you use common approximations, roughly by estimation of trunk volume by DBH and tree height, the specific conversion factor for trunk wood weight and multiplication (about 2 for temperate zone trees).

Article Nitrogen and phosphorus limited production of cereals and se...

Article Carbon Stock of European Beech Forest; A Case at M. Pizzalto, Italy

Regardless of ecosystem, soil C: N ratio is nothing soil organic carbon to total N , but in plants , there is nothing like this..

To know the C:N ratio, soil carbon you may determined by either Dry Combustion Method or Walkley and Black Rapid Titration Method or Edens Method: for Nitrogen you may determined by modified Kjeldahl method.

For plant carbon and N analysis first you have to go for dry ashing or wet digestion technique followed by above methods for estimation.

Simplest protocol for estimation of carbon in soil is by dry combustion. Finely ground and sieved soil sample is treated with sufficient quantity of o-phosphoric acid ( 2.5 % v/v) to remove the carbonate carbon before combustion with gentle heating at 60 oC for 1-2 h. The SOM is calculated by loss in weight after burning the soil sample at 500o C for  4 h.The carbon is calculated by dividing soil organic matter (SOM)  by 1.724. Another method is by Walkley and Black rapid titration method. Nitrogen is estimated by Kjeldahl's method.

Plant samples need to be wet digested before estimation of C and N  by the cited methods

The simplest approach for both matrices is a C/N combustion analyser 

All

I rad with interest this discussion.  The discussion seems to look at both organic C to N and total C to N. I’ve attached an article describes a simple method for measuring inorganic C. I would like some addition thoughts on use of total C vs. organic C for calculation of the C to N ratio.

I’ve also attached a review article discusses the conversion of organic C to SOM. The article discusses the origins of the 1.724 conversion factor and proposes a conversion factor closer to 2.0 (OC to SOM).

Bob

Thank you so much all of you!!

All of the answers above are useful in answering Debarshi's question. However keep in mind that total C does not equal organic C in many soils, especially, those that have a pH above 7, are calcareous (have free lime) or soils that have been limed in the past. Total C needs to be converted to organic C by subtracting out inorganic C (lime, calcium carbonate or calcite). The paper by Sherrod et al. (2002) above is a good method to determine inorganic C among other methods in the literature. Once inorganic C has been accounted for, then a soil C:N ratio can be calculated.

For plants, using a dry combustion analyzer that determines C and N is the quickest way to determine the C:N ratio of plants. However, as mentioned above, determining the C:N ratio of plants is of little use during the growing season. But, knowing the C:N ration of plant residues as well as the quantity of residues left in the field after harvest can be useful in in determining N fertilizer needs of succeeding crops. This can be more useful in areas with long growing seasons (long residue mineralization periods) than in areas with short growing seasons limited by cold soils.

Is there any relation between soil C:N ratio and plant C: N ratio...how do you see , these strata of C: N ratio operate at various levels of

crop performance.

Dr. Srivastava, there is no relation between soil C:N ratio and plant C;N ratio. Plant C:N ratio depends mainly on the type of plant, genetic character and their N uptake efficiency from the soil.

Is there any specific formula for C/N ratio ? how we can calculate C/N ratio of complex microbiological media having different substrate with carbon and nitrogen e.g if the media contain all the 20 amino acid ? how to calculate C/N ratio of the 5 g/L of all 20 amino acid?

Good and discussion and members suggest answer in perfect way