You need to measure the concentrations of both C and N then calculate the ratio.  The methods for measuring soil C and N are well documented.  For example, in the recent past in the 2011 Australian text by Rayment and Lyons. 

I agree, I would like to specify, that C refers rather to organic carbon (most frequently determined by wet combustion - Walkley-Black method) and total nitrogen - for example with Kjeldahl method. Both methods were described the papers I attached.

In recent years the method of dry combustion or distillation is frequently use, because it is quicker and simpler. However, this method determines total C, which in many soils is formed by organic carbon. However, many soils in arid areas, or developed from carbonate rocks contain a lot of calcium carbonates. It is possible to remove carbonates before analysis using HCl, but it is not always efficient, In the Walkley-Black method, the carbon from carbonates is not measured, because it is oxidized.

In this case, You may go for the Total Carbon (TOC+TIC) and Nitrogen estimation with the help of CHNS analyser...With this can predict the C:N ratio of the soil is concerned.

Draw your attention. In the calculation of the C/N, you should use the atomic ratios. I mean С/N = (C/12)/(N/14) were C and N content of elements in mass %, 12 and 14 their atomic mass.

Sorry, I don't agree with Vladimir. At least in Polish publications and the ISRIC calculations (i have one sheet with profile data received this year during ISRIC spring School) the C:N ratio is calculated on the mass basis. For example, the soil contain 1,72% C and 0,19%N, it makes C/N ratio of 9 (or 9/1), as specified in the table. If we use atoic ratios, it would be about 10.

The English version of Wikipedia confirms this:

https://en.wikipedia.org/wiki/Carbon-to-nitrogen_ratio

as the papers of USDA:

http://www.nrcs.usda.gov/wps/PA_NRCSConsumption/download/?cid=nrcs142p2_052823&ext=pdf

Clearly, I don't write, that the expression of C/N ratio is more proper from scientific view than use of atomic ratios, I refer to the common practice.

Now it is clear for me, that in every publication it should be specified, if  mass, or atomic C/N ratio is used.

Thank you Michal for comment. I totally agree that in every publication it should be specified, if mass, or atomic C/N ratio is used. I guess for tasks of soil classification we may use C/N mass ratio, but if we need to discussion properties or changes in chemical structure of organic matter we need use atomic ratio. Howbeit we always can to recalculate mass ratio to atomic.

I agree now :-)

Hi Shah ji,

I endorse all the answers given above. I would like to add little more. Estimation of total C by  wet oxidation method is not possible. You may have to multiply by 1.33 for the Total C content estimated by the above method.  Similarly it is necessary to estimate Total N in soil.  After this, these have to be converted to gram atoms of C and gram atoms of N, respectively in 100 grams of Soil.  Then workout the ratio.

Thanks for putting such question on this floor!

Yes , I agree with Junhong , C/N ratio of soil is organic-N/total nitrogen only. Comments of Michal in this matter,  are worth appreciating. During total C estimation , soils having plenty of calcium carbonate , will over-estimate the soil C . And C/N is based on mass ratio conventionally.

‘C/N ratio of soil’ and ‘C/N ratio of soil organic matter’ are two different parameters.  The difference between the two can be substantial depending on the calcium carbonate content in the soil (as already commented by many in this discussion) as well as the extent of ammonium fixation.  Note that in some soils, fixed ammonium may account for as much as 50% of the total N in top soil as reported in the book chapter cited below.  The extent of ammonium fixation varies depending on factors such as the type of clay mineral, soil texture, and soil pH.

Qi-Xiao Wen, Li-Li Cheng (1997) Fixation and release of ammonium (book chapter) In: Nitrogen in Soils of China. Volume 74 of the series: ‘Developments in Plant and Soil Sciences’. pp 67-86

Interesting question Tufail, As many of the preceding comments mention, much depends on what fraction you wish to measure OM, mineral soil or both and secondly what type of soil are you working with. I have about 400 soil samples analysed each year and for carbon we use Loss on ignition and also the Dumas combustion. Because the soil is mainly of volcanic origin high in Al & Fe sequioxides  we found the LOI test to be over-estimating Carbon and because we do not have Carbonates the Dumas Combustion works very well. We also include in the analysis Ammonium and Nitrate but to produce a C:N ration you will really need organic N also therefore probably the best N test would be the Kjeldahl method. I look forward to other comments relating to my recommendation.

Philip , your  interesting response  has generated yet another question , as what l  analytical method should we follow to determine the C:N ratio of calcareous soils.

I agree with Philip.

On the question on what is the most appropriate for determining the soil organic carbon (SOC) in calcareous soils, modified Mebius method is one option.

There are alternative ways to remove carbonates from soil samples prior to analysis of SOC by Duma’s combustion. There was a recent discussion on this topic which can be found in this link:

https://www.researchgate.net/post/Any_better_methods_to_remove_carbonate_from_soils

Dr. Pere Rovira (has a profile in RG) has been commenting on the modified Mebius method many times before and his comments may be very useful.

Hello to all,

While estimating Organic C, the lime will not interfere; however we treat with sulfuric acid, hence it will be neutralized and sent as CO2.  Only the organic C will interact with Chromium ion and later it will be determined with Ferrous ammonium sulphate.  So, one should confuse with lime content and estimating total C in my opinion. After the determination of  organic C, it should be multiplied with 1.33 to get Total Organic C.

Thanks

Hello to all,

Estimation of total C in soil, i.e. Total Organic C does not include lime content in soil. Only the organic C will interact with Chromium ion in the reaction with potassium dichromate, later it will be determined by FAS. After organic C is known, it should be multiplied with 1.33 to get Total Organic C. The C:N ration refers to organic source only..

Thanks

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

Excellent response from Michal. Soil C:N  ratio by and large refers to organic carbon to total N only . Dr Rajakumar has given some valid points . we dont need to have total carbon of the soil  in order to interpret soil C:N ratio. 

Hi scientist, this thing is simple. You find your OC and N using Leco machine or any other method which will give you the concentrations of the above as %, then you divide C by N to get your C:n ratio.

Hi Hari

Definitely yes. calculated as (C/12)/(N/14) where C and N are the amounts you obtain from the elemental analyzer.

Yes agree with Dr. Junhong Bai:

You should determine the SOC content and total nitrogen content to calculate the C/N ratio in soils. I do not think it is the ratio of TC and TN.

Yes, you can calculate directly C:N from what you obtained from CNS elemental analyzer (C%) and (N%). Just divide (C%)/(N%)

Dear all, greetings.

any ACCEPTED biochemical method to estimate C?

I need to estimate in rice crop. plz.

Dear all, greetings.

any ACCEPTED biochemical method to estimate C?

I need to estimate in rice crop.

What ranges of C and N % and C/N do you expect in the surfaces of peat bogs?

is it possible to use Total Organic Carbon (for plant material samples) from loss on ignition test to estimate C/N ratio?

It’s total carbon divided by total nitrogen (C/N)

To calculate carbon to phosphorous is the same procedure?