Be careful of using the WB method. It is now superceded by combustion C. I'm not sure even a correction factor is sufficient. Keep in mind it has always been a near qualitative measure of soil C. Try us combustion C if at all possible. (Some publications are available to support these statements.

Usually the WB method gives a value that is lower than Springer-Klee or that of elemental analyzer. The correction of the WB value in Italian Official methods of soil analysis is obtained multiplying the fianl value by 1.3, that consider that partial oxidation of carbon (70%). In my experience, comparing the values obtained by both corrected WB and elemental analyzer, I obtained roughly the same values, for samples from a Mediterranean semiarid climate. However, it is important that in the description of the methods you indicate the analysis method that you used and if your final value is the corrected one.

Sorry, I add that as a matter a comparison, it is important only that use the same type of values

I think you should rephrase your question! Everything people are saying here is correct for WB method. But, if I understood you (and correct me if I'm wrong), what you really wanna know is that if the same factor applied to soil C content by WB should also be applied as a N index, is that correct?

If this is right, which N index are you talking about? If you wanna estimate N from the C content, than you should do it after applying the correction factor to the C content. Because that is the correct amount of C you really have, assuming that the 77% is the correct factor.

A factor of 1.3 is usually used to convert Walkely-Black values to SOC. The SOC then needs to be converted to organic matter to estimate to contribution of OM to soil N. A factor of 1.724 is usually used to convert SOC to OM. Other factors such as 1.9 or 2.0 have also been used to convert SOC to OM in surface layers of mineral soils that are commonly found in agricultural applications. Organic matter is the form of carbon that is reported on soil testing reports, for example, and is used to make fertilizer recommendations based on yield goal, crop, and residual nitrate-N. Nitrate-N is commonly measured in arid or semi-arid soil since residual N in the soil needs to be accounted for in fertilizer recommendations. Usually, for every one percent OM in the soil, there will be 30 lbs of nitrogen released into the soil from mineralization or decomposition of the OM. This conversion may vary from lab to lab, depending on geographical location that can affect soil type. The organic matter content, and residual nitrate-N is used (along with yield goal and crop type) to make fertilizer recommendations. Most soil labs use a modified version of the WB method where the soil is treated with sulfuric acid and potassium dichromate solution. A specific volume of water is added to the mixture and then the absorbance of the solution is determined at 610nm using a colorimeter. The absorbance is then converted to %OM. The conversion is based on calibrating the relationship of SOC to OM using C analysis by combustion or wet oxidation and CO3-C analysis to account for inorganic C.

Organic carbon estimation in soils having a range of organic carbon from 0.25 to 3.5% is best possible to recover 100% oxidizable organic carbon by Tiurin (1951) method and

there is no need for any correction factor. This method is a modified wlkeley & Black Method wherein 0.50 g to 1.0 g of 0.5 mm sieved soil is heated with 10 ml of 0.33 M1N Potassium Dichromate solution treated with 20 Ml of concentrated sulphuric acid and heated on hot plate for 5minutes and cooled and titrated against 0.5 M Ferrous sulphate or Ferrous ammonium sulphate using Diphenyl amine as indicator and the end point will be dull green colour. This can be referred through the follwing citation.

Citation 1) TIURIN N.W. K metodikie analiza dla sprawnitielnogo izuczenija

sostawa poczwiennogo pieregnoja ili gumusa. Rabota

po organiczeskomu wieszczestwu poczwy. Moskwa 1951.

The other two following citations will help estimation of Total Organic carbon in soils.

Citation 2: Evaluation of methods for measuring soil organic carbon in West African soils Gregory W. McCarty1*, James B. Reeves III2, Russell Yost3, Paul C. Doraiswamy1 and

Mamadou Doumbia4. African Journal of Agricultural Research Vol. 5(16), pp. 2169-2177, 18 August, 2010. vailable online at http://www.academicjournals.org/AJAR

ISSN 1991-637X ©2010 Academic Journals

Citation 3: Gaublomme, E., De Vos B. & Cools, N. 2006. An indicator for Microbial Biodiversity in Forest Soils. INBO. R.2006.40 Instituut voor Natuur- en Bosonderzoek, Brussel. D/2006/3241/308 INBO.R.2006.34 ISSN: 1782-9054

Whether a correction factor should be used for WB C depends on the purpose of your application. For example, if it is for C accounting purpose, then a correct factor would be necessary, bearing in mind that such a factor may not be accurate for all soils because the recovery of total SOC by the WB method differ for different soils as correctly pointed out by others above. As you want to use the measurements for "soil fertility assessment", you only need a relative index. In this case, whether a constant correction factor is used or not does not affect the relative assessment. And WB C may be a better index than TOC measured with the combustion or external heating method because the TOC tends to include more recalcitrant components. Here use or not use of a correction factor is your personal choice. However, you do need to clearly indicate whether the values are corrected or not in reporting so as to help readers assess and compare with other reported values. If you want to use WB C as an "soil N index", again a correction factor would not make difference for predicting mineralisable organic N as long as the values are reported consistently. Given that the WB-oxidisable organic matter has different C/N ratios in differnt soils, an alternative index (e.g. easily oxidisable organic N or mineralisable N during incubation) may be an better option.

A correction factor is necessary only if WB results are combined with those obtained by other methods (e.g. determination of Total C). Anyway, a correction that is valid for all types of samples, or even for all horizons of one profile, does not exist. There are two reasons for this statement: C contents of SOM are not equal in all soil horizons (it depends on the relative amounts of aromatics, carbohydrates, and aliphatics). The second reason is that the fraction oxidized by the WB method decreases from around 90 % in topsoils to 50-75% in many subsoils. This kind of information is not even available for most soils.

Thanks to the overwhelming response on my question.

Let me explain the background of this question.

Laboratories use several methods to estimate soil C. Each lab select a method based on convenience as well as cost. Particularly I ask the question whether WB carbon is an index of N. Most of the labs in the third worls still follow WB protocol without applying ext. heat. The C value obtained is directly treated as the N index and fertilizer recommendations are offered. Knowingly or unknowingly soil scientists use WB - carbon in such a manner even to calculate stock-C.

Of course I agree depending on various factors the WB carbon varies, however, those labs that still follow merely WB protocol may not take it as the N index, instead at least for a set of samples they deal with, the true carbon may be estmated by dry combustion method, derive a factor, apply on the WB carbon. And preferably, for stock C estimations, follow the dry cumbustion method.

Also, from my personal experience in the tropical soil (ultisols), The WB carbondoes not represent the labile C. The labile C in these soils are much much lower which was verified through soil respiration and mineralization studies. Also it is pointed out here that even the modified WB protcols (with external heating) do not completely recover C from our soils.

I feel I should emphasize a point here care to be taken while WB carbon is used as mere N index and for objectives that require more accuracy it can not be used.

However, the fact is that, still many labs prefer WB protocol for its simplicity and less cost. It may be followed who can not afford other protocols, but with a little more care such as deriving and utilizing a correction factor.

Once again I thank all the peer group came up with invigorating answers and collectively we can derive something useful to all.

Thanks

Joshua

You can have a look at these related papers:

De Vos, B., Lettens, S., Muys, B., Deckers, J.A., 2007. Walkley-Black analysis of forest soil organic carbon: recovery, limitations and uncertainty. Soil Use and Management 23: 221-229.

Lettens, S., De Vos, B., Quataert, P., Van Wesemael, B., Muys, B., Van Orshoven, J., 2007. Variable carbon recovery of Walkley-Black analysis and implications for national soil organic carbon accounting. European Journal of Soil Science 58: 1244-1253.

De Vos, B., Vandecasteele, B., Deckers, J. and Muys, B., 2005. Capability of loss-on-ignition as a predictor of total organic carbon in non-calcareous forest soils. Communications in Soil Science and Plant Analysis 36: 2899-2921.

 Very valid points Buurman ,,,your reason is worth considering ...

Dear Abraham - Please refer to the attached pdf. Hope you may get  the latest  idea about the topic.

Dear Pal gi,

Indeed  great pleasure in having an update from your team.

Hopefully you remember me! We met during IPI conference GB Pant university in Ludhiana.

I did the work for Rubber Soils in Kerala as early as in  2013. Please refer my publication in Rubber Science (can have it from my research gate site). But you can imagine the trouble I had undergone to convince the tradionalists !!!!!

Great effort . Congratulations!

Joshua ABraham

Dr.Abraham, the following two papers published from IISS by my coworkers may be of interest to you.Full texts of these publications can  be seen under my publications/contribution.

Relation between dichromate oxidizable and total organic carbon and distribution of different pools of carbon in Vertisols of Central India. Kundu et al. 2014 .Indian Journal of Agricultural Sciences.

Predicting total organic carbon content of soils from Walkey and Black analysis.  Jha et al. 2014 Communications in Soil Science and Plant Analysis.