Three important points need to be noted when we  go for Walkley and Black modified method.First point, for  grinding  sample we can go up to fine grinding of 0 .2-0.5 mm in case of calcareous soil but in case of sandy soil we may restrict grinding up to 2 mm only as we can not break particles(we can break aggregates).Second point is sample size ,in case of fine  texture soils having somewhat more amount  of organic carbon we can take 0.5-1.0 g sample whereas in sandy soil with low organic carbon one has to take 2-3 g sample .Third point ,whether CaCO3 content influences the analysis .Normally calcium carbonate concretions having coarse texture may not react or slightly  react with con.H2SO4.But  if calcium carbonate is more like in the young alluvial soils of Bihar having 30-35% fine/silt sized CaCO3,it may react with conH2SO4 and may lower is strength. This point need to be examined. Whether we have to add more con H2SO4 to compensate for the loss in neutralizing CaCO3?

Dear Rishikesh! I think if you want to analyse carbonate soils with higher sand content it is better to use Walkley-Black method (wet combustion), because it does not measure the cabonate C content (with oxidation of C +2). In the case of elemental analyzer (dry combustion method) it would be necessary to remove carbonates using HCl or other reagent and it is not always easy. High sand content may affect repeatability of analysis, it is recommendable to grind such soil, however the ground sample will not be suitable for many other analyses (well - elemental analysis and N tota using Kjeldahj method can be performed).

I concur with Michał Stępień. Another method that is generally not affected by carbonates is loss on ignition (LOI, 360oC), although it is best to run parallel analyses with Walkley Black or another method and use a regression to correlate LOI-C to the other method. Note that LOI is actually an estimate of organic matter content, and conversion to C content requires a correction factor that is an estimate of the C content of organic matter.

Wet digestion method(modified Walkley and Black procedure) can safely be used for sandy soils and calcareous soils.For fine textured soils we use 0.5 g soil sample for analysis but in case of sandy soil we have to use 2 g sample.

W&b method goes well with sandy and calcareous soils with specific mesh size. Would you  please elaborate  the difficulty you might have faced following the W&B method?

Soil organic carbon content can be measured as different fractions of carbon. In general Walkley and Black method is good for routine analysis. For TOC content, carbonate has to be removed by treating the soil with HCl or other sutaible reagent. Approximate value of total carbon content can be measured by the loss on ignition method, however these value should be converted in organic carbon content.

In these cases, I rather prefer WB method, taking 2 - 3 g of sample.

Generally, Walkley and Black method is suitable for analyzing organic carbon.

Three important points need to be noted when we  go for Walkley and Black modified method.First point, for  grinding  sample we can go up to fine grinding of 0 .2-0.5 mm in case of calcareous soil but in case of sandy soil we may restrict grinding up to 2 mm only as we can not break particles(we can break aggregates).Second point is sample size ,in case of fine  texture soils having somewhat more amount  of organic carbon we can take 0.5-1.0 g sample whereas in sandy soil with low organic carbon one has to take 2-3 g sample .Third point ,whether CaCO3 content influences the analysis .Normally calcium carbonate concretions having coarse texture may not react or slightly  react with con.H2SO4.But  if calcium carbonate is more like in the young alluvial soils of Bihar having 30-35% fine/silt sized CaCO3,it may react with conH2SO4 and may lower is strength. This point need to be examined. Whether we have to add more con H2SO4 to compensate for the loss in neutralizing CaCO3?

The unique limitation that you can face when using Walkley-Black is a lack of a complete oxidation of the most recalcitrant fraction of Organic Carbon. To improve it, it is useful to use a block digestor and heat the samples at 150ºC for 30 min. The production of potentially dangerous residues (i.e. chromium) is another drawback of the method.

Elemental analyzers are usually limited by the presence of carbonates, since most of them heat at high temperatures resulting on total carbon oxidation (i.e. organic + inorganic). In some labs carbonate is previously eliminated with a fumigation of HCl. However, that procedure is tedious and can affect the analyzer if residues of HCl remain in the sample.

Analyzers able to differentiate SOC and SIC can be found in the market. They usean inert or oxidizing atmosphere and furnace temperature ramping to differentiate the C forms.

I attach a file that can be helpful.

Regards,

Daniel

Analytik Jena from Germany proposes a Total Analyser (Multi EA4000) who can very effectively measure TC and TIC separately so that you can calcutae TOC = TC-TIC.

TIC is removed by concentrated P-Acid and then measured. Works for carbonates and even dolomites.

I just can recommend the apparatus if you wand to separate and measure proporly carbonates.

With LECO this was not possible.

Dear Rishikesh

Are you planning to set up the methodology and by equipment for a lot of samples, or are you looking for a solution for few samples?

In the first case, I would choose na elemental analyser: Skalar, Breda, Netherlands have solutions which we have in our Lab.

In the second case, I would recommend the WB method.

Dear Sir (Dr. Joao Coutinho),

Basically I performed the WB method for soil samples (rich in sand content). However, after going through the literature I doubt about the underestimation or overestimation of SOC, therefore, I asked this to the present platform.

The sample size is about 70-80 at present. I am thankful for your suggestions. 

Regards,

Rishikesh

Thank you Dr. Michał Stępień and Prof. Brian Karl Richards. 

I will try to see the regression lines by performing both the recommended method. If possible, can you send me a pdf having details of the LOI method (360 oC) because I have a protocol on LOI with 550 oC for 2-4 hours.

Thanking you once again in advance.

Rishikesh

Thank you Sir (Dr. A. Subba Rao, Dr. Dilip Kumar Pal, Dr. Bibhash Chandra Verma) for your critical insight on my problem.

Actually I doubt about the underestimation or overestimation of SOC from the samples by using this method. Now I will once perform the suggested methods to verify it.

Regards,

Rishikesh

Thank you Dr. Daniel Plaza-Bonilla for your suggestions and sending a pdf for the same.

Regards,

Rishikesh

If you want to set up methodology and purchase equipment for your lab, I would recommend an Elemental Analyser, which takes care of inorganic carbon presence.

WB method "only" mesures oxidizable carbon, not total organic carbon, with a higher or lower efficency depending on the type of digestion (time and temperature)

Thank you Sir.

We would see the suggested elemental analyzer (Skalar, Breda, Netherlands).

And also look for other such equipments. We would follow the other alternatives such as extended digestion and loss on ignition (LOI) and if required then try for it via any project.

Regards,

Rishikesh

Dr.Singh, You  can also make a small study  to see the effect of calcium carbonate on organic carbon estimation by Walkley and Black method,by selecting a few noncalcareou soils varying in organic carbon and adding different  levels of calcium carbonate.You may find up to what concentration  the CaCO3  will not interfere with the organic carbon estimation and if it interferes beyond certain level what can be done(increasing the amount of H2SO4 to be added or any other alternative)

Thank you Dr. A. Subba Rao Sir.

Regards,

Rishikesh

Hello, you can use the Rock Eval pyrolysis to measure both organic and mineral carbon, without further treatment as drying and crushing.

HI, we have used a LOI method to determine SOM for similar soils:

https://www.researchgate.net/publication/230805511_Comparisons_of_Three_Methods_for_Organic_and_Inorganic_Carbon_in_Calcareous_Soils_of_Northwestern_China

Article Comparisons of Three Methods for Organic and Inorganic Carbo...

Thank you Dr. Wang for your suggestion and attaching the article on the same.

Regards,

Rishikesh