All comments have been good, but which method you might prefer will depend on soil type as well as which form or forms of organic carbon you are attempting to measure. Organic matter in well structured soils with relatively minor clay contents may not be fully oxidized in Walkley-Black without prior dispersion procedures needed to assure adsorbed and sequestered organic matter in soil aggregates (especially well developed micro-aggregates) are adequately exposed, especially if organic carbon content is generally high. Loss on ignition results can also be affected by organic carbon/matter association with clay particles. In the case of LOI, losses I have observed appeared to be due to ejection of fine particulates during combustion. This is often not taken into account as a potential error source, but I have observed such losses under presumably quite gentle conditions. For example, I have observed such clay particulates with associated organic matter being swept out of freeze-drying vials in the very low density stream of water molecules exiting through the neck of the vial. One cannot practically observe such movement during the LOI combustion process, but unless the process is carried out with appropriate consideration of rate of heating and max. temperature of the furnace, one can reasonably expect such random losses to occur.

For such reasons you should be cautious about applying a SOM/SOC conversion factor from the literature. If you need accuracy even somewhat beyond routine soil analysis, then you should adjust and calibrate whichever method you choose for your soils, and if you want to make cross-comparisons, calibrate the two methods against each other. I suggest the same cautions for any of the numerous adaptations of either method. It all depends on the accuracy and precision you require for your purposes. Good luck.

These methods don't evaluate the same thing. While LOI allows you to quantify soil organic matter (not only organic carbon), Walkley-Black quantifies directly organic matter by oxidation. Nevertheless I would advise you to use CHN Analyzer instead it is a more accurate and reliable method.

Important question. It really depends on your purpose, funds, lab availability, and intended audience or research stakeholders. Walkley-Black method is more recognizable and accepted among tenured soil scientists, but the trend could be changing towards loss-on-ignition in recent years.

The Walkley-Black method is more effective and accurate on soils with very low OM, but on soils with high OM, it may produce low test results due to incomplete oxidation. The major concern with Walkley-Black is the disposal of hazardous acid material.

In soils with high OM, loss-on-ignition will provide more accurate and consistent results. However, loss-on-ignition may also be impacted by the presence of carbonates, which may inflate values depending on their abundance. There is also concern about water losses from clay mineral matrices. There are ways to handle that, like the use of HCl acid, but that will dissolve a small portion of the OM prior to testing.

If you fear that soils are low in OM and will be very near each other in values, Walkley-Black method may be preferred, assuming laboratory space or funds for testing are available. If you are dealing with higher OM soils, loss-on-ignition may be preferred and is less labor intensive, and no acid material to dispose of after. If soil OM is the main component of your study, you probably need to find best method for your given situation (there are more than the two we've talked about here). If soil OM is a part of a larger study in which you are dealing with a lot of soil measures, loss-on-ignition could be the effective route in light of the considerations listed above. Regardless of which one you use, both methods are generally well accepted in most soil science circles. Whichever you chose, just know the strengths and weaknesses of that method, and state those in your reporting, you should be fine.

Hope this helps.

BT

Again one thing that is very important to have in your mind is that soil organic matter is not equal to soil organic carbon. There is a relationship between SOM and SOC but they are not synonyms.

Thanks Bernardo, that is a very important thing to remember. Depending on what someone is after and what they have measured, they can use a conversion factor to compare SOM/SOC levels. A lot of times its probably discipline specific. In my disciplines we talk more about SOM rather than SOC, which is how I think about Walkley-Black vs. loss-on-ignition. Such considerations need to be addressed in any study depending on its objectives.

Thank for all these valuable answers ...kind regards for all of you.

All comments have been good, but which method you might prefer will depend on soil type as well as which form or forms of organic carbon you are attempting to measure. Organic matter in well structured soils with relatively minor clay contents may not be fully oxidized in Walkley-Black without prior dispersion procedures needed to assure adsorbed and sequestered organic matter in soil aggregates (especially well developed micro-aggregates) are adequately exposed, especially if organic carbon content is generally high. Loss on ignition results can also be affected by organic carbon/matter association with clay particles. In the case of LOI, losses I have observed appeared to be due to ejection of fine particulates during combustion. This is often not taken into account as a potential error source, but I have observed such losses under presumably quite gentle conditions. For example, I have observed such clay particulates with associated organic matter being swept out of freeze-drying vials in the very low density stream of water molecules exiting through the neck of the vial. One cannot practically observe such movement during the LOI combustion process, but unless the process is carried out with appropriate consideration of rate of heating and max. temperature of the furnace, one can reasonably expect such random losses to occur.

For such reasons you should be cautious about applying a SOM/SOC conversion factor from the literature. If you need accuracy even somewhat beyond routine soil analysis, then you should adjust and calibrate whichever method you choose for your soils, and if you want to make cross-comparisons, calibrate the two methods against each other. I suggest the same cautions for any of the numerous adaptations of either method. It all depends on the accuracy and precision you require for your purposes. Good luck.

Thanks Bryce. In regards to your last paragraph, I recently have been doing some reading about the conversion factor and have found this resource helpful:

Pribyl, D.W. 2010. A critical review of the conventional SOC to SOM conversion factor. Geoderma 156: 75-83.

Ben,

Been a number of years since I did my considerable work on OM. Following up on your comment I ran into:

Assessment of TOC-SOM and SOM-TOC Conversion in Forest Soil by A. Ostrowska and G.Porebska in Pol.J. Environ. Stud. V 21, No.6 (2012) 1767-1775

They state, "... (other) authors agree that the LOI method is simple and cheap and may be commonly used, especially to determine the content of organic matter in acid soils developed from sands and light clay, which are characterized by the very low content of particles smallet than 0.002 mm."

After my years of separation from direct work on the subject it is interesting to see others have reached conclusions similar to mine in those days. Unfortunately with respect to Payman's question, Ostrowska and Porebska seem to have used two different LOI methods (SOM by LOI at 450 deg C in muffle for 4 hours, and TOC by LOI at 1000 deg C in TOC autoanalyzer), but no Walkley-Black (or other wet combustion). Their results are somewhat foreseeable given is related to comparison of two differently intensive LOI procedures. Regardless, it is an interesting paper that still speaks to Payman's concerns, even if not so directly.