Organic matter can be estimated quickly and rather easily by burning a soil sample at 300 oC in a furnace for 4-6 hours. How well correlated is such a value with soil organic carbon as measured by other methods, such as Walkley-Black H2SO4-Dichromate method or Ignition at 1000 oC (CO2 released and measured) IR method (say, using a LECO carbon analyzer)?
first of all 300 degree C is not a good measure. You should do it at a minimum of 390 or 400. In order to corelate with other methods you should calibrate between the methods. However, the muffle furnace is a well known method and in case that you have many samples is a very useful method which yield good data.
Dear Nguyen,
we have done only a little LOI experiment to compare some different ignition temperatures and some soil properties. The data-set has been small/narrow (n=42), the soil properties range rather extensive). May be therefore was a correletion between "optimal" ignition temperature (373 °C) and Walkley-Black method not very high (a some improvement results from curvilinear dependence). Most the authors found very high correlation between LOI and W-B, or other (e.g. Leco total carbon analyzer) methods. Please, look at: https://www.researchgate.net/profile/Petr_MRAK/contributions?ev=prf_act. If would not be successful, I can send to you this poster on your e-address. I am sorry I can't help you more.
Cordially, Karel FIALA, alias Petr MRAK.
It is important to keep in mind that not only the organic fraction will be lost at higher temperatures, but also carbonates for instance. We have used a nice method in which the weight is measured over a burning-temperature-curve, and in that way one can distinguish between different soils and their origin.
I guess a temperature around 400 to 450C should give you standard results, but if you want to be sure: check what others have used as temperatures, but one of the Dutch standards of soil description even uses 550C (see link, in Dutch, page 10)
http://www.sikb.nl/upload/documents/AS3000/Protocol_3210-3290_v2.pdf
Nguyen,
My LOI for soil carbon work was years ago now, but my impression is not much has changed. That is one advantage of a simple, basic procedure, it tends not to change much. The disadvantage, of course, you have to use it only for very general information, or you have to adapt and refine it to your specific purposes.
I agree generally with the previous 3 answers. I note, however, that you are in Hawaii, and presume, therefore, that you may be working on volcanic ash soils. The spatial/structural occlusion and surface adsorption relationships between minerals and OM in tropical soils, especially those that have developed in volcanic parent materials, can be quite different than in soils from other milder climate regimes and non-volcanic areas. Occlusion of OM by (primarily) aluminum (oxy)hydroxides can be the dominant form of OM stabilization, and it can form rapidly and be quite effective. Historically, the rapid accumulation of OM that results from this stabilization mechanism has been regarded as the basis of the notable fertility of some volcanic soils. Hence, the objective of your work should be considered when looking into LOI as a method for measuring OM. If you are concerned with total organic C, for example in regard to bio-sequestration of atmospheric carbon, then you should be using a higher combustion temperature. If you are concerned with more detailed aspects of soil quality/productivity/fertility, then you will need to be more cautious in selecting a combustion temperature and conditions.
Al oxyhydroxide/OM aggregates may not respond as expected during combustion, and may combust at lower or higher temperatures than typical for other soils. In addition, be aware that the Al oxyhydroxides are functionally amorphous mineral clays. As such, you need to be concerned about loss of mass due to entrainment of fine mineral particulates in gasses exiting the crucible during combustion. My recollection is that there is a reasonably thorough introduction and discussions of adaptations of the procedure in the ASA/SSSA Methods of Soil Analysis (I am not sure what the latest available edition is now). Good luck.
These discussions are really interesting. I really appreciate your comments. Thanks, NVH.
I do not find the LOI method very accurate. It is an old method and modern methods using Carbon analyzers are more accurate. As one of the other colleagues noted, if the temperature is too low, you risk not burning off all of the organic material but if the temperature is too high, you also risk burning off some of the inorganic materials.
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