One of the simplest (though not very accurate) methods of is by Loss on Ignition i.e. determining the weight loss after igniting the soil at ~ 450 celsius to determine SOC. Thereafter, SOM is equated to be 57% of SOC

I have not seen Ronaldo answer, but came to the same answer. It is necessary to evaporate a lot, it is best to do this in a vacuum post. The difference of the masses and will be what we need. With the evaporation temperature is desirable to experiment.

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Article Soil organic matter determination