What method do I use to derive the organic material content (as a percentage) per layer in SoilGrids
All weight percentages are calculated using the same formula:
Mass Fraction/Total Sample Mass
There are two different values for organic material in a soil and they're derived by different techniques:
If you want to know how much bioavailable organic material is in the soil I was taught to use a chemical reagent to oxidise and strip organic carbon from the soil, saturated soil samples are weighed before and after the test to estimate the weight of organic matter so removed or a titration can be performed on the soil leachate to determine the exact amount of carbon oxidised.
If you want the total soil carbon weigh a dried sample then crush it and put it in a crucible over a Bunsen Burner until it stops smoking, the sample also darkens as it starts to burn and then turns pale once all the carbon has been burned out of it, and reweigh it the difference is all the soil carbon.