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I am doing soil related calculations for moisture content and the soil characteristics I need (among others) is the content of the soil as a percentage for:

  • sand
  • clay
  • organic matter

I am thinking I take the the sand content (in g) divide by 1000 to get sand percentage content and like wise for clay (except use the clay content value in g). For organic matter do I take the soil organic content they provide in dc/kg and divide by 10000? It seems some areas will have a very high organic material content then. I would expect somewhere between 1% and 6% but some areas if I use that method would be like 44%.

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    $\begingroup$ I think that this is a question that should be asked at the Earth Science Stack Exchange rather than here because there does not appear to be a GIS component to it. $\endgroup$
    – PolyGeo
    Jan 18 at 4:46
  • $\begingroup$ @PolyGeo: Could you move this question into opendata, since this is an open dataset transformation request, and the soilgrids community can answer it there $\endgroup$ Jan 18 at 9:20
  • $\begingroup$ Your question puzzles me. (1) What does the dc in dc/kg mean - deci* 'something'* per kilogram? (2) Why are you dividing the mass of sand, in grams, by 1000 to get sand percentage ? (3) Likewise, why are you dividing soil organic content by 100000? Could please elaborate. $\endgroup$
    – Fred
    Jan 18 at 9:40
  • $\begingroup$ @JorgeMendes too late - two users and a moderator already migrated it to Earth Science. $\endgroup$
    – PolyGeo
    Jan 18 at 10:13
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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.

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If you are not in the position of following the steps suggested in Ash's answer, I'd suggest you to rely on existing data, as the ISRIC dataset "WISE derived soil properties on a 30 by 30 arc-seconds global grid", developed at Wageningen university and available here. This dataset is originally based on the FAO-Unesco Soil Map of the World (DSMW). Of course, the 5 arc-minutes resolution (about 9.2 km at the equator) might be a little coarse if you are conducting a specific site project, but it could be a useful reference if your domain is broader.

Additional information about the project available here.

Methodology and main variables described here.

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