Depth sum of organic matter weight

[kevinwolz]

The Soil Properties Map shows "Soil Organic Matter" in kg/m3 as the "depth sum of all horizons". SSURGO has the om_* variables which are OM percentages, but I cannot find a variable for bulk density with which to calculate actual weights. Am I missing something? The Soil Properties Map also maps bulk density, so it must be in there somewhere!

[dylanbeaudette]

This calculation uses the Db at 1/3 bar water retention (dbthirdbar_r ) column from the chorizon table.

[kevinwolz]

Ah, so "db" is bulk density? That's what I get for just searching for "bulk density" in the metadata!

So that map is summing om_r / 100 * dbthirdbar_r * (hzdepb_r - hzdept_r) * 10 across horizons?

[dylanbeaudette]

Yes, in the soil science and especially soil physics literature bulk density is represented as $D_{b}$. Hard to use subscripts in a database, so it is abbreviated as "db" or "Db". I've asked that the word "bulk density" be added to the SSURGO metadata, it is kind of confusing.

The calculation includes an additional term, "soil fraction", which is (1 - coarse fragment volume fraction.) Same as the calculation of SOC stock:

[kevinwolz]

Ah yes. Here in the mollisols of Illinois I always forget about those things called rocks...

Thanks!