Dear Jakab,

U can add activated charcoal to get rid of yellowish brown development by trail and error. 

Dear Gergő,

According to Morgun and Makarov, it does separate SOM from other soil particles: http://link.springer.com/article/10.1134%2FS1064229311040077#/page-1

However they are not talking/writing about solving it.

But these guys are wqriting about C loss in PTS solution: http://andrewsforest.oregonstate.edu/pubs/pdf/pub4332.pdf

See page 77: "We calculated that between 17% and 26% of total soil C and N was mobilized in the SPT solution and subsequently discarded during the density fractionation process in both soils (Fig.2)."

Cheers, Csaba

Dear Csaba,

Thanks for the links.

So the point is that this small problem of PTS was published 9 years ago and since then this method is still widespread for fractionation and SOC quantification.

Amazing.

Cheers,

G