We are attempting to determine the amount of Al, Si, and Fe in amorphous materials in a soil sample, hence the oxalate. However, we want to remove the contribution from humus complexes (hence the pyrophosphate). Is there any reason to run these dissolutions separately, or can they be performed sequentially on the same sample?
I agree with Andre, but would also like to point out that the interpretation of the pyrophosphate extraction results may not be so straightforward in the case of Fe and Si. For Fe it is probable that some Fe (hydr)oxides are dissolved by pyrophosphate - this would mean that this fraction is not only composed of Fe-SOM complexes (see e.g. Parfitt & Childs, Austr. J. Soil Res. 26, 121-144, 1988). In the case of Si, pyrophosphate may in fact release much higher contents of Si than oxalate in organic-rich horizons due to partial dissolution of biogenic Si. We have observed this phenomenon several times but only documented it once in a publication (for the Horröd site; Bain et al., Water Air Soil Pollut. Focus 3, 29-47, 2003). However, for Al the assumption that pyrophosphate is specific for Al-SOM complexes seem to hold reasonably well (again see Parfitt and Childs, 1988).
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