I am analyzing soil samples from a volcanic area (relatively young), formed by Andosols (non-allophanic), I have run all the routine analyzes (oxalate, DCB, pyrophosphate extractions, CEC....) however I've realized that the Alo content is smaller than the Alp for some samples (all of them A horizons with high organic matter content while for the 'less' organic horizons the Alo is always higher than the Alp). As far as I understand the Alo content represents the organic-bounded + the amorphous Al content, while the Alp represents only the organically-bounded Al, hence the Alo content should be higher than the Alp. Is there any mechanism that could help me explaining this behavior?
Similar results have often been reported in the past, for example,
https://oce.ovid.com/article/00010694-199607000-00005/HTML
https://www.ars.usda.gov/ARSUserFiles/20360500/pdf_pubs/P1103.pdf
These suggest that Alp and/or Alo are over- and/or under-estimated. Fractionation based on extraction is often less quantitative.
Dear Marlon
Andisol with a high content of humic compounds, the majority of the proportion of Al-amorphous will be complexed to Al-humic, so it makes sense that in Andisol with high organic content, Al-humic> Al-amorphous. Conversely, at andisol with low OM content, only a small portion of Al-amorphous complexed to Al-humic so that Alo> Alp content
I always thought ammonium oxalate extraction (dark) extracts amorphous and poorly crystalline aluminium oxides, while sodium pyrophosphate extracts organically bound Al.
In your case, due to the high content of SOM, much of the Al is bounded to organic compounds.
I would appreciate any considerations done by other collaborators.
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