DEFECTS IN ESTIMATION OF ALUMINUM IN HUMUS COMPLEXES OF PODZOLIC SOILS BY PYROPHOSPHATE EXTRACTION

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Abstract
In several experiments with soils and mineral phases, extraction of Al (and Fe) by pyrophosphate reagent (0.1 M Na4P2O7, pH 9.5 - 10.5) was tested with regard to its use for the characterization of Al in humus complexes. Results showed that, independent of the soil content of organic C, pyrophosphate reagent removed nearly the same amount of Al from illuvial horizons of podzolic soils as did acid NH4-oxalate reagent. In some horizons, pyrophosphate extracted even more Al than the NH4-oxalate and dithionite-citrate bicarbonate reagents. Pyrophosphate-extractable Al (mmol kg-1) also exceeded the content of organic acidity (mmolc kg-1). Experiments with amorphous Al(OH)3 and gibbsite showed high release of Al from these phases by pyrophosphate because of the high pH of the extractant. Adsorbed organic matter increased the solubility of amorphous Al(OH)3, perhaps by enhanced peptization. Pyrophosphate completely removed synthetic coatings of amorphous Al(OH)3 from soil material. These results indicate that pyrophosphate-extractable Al may not be attributable only to Al bound in humus complexes, but also to alkaline dissolution of Al hydroxide phases and peptized Al hydroxide associated with adsorbed organic matter. Thus, pyrophosphate should not be used to estimate Al in humus complexes.