Wet-chemical extractions to characterise pedogenic Al and Fe species – a critical review

Abstract

Wet-chemical extraction of soil is a standard procedure to characterise pedogenic aluminium (Al) and iron (Fe) species, especially oxides, allophanic minerals and metal–organic associations. This article critically reviews the suitability of commonly used extractants (e.g. dithionite, oxalate and pyrophosphate) and the potentials and restrictions in their use for species identification and in soil classification. None of the commonly used extractants is completely selective and quantitative. The degree of completeness differs between the extractants and depends on soil composition. Dithionite-based methods provide a ‘pseudo-total’ content of pedogenic Fe oxides, as they are not always completely dissolved. Oxalate may attack further non-target species, releasing additional Al and Fe. Therefore, the extraction of Al and Fe exclusively from poorly crystalline species is not always guaranteed. As a consequence of dispersion of aggregates, pyrophosphate solubilises both mineral particles and metals from organic associations. Thus, quantification of species based on these extractions and their implementation in pedogenic thresholds may be questionable. Alternative extractants such as citrate–ascorbate and dithionite–citrate–oxalate could be used in addition, as applicable and reliable wet-chemical extractions will be still demanded for research and practical applications. The examination of the effectiveness and selectivity of wet-chemical extraction methods by spectroscopic techniques is recommended.

Speciation and sorption of phosphorus in agricultural soil profiles of redoximorphic character

“…Phosphorus sorption capacity calculated from the oxalate extract ranged between 27 and 32 mmol P kg -1 , suggesting that only a maximum of 1.5-2.1 times more P could be adsorbed compared with the present amount of P (15-18 mmol P kg -1 ). Although oxalate extraction neglects some clay binding sites for P as well as binding sites offered by crystalline pedogenic oxides (Rennert 2019), PSC calculated from oxalate extraction is commonly used to estimate P sorption capacity of Northern European soils because an empirical linear relationship between the sorption maximum of P and the sum of Al ox and Fe ox was reported by Beek (1979) (Maguire et al 2001). A relatively low binding capacity for P was also demonstrated by low K f coefficients (20.2-33.1 mg 1Àn f L n f kg À1 , Fig.…”

Podzolisation affects the spatial allocation and chemical composition of soil organic matter fractions

“…We extracted the soils with citrate (Reyes and Torrent 1997), targeting organic-metal (Al and Fe) complexes, with ammonium oxalate-oxalic acid in darkness (Schwertmann 1964), targeting poorly crystalline Fe oxides and aluminosilicates, and partially organic Al and Fe complexes (Rennert 2019) and with dithionite-citrate-bicarbonate (Mehra 1958), targeting all pedogenic Fe oxides. Additionally, we extracted the S + A fractions with ammonium oxalate-oxalic acid.…”

Review Article. What are the nature and formation conditions of hydroxy‐interlayered minerals (HIMs) in soil?

“…However, DCB is based on the reduction of Fe(III) and does not necessarily mobilize Fe 2+ potentially present in the interlayers. In addition, ferric oxides and hydroxides are dissolved by DCB, and further silicates are partially attacked by DCB, potentially releasing Fe ( Rennert , ). Thus, these analyses are no ultimate proof that ferrolysis was a process to affect the composition of the clay fraction in the soils under study.…”