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REVIEW (Open Access)

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

Thilo Rennert https://orcid.org/0000-0003-1435-2157
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Fachgebiet Bodenchemie mit Pedologie, Institut für Bodenkunde und Standortslehre, Universität Hohenheim, D-70593 Stuttgart, Germany. Email: t.rennert@uni-hohenheim.de

Soil Research 57(1) 1-16 https://doi.org/10.1071/SR18299
Submitted: 05 October 2018  Accepted: 13 November 2018   Published: 5 December 2018

Journal Compilation © CSIRO 2019 Open Access CC BY-NC-ND

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.

Additional keywords: dithionite, organic complexes, oxalate, oxides, pyrophosphate, soil classification.


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