Sequential Extraction and Separation

The conventional methods to differentiate organic and inorganic P components in soils basically rely on extraction with alkaline or acid reagents. They go back to the procedure originally developed by Chang and Jackson (1956), later further modified (Hedley et al. 1982; Kuo 1996). Many studies use NaOH, often in combination with chelating agents, such as EDTA, or resins. It is assumed that the chemical extractants sequentially remove dis-cretegroupsofPcompounds. TheinorganicPfractionintheextractisthen determined by a colorimetric procedure and the amount of organic P is obtained by difference (Condron et al. 1985; Guggenberger et al. 1996; Sui et al. 1999; Koopmans et al. 2003). Another approach is to identify and quantify organic P forms with enzymatic mineralisation. The sample is incubated with specific P-releasing enzymes, such as alkaline or acid phosphatases, phytases or nucleases. The orthophosphate released is determined by col-orimetric procedures, most often the molybdate blue method (Kuo 1996; Shand and Smith 1997; Hayes et al. 2000; He and Honeycutt 2001) and can be assigned to the organic binding form from which it was released.

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