A quantitative assessment of phosphorus forms in some Australian soils

Solution P-31 nuclear magnetic resonance (NMR) spectroscopy is the most common technique for the detailed characterisation of soil organic P, but is yet to be applied widely to Australian soils. We investigated the composition of soil P in 18 diverse Australian soils using this technique. Soils were treated with a mixture of sodium hydroxide-ethylenediaminetetra-acetic acid (NaOH-EDTA), which resulted in the extraction of up to 89% of total soil P. It was possible to identify up to 15 well-resolved resonances and one broad signal in each P-31 NMR spectrum. The well-resolved resonances included those of orthophosphate, alpha- and beta-glycerophosphate, phytate, adenosine-5'-monosphosphate, and scyllo-inositol phosphate, as well as five unassigned resonances in the monoester region and two unassigned resonances downfield (higher ppm values) of orthophosphate. The majority of P-31 NMR signal in the NaOH-EDTA extracts was assigned to orthophosphate, representing 37-90% of extractable P. Orthophosphate monoesters comprised the next largest pool of extractable P (7-55%). The most prominent resonances were due to phytate, which comprised up to 9% of total NaOH-EDTA extractable P, and alpha- and beta-glycerophosphate, which comprised 1-5% of total NaOH-EDTA extractable P. A substantially greater portion of organic P (2-39% of total NaOH-EDTA extractable P) appeared as a broad peak in the monoester P region; we propose that this is due to P found in large, 'humic' molecules. Orthophosphate diesters (1-5% of total NaOH-EDTA extractable P) and pyrophosphate (1-5% of total NaOH-EDTA extractable P) were minor components of P in all soil extracts. These results suggest that organic P in large humic molecules represents the second most abundant form of NaOH-EDTA extractable soil P (behind orthophosphate). Furthermore, small P-containing compounds, such as phytate, represent a much smaller proportion of soil P than is commonly assumed.