Spectral sensitivity of solution 31P NMR spectroscopy is improved by narrowing the soil to solution ratio to 1:4 for pasture soils of low organic P content

•Organic P speciation using NMR can be hindered due to poor spectral sensitivity.•Metal and P content extracted at two ratios (1:4 and 1:10) were highly correlated.•Spectral sensitivity was considerably better at the 1:4 ratio than at 1:10.•Concentrations of organic P at both ratios are quantitatively comparable.

Solution phosphorus (P) nuclear magnetic resonance (NMR) spectroscopy following extraction with sodium hydroxide–ethylenediaminetetraacetic acid (NaOH–EDTA) is the most powerful and widely used technique for characterising soil organic P. However, poor spectral sensitivity (related to the signal to noise ratio) can limit its applicability for soils of low organic P content, especially in subsurface layers. Sensitivity can potentially be improved by employing a much narrower (1:4) soil to solution ratio than usual (1:10 or 1:20), which increases the density of P in the NMR tube. Pasture soils were collected at two depths (0–4 cm and 4–10 cm) from across the high rainfall zone of eastern Australia, and were extracted with NaOH–EDTA at a 1:4 and 1:10 extraction ratio. The extracts were analysed for molybdate-reactive P (i.e. an estimate of inorganic P), and total extractable concentrations of aluminium (Al), iron (Fe), magnesium (Mg), manganese (Mn) and P using inductively coupled plasma optical emission spectroscopy (ICP-OES). Total concentrations of P in soil were determined by laboratory X-ray fluorescence. Extraction efficiency of total soil P was on average only slightly lower for the 1:4 extracts (41%) than for the 1:10 extracts (47%). Extraction efficiencies for paramagnetic (Fe, Mn) and non-paramagnetic (Al, Mg) metals were also similar at the two extraction ratios. Solution 31P NMR spectra of 1:4 extracts were considerably better in terms of spectral sensitivity than those of the 1:10 extracts, particularly for soils of low organic P content. This study demonstrates that in soils of low organic P content, NaOH–EDTA extraction using a 1:4 soil to solution ratio is a viable alternative to the more commonly used 1:10 soil to solution ratio.