Communications in Soil Science and Plant Analysis, 46:1034–1043, 2015 Copyright © Taylor & Francis Group, LLC ISSN: 0010-3624 print / 1532-2416 online DOI: 10.1080/00103624.2015.1019081 Improving Sensitivity of Solution 31 P NMR Analysis in Australian Xeralfs MELINDA R. S. MOATA, 1,3 RONALD J. SMERNIK, 1 ASHLEA L. DOOLETTE, 1 ANN M. MCNEILL, 1 AND LYNNE M. MACDONALD 1,2 1 Soils Group, School of Agriculture, Food and Wine and Waite Research Institute, University of Adelaide, Urrbrae, Australia 2 Commonwealth Scientific and Industrial Research Organisation, Agriculture Flagship, Glen Osmond, Australia 3 Department of Dryland Management, Kupang State Polytechnic for Agriculture, Kupang, Indonesia Phosphorus-31 nuclear magnetic resonance ( 31 P NMR) spectroscopy is widely used to identify and quantify phosphorus (P) forms in soil. This study aimed to determine whether narrowing the soil to extractant sodium hydroxide–ethylenediaminetetraacetic acid (NaOH-EDTA) ratio from 1:20 to values as low as 1:4 would improve sensitiv- ity of solution 31 P NMR spectroscopy without degrading resolution or quantitation. Four Australian soils were tested using four ratios. The narrowest ratio of 1:4 gave the best quality NMR spectra in terms of signal-to-noise ratio. Peak resolution was not degraded on narrowing the ratio. There was no clear effect of narrowing the extraction ratio on extraction efficiency or the distribution of signal among chemical shift regions (orthophosphate, monoester P, diester P, and pyrophosphate). We conclude that a ratio of 1:4 improved NMR analysis for these particular soils and should be considered for other soils, particularly low-P soils, where NMR sensitivity is limiting. Keywords 31 P nuclear magnetic resonance spectroscopy, soil phosphorus, soil to extractant ratio Introduction Solution phosphorus-31 nuclear magnetic resonance ( 31 P NMR) spectroscopic analysis of sodium hydroxide–ethylenediaminetetraacetic acid (NaOH-EDTA) extracts has become the most widely used method to identify and quantify phosphorus (P) forms in soil (Cade-Menun 2005; Doolette and Smernik 2011). When NaOH-EDTA extraction was first introduced by Bowman and Moir (1993), a soil/extractant ratio of 1:50 was used. It should be noted that this extraction was not originally designed for NMR analysis but as an improvement on NaOH alone as a strong extractant of soil P. Subsequently, Cade- Menun and Preston (1996) reported that extraction of an organic soil (forest litter layer) with NaOH-EDTA at a 1:20 ratio resulted in better P extraction and a greater diversity of P forms detected by subsequent solution 31 P NMR analysis than other extraction protocols Received 14 February 2014; accepted 3 November 2014. Address correspondence to Melinda R. S. Moata, Davies Building, Waite Campus, Glen Osmond, SA 5064, Australia. E-mail: melinda.moata@adelaide.edu.au 1034