A comparison of soil extraction procedures for P-31 NMR spectroscopy

The effect of extractants on phosphorus determination by P-31 NMR spectroscopy was examined using five forest door samples, The extractants used were: 0.25 M NaOH, 1:6 soil to Chelex in water, 1:6 soil to Chelex in 0.25 M NaOH, and a 1:1 mix of 0.5 M NaOH and 0.1 M EDTA, The broadest peaks were produced by the NaOH + EDTA extraction, However, NaOH + EDTA extracts contained the highest percentage of total phosphorus and the greatest diversity of P forms, These extracts were the only ones to show peaks for polyphosphates. Metals analysis indicated that NaOH + EDTA maintained Mn in solution, which seemed to be responsible for the line broadening, The sharpest peaks, with the best separation, were produced with Chelex + NaOH, and these were improved further by increasing the pH with NaOH prior to NMR analysis, Chelex + NaOH extracted 23 to 35% of the total soil P, Chelex in water extracted 10 to 13%, NaOH alone extracted 22 to 34%, and NaOH + EDTA extracted 71 to 90%. This work suggests that, because the extractant used will affect the P forms, care must be taken when interpreting studies of P cycling in soils using P-31 NMR spectroscopy and when comparing studies using different extractants.