A COMPARISON OF SOIL EXTRACTION PROCEDURES FOR 31P NMR SPECTROSCOPY

The effect of extractants on phosphorus determination by 31P NMR spectroscopy was examined using five forest floor 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 o.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 31P NMR spectroscopy and when comparing studies using different extractants.