An Examination of Spin–Lattice Relaxation Times for Analysis of Soil and Manure Extracts by Liquid State Phosphorus‐31 Nuclear Magnetic Resonance Spectroscopy

Phosphorous (P)‐31 nuclear magnetic resonance (NMR) spectroscopy is used in the analysis of P forms in extracts of soils and manures for environmental and agronomic purposes. Quantitative spectra require knowledge about spin–lattice relaxation times ( T 1 ) to ensure adequate delays between pulses. This paper determined T 1 values of P forms in reconstituted (0.2 g in 0.7 mL −1 ) samples of freeze‐dried 0.25 M NaOH plus 50 m M EDTA extracts of eight diverse soils (Aquept, Dystrochrept × 2, Hapludand, Rendoll, Udand, Haplostoll, and Orthod), three different manures (dairy cattle, deer, and sheep), and one epiphyte moss. Total concentrations in the reconstituted samples ranged from 5 to 175 mg Fe mL −1 , 2 to 62 mg Mn mL −1 , and 72 to 837 mg P mL −1 Values of T 1 for orthophosphate monoesters, orthophosphate diesters, and pyrophosphate varied from 0.42 to 1.69 s in soils and from 0.89 to 2.59 s in manures and the epiphyte. In contrast, T 1 for orthophosphate varied from 0.78 to 1.94 s in soils and 1.45 to 5.82 s in manures and the epiphyte. For quantitative 31 P NMR, delay times should be three to five times the T 1 value, translating to delays of 3 to 5 s for soils and up to 25 s for manures. If the required delay is too long then strategies such as adding paramagnetics could shorten T 1 , provided this does not increase line‐broadening too much. A regression relationship was obtained between orthophosphate T 1 values and the ratio of P concentration to Fe and Mn concentration on a w/v basis ( r 2 = 0.97, P < 0.001), and between the T 1 for all other compound classes and the ratio of P to Fe and Mn ( r 2 = 0.70, P < 0.01). By combining measurement of Fe, Mn, and P in the reconstituted extract and these relationships, T 1 can be estimated and the appropriate delay time used. If T 1 is not considered and the delay time is too short, some peaks will be under‐ or over‐represented and the relative distribution of P forms not quantitative.