Identification of specific phosphorus compound groups by NMR (nuclear magnetic resonance) spectroscopy

Abstract

Only a small proportion of soil phosphorus (P) can be utilised readily by plants, and knowledge of the quantity of readily-available P in the soil is necessary for crop management. Current analytical methods for quantifying this P use different chemical reactions to extract P from the soil. The amount of P in the soil, that is of interest, is then defined by the extractant used (e.g. Olsen's P). Whilst good relationships have been established between Olsen P levels and crop yields, the composition of Olsen P is not known. Knowledge of the actual P compounds (e.g. orthophosphate) or compound groups (e.g. monoesters) could enhance our understanding of P dynamics in the soil increasing the possibilities for manipulation to the advantage of crop nutrition.

The aim of this study was to investigate the use of Nuclear Magnetic Resonance (NMR) spectroscopy to identify different P compounds in soil. The objectives were to investigate the number of different P compounds and/or compound groups in three soil types, and their relative proportion as influenced by extraction method (EDTA, CaCl2, Olsen), soil type, initial Olsen P level and fertiliser addition.

EDTA extracted more P in total than the other extractants and six compounds/groups were extracted. Orthophosphate (immediately-available) and monoesters (labile) accounted for the majority of the P extracted, but in one soil type (Cholsey) the proportion of diesters (labile) was significantly greater; this soil had the lowest Olsen P. After fertilisation there was an increase in orthophosphate and monoesters, but there was some variability and there was an increase in orthophosphate in an unfertilised Cholsey soil sample, possibly due to mineralisation. The extractant (CaCl2) is normally associated with representing highly available P forms. NMR identified that this included organic P.

Existing literature was also reviewed relating to P compounds in manures. Orthophosphate was the primary P compound in the manures but there was some indication that poultry manure may have less immediately-available P than dairy manure and the results for pig manure were inconclusive.

This study demonstrated that NMR could be a useful tool to identify specific P compounds or compound groups in manure and soil and so enhance our understanding of what happens to P contained in inorganic and organic fertilisers after application. Identification of different P compounds/groups could assist with explaining why different soil/crop interactions occur in relation to P demand. The study can be used to provide focus for more specific research in relation to crop utilisation of different P compounds/groups, especially organic forms, how these are influenced by soil type, and crop response to fertilisers. The study is potentially a precursor to the development of in-field testing kits.