Characterizing phosphorus forms in cropland soils with solution 31P-NMR: past studies and future research needs

Understanding the forms and dynamics of soil phosphorus (P) is essential to maintain agricultural productivity while minimizing environmental risks. Since it was first used on soil extracts in 1980, P-31-nuclear magnetic resonance spectroscopy (P-NMR) has emerged as the leading technique to characterize extractable soil organic P forms. However, it is still underutilized in agriculture; of the more than 200 soil P-NMR papers published to date, only 44 have been conducted in non-pasture soils used for the production of annual or perennial crops, and only nine of those have linked identified P forms to agronomic parameters such as yield. This paper reviews these prior studies, suggesting gaps in research with respect to cropping systems and geographical regions. In particular, there have been few recent P-NMR studies that have fully identified P forms in African soils, and few studies of permanent crops such as orchards and vineyards. There is a need to link future P-NMR studies of cropping systems to agronomic parameters, and combine P-NMR with other techniques to fully capture P dynamics in cropping systems.