Solid-state nuclear magnetic resonance at low-field as an approach for fertiliser dissolution monitoring

The dissolution of fertilisers is the initial process that takes place in soils following fertiliser application and influences the fate and effectiveness of fertilisers. Currently, there are only a few methods for studying fertiliser dissolution in soil. These approaches typically do not accurately represent real soil-fertiliser systems and are susceptible to errors, since they are influenced by processes associated with the loss or retention of the trace ions of the fertiliser. Low field NMR or time-domain NMR (¹H-TDNMR) is typically employed for studying ¹H in fluids (or mobile ¹H), however, special pulse sequences enable the selective detection of ¹H in solids. Furthermore, it is possible to filter out undesired signals like ¹H from minerals and from soil organic matter. This allows for the detection and monitoring of ¹H only from protonated fertilisers (e.g., ammonia, (di)-hydrogen phosphates, etc.). The aim of this study is to present an efficient procedure which monitors the dissolution of fertilisers in soils using ¹H-TDNMR. For this, six contrasting New Zealand soils and four protonated fertilisers - NH₄Cl, NH₄NO₃, NaH₂PO₄.H₂O, and (NH₄)₂HPO₄ - were utilised. The proposed method efficiently, accurately, and precisely, monitored the dissolution of the studied fertilisers in all the tested soils under different rain regimes, from violent rain (60 mm h^− 1) to light rain (2 mm h^− 1) with a time interval (temporal resolution) as short as 5 s.