Anion Competition By Sulphates in a P Fertilizer Band Investigated Using 31 P MAS NMR, P L – Edge XANES and S K - Edge XANES.

Precipitation reactions of P with Ca decrease the P fertilizer efficiency in calcareous soils. The objective of this study was to determine the effect of applying K₂SO₄, (NH₄)₂SO₄ and MgSO₄ salts with monopotasssium phosphate (MPP). The hypothesis was that the anion competition of sulphate with phosphate for precipitate with Ca will increase P solubility. A laboratory incubation experiment was conducted using a model calcareous soil and Dezwood Loam (DL) soil. Phosphorus - 31 magic-angle spinning nuclear magnetic resonance (³¹P MAS NMR), P – L edge and S – K edge X – ray absorption near edge spectroscopy (XANES) were used to identify the P and S compounds formed in soils. Addition of sulphate salts increased the water-soluble P concentrations in soils compared to the treatment with only MPP (MPP only).  The enhanced P solubility at the site of application of P in both soils was in the order of K₂SO₄ > (NH₄)₂SO₄ > MgSO₄. The results of S – K edge XANES showed that the CaSO₄•2H₂O was formed in the DL soil. Dicalcium phosphate dihydrate (DCPD) was formed in both soils with all treatments as shown by MAS NMR. The relative percentage of DCPD decreased with the presence of sulphate salts due to the formation of a new P compound. The P L – edge XANES spectra of model soil samples did not confirm the formation of DCPD. The P L - edge spectra were affected by low P concentration and the heterogeneity of DL soil. Results of this study confirmed that the addition of sulphate salts increased the P solubility in calcareous not only due to the anion competition but also due to the influence on P precipitation reactions by altering the P reaction products. Application of sulphate salts together with P fertilizer is a promising agronomic strategy to enhance the P solubility in calcareous soils.