Phosphorus solubility and solid - state speciation in fertilizer bands applied to calcareous soil systems
Manimel Wadu, Mihiri Chathurika Wilasini
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Precipitation reactions of phosphorus (P) with Ca decrease the P fertilizer efficiency in calcareous soils. The hypothesis was that anion competition of sulphate with phosphate to precipitate with Ca will increase P solubility in calcareous soils. Initial experiments were conducted to investigate the effect of co-application of K2SO4, (NH4)2SO4, MgSO4 and (NH4)2CO3 on the solubility of monopotassium phosphate (MPP) and monoammonium phosphate (MAP) in different Manitoba soils. Co-application of sulphate salts enhanced P solubility by a factor that ranged from 6% to 44% in soils with greater exchangeable Ca and smaller acid-extractable Ca content (i.e. Osborne, Red River, Balmoral and St. Claude soil series). Solubility and solid-state speciation of P were investigated when MPP was banded with sulphate salts in both model and natural (Dezwood Loam soil series, DL) calcareous soils. The results of S K-edge X-ray absorption near edge structure spectroscopy (XANES) showed that CaSO4•2H2O was formed in DL soil when either K2SO4, (NH4)2SO4 or MgSO4 was applied with MPP. Phosphorus-31 magic-angle spinning nuclear magnetic resonance (MAS NMR) analysis showed that dicalcium phosphate dihydrate (DCPD) was formed in both soils, but the relative proportion of DCPD decreased in the presence of sulphate salts due to the formation of a new P compound. The results also showed that decreasing Ca saturation on the cation exchange complex increased the solubility and mobility of MAP in model calcareous soils and Ca played a more dominant role in forming P precipitates than Mg. According to 31P MAS NMR results, DCPD was the dominant P species in soils with 100% to 50% Ca saturation on the cation exchange complex while poorly ordered magnesium phosphate trihydrate was the prominent P phase with 70% to 100% Mg saturation. This study has demonstrated that the combination of solid state NMR and XANES provided complimentary information concerning anion competition and changes in P speciation due to the co-application of sulphate salts in fertilizer bands. Application of sulphate salts is a promising agronomic practice to enhance P solubility in soils with greater exchangeable Ca content due to the anion competition by sulphates with phosphate to precipitate with Ca.