Soil Mineralogy and Applied Phosphate Transformation in a Toposequence of a Tropical Semi-arid Landscape: Montgomeryite [Ca₂Al₂(PO₄)₃(OH).7H₂O] as the Possible Metastable Phosphate Transformation Residue

ABSTRACT

The residual responses of crops to phosphate fertilizers in weathered soils depend on the mineralogy and the nature of the phosphate reaction residue formed by soil–phosphate interactions. The study examined the metastable phosphate residues formed from soil-phosphate interactions along a tropical semi-arid toposequence by solubility criteria principles after soil-phosphate incubation for 28 weeks. The powder X-ray diffraction of the sand and silt-sized particles showed the dominance of K-feldspars in association with quartz and muscovite, while the oriented clay fractions indicated a preponderance of kaolinite and illite derived from the weathering of K-feldspars that were the surviving primary silicates in the soils. Soil-phosphate incubation with 12.5 mmol P kg⁻¹ for 28 weeks showed, by the 7^th week, phosphate, ${\mathrm{H}\mathrm{P}\mathrm{O}}_4^{2-}$, activities in apparent equilibrium with montgomeryite, Ca₂Al₂(PO₄)₃(OH).7(H₂O), solubility at pH ≥ 6.0. After 28 weeks, the HPO₄^2- activities were in apparent equilibrium with hydroxyapatite, Ca₅(PO₄)₃OH at the pH range of 6.5 to 7.2. The study provides an indirect evidence of the existence of montgomeryite as the potential metastable phosphate residue of soil-phosphate reactions in these slightly acid tropical semi-arid soils, before reverting to the more stable hydroxyapatite with time.

KEYWORDS:

• Hydroxyapatite
• ion-activity-products
• mineralogy montgomeryite
• solubility

Acknowledgments

The author gratefully acknowledges the financial support provided by the Canadian International Development Agency (CIDA) that facilitated a field trip as a doctoral student at the University of Saskatchewan, Saskatoon, Canada.

Data Availability

The data for this paper are contained as supplementary data of a PhD thesis submitted to the University of Saskatchewan, Saskatoon, Canada.

Author’s Declaration

The author has no competing financial interest and relationships that could have influenced this submission directly or indirectly; and the paper was checked through plagiarism and grammar before submission.

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This article has been republished with minor changes. These changes do not impact the academic content of the article.

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Funding

The author gratefully acknowledges the financial support provided by the Canadian International Development Agency (CIDA) that facilitated a field trip to Brazil for this study.