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Published in Soil Sci Soc Am J 34:735-740 (1970)
© 1970 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Crystalline Phosphates Produced By Interaction of Orthophosphate Fertilizers With Slightly Acid and Alkaline Soils1

L. C. Bell and C. A. Black2

ABSTRACT

Petrographic and X-ray diffraction methods were used to identify the phosphates formed when Ca(H2PO4)2 · H2O, NH4H2PO4, and (NH4)2HPO4 were allowed to move individually into columns of slightly acid and alkaline soils from a layer of the solid phosphate salt placed at the bottom. The soil columns were examined after 4, 16, and 48 weeks of incubation. With Ca(H2PO4)2 · H2O as the fertilizer, the dominant phosphate identified in all soils was CaHPO4 · 2H2O. Minor amounts of CaHPO4 were found in the soils within 2 mm of the fertilizer. With NH4H2PO4 as the fertilizer, CaHPO4 · 2H2O was again the dominant phosphate identified in all soils, the amount increasing with the amount of calcium present initially in exchangeable and carbonate forms. No CaHPO4 was found. In one soil containing 12 meq of exchangeable magnesium per 100 g, the CaHPO4 · 2H2O was accompanied by MgNH4PO4 · 6H2O, which, with time, dissolved to form Mg3(PO4)2 · 22H2O. With (NH4)2HPO4 as the fertilizer, the amounts of crystalline phosphates found were smaller, but the number of different phosphates was larger than with the other fertilizers. Phosphates identified were Ca(NH4)2(HPO4)2 · H2O (dimorph B), Ca8H2(PO4)6 · 5H2O, CaHPO4 · 2H2O, and MgNH4PO4 · 6H2O, but no one phosphate was identified in all soils. CaHPO4 · 2H2O was the dominant phosphate in high-calcium soils (except for a highly calcareous soil in which only Ca8H2(PO4)6 · 5H2O was identified), but was present only in traces in low-calcium soils. Ca(NH4)2(HPO4)2 · H2O (dimorph B) was identified in most soils at 4 weeks; it disappeared with time, leaving a residue of CaHPO4 · 2H2O. MgNH4PO4 · 6H2O occurred at 4 weeks in a soil high in exchangeable magnesium; only a trace remained at 48 weeks. Ca8H2(PO4)6 · 5H2O occurred as an initial product in a soil containing 22% CaCO3 equivalent and as a product of alteration of CaHPO4 · 2H2O in a number of soils with pH values of 6.3 or higher. Once formed, it seemed stable with time.

NOTES

1 Journal Paper no. J-6446 of the Iowa Agr. & Home Econ. Exp. Sta., Ames, Iowa. Project no. 1183.

2 Former Graduate Assistant and Professor. The senior author is now a Queen Elizabeth Fellow, Dept. of Soil Science and Plant Nutrition, Univ. of Western Australia, Nedlands, Western Australia.

Received for publication December 22, 1969. Accepted for publication May 13, 1970.






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