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Chemical and Physical Properties of Iron and Aluminum Phosphates and Their Relation to Phosphorus Availability¹

ABSTRACT

Synthetic, colloidal ferric and aluminum phosphates were crystallized in aqueous medium to form variscite, AlPO₄ · 2H₂O and strengite, FePO₄ · 2H₂O. The compounds were identified and characterized by X-ray, infrared analysis, and electron microscopy. Iron phosphate crystallized at a much faster rate than aluminum phosphate under the same temperature and pressure conditions. Variscite and strengite which were crystallized at 105C for 40 days had specific surface areas of 24.9 m²g^-1 and 1.98 m²g^-1, respectively. The strengite consisted of single spherical crystals with an average diameter of 6 microns and the variscite consisted of reniform crystalline aggregates with an average diameter of 1.5 micron. In quartz sand cultures, with sudan-grass (Sorghum vulgare sudanese) as the indicator crop, the relative availability of P from the synthetic compounds was: strengite < variscite <<< colloidal Fe-P colloidal Al-P.

¹ Published with the approval of the Director of the Michigan Agr. Exp. Sta. as Journal Article no. 4099. Presented before Div. S-2, Soil Science Society of America, Aug. 22–25, 1966 at Stillwater, Okla.

² Formerly Graduate Research Assistant (presently Post Doctoral Research Associate, Agronomy Department, Purdue University, Lafayette, Ind.) and Associate Professor, Department of Soil Science, Michigan State University, East Lansing, respectively.

Received for publication January 26, 1967. Accepted for publication November 2, 1967.