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Phosphorus Sorption as Influenced by Added Urea in Two Eastern Canadian Soils

Dep. of Renewable Resources, Macdonald Campus of McGill Univ., 21,111 Lakeshore Road, Ste Anne de Bellevue, PQ, Canada H9X 3V9

*Corresponding author.

ABSTRACT

The interaction of urea with P in soil-fertilizer microsites may have an important effect on P availability to crops. A laboratory study was conducted to evaluate the effect of urea on P reactions in two eastern Canadian soils (Typic Humaquepts). Phosphorus sorption was studied by equilihrating soil samples pretreated with four levels of urea for 24 h with six rates of P in either 0.01 M CaCl₂ or 0.03 M KCl solutions. Phosphorus desorption was determined by sequentially extracting the residual soil with 0.01 M CaCl₂ or 0.03 M KCl, and 0.5 M NaHCO₃. Urea application increased the soil pH (0.01 M CaCl₂) in the fertilizer microsites from 5.2 to 7.3 in the Ste Rosalie soil and from 5.9 to 7.4 in the Ormstown soil. These pH changes had variable effects on soil P sorption characteristics, depending on soil and electrolyte. Phosphorus sorption increased with increasing urea and pH when using CaCl₂ as the electrolyte. Added urea and pH had little effect when KCl was used, indicating that the urea effect on P sorption was influenced by Ca content. The effect of urea was not significant at P rates <320 mg P kg^–1 soil. The observed increase in P sorption and decrease in solution P with added urea was probably related to precipitation of Ca-P compounds and the shift from H₂PO^–₄ to HPO^2–₄ at higher pH values. Urea application increased 0.5 M NaHCO₃ extractable P and reduced nonextractable P in both soils. For soils with high Ca content, urea application with P fertilizer could reduce P concentration in soil solution and P mobility because of the increase in P sorption and P buffer capacity, but increase available P as expressed by NaHCO₃-extractable P.

Received for publication February 21, 1992.