PHOSPHORUS SORPTION AS INFLUENCED BY ADDED UREA IN 2 EASTERN CANADIANSOILS

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 easter n Canadian soils (Typic Humaquepts). Phosphorus sorption was studied b y equilibrating soil samples pretreated with four levels of urea for 2 4 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 mic rosites 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 so rption characteristics, depending on soil and electrolyte. Phosphorus sorption increased with increasing urea and p;H when using CaCl2 as th e electrolyte. Added urea and pH had little effect when KCI was used, indicating that the urea effect on P sorption was influenced by Ca con tent. 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 compou nds and the shift from H2PO4- to HPO42- at higher pH values. Urea appl ication increased 0.5 M NaHCO3 extractable P and reduced nonextractabl e P in both soils. For soils with high Ca content, urea application wi th P fertilizer could reduce P concentration in soil solution and P mo bility because of the increase in P sorption and P buffer capacity, bu t increase available P as expressed by NaHCO3-extractable P.