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Urea and Phosphate Interactions in Fertilizer Microsites: Ammonia Volatilization and pH Changes

Department of Renewable Resources, McGill Univ., Macdonald Campus, 21111 Lakeshore Road, Ste Anne De Bellevue, Quebec, Canada H9X 3V9

*Corresponding author.

ABSTRACT

Ammonia volatilization from urea fertilizer reduces N fertilizer efficiency by crops. Reduction of NH₃ loss may be possible through addition of acidic materials. The objectives of this study were to compare the effects of three phosphate fertilizers on NH₃ volatilization, urea hydrolysis, and pH changes with surface-applied urea. Surface soil samples from two Typic Humaquepts, a Ste Rosalie clay (very-fine-silty, mixed, acid, frigid) and an Ormstown silty clay loam (fine-silty, mixed, nonacid, frigid) were used. Mixtures of two rates of urea, four rates of P, and three P fertilizers (triple superphosphate, TSP, monammonium phosphate, MAP; diammonium phosphate, DAP) were compared. Ammonia volatilization increased on both soils as application rates increased from 0.5 to 2.0 g N kg^–1 soil. Adding TSP and MAP to urea reduced NH₃ loss from 30 to 90% on both soils compared with urea alone. Ammonia loss decreased as P/urea ratios increased. No significant difference was found in NH₃ loss between TSP and MAP. Ammonia loss from urea was increased with added DAP, because of a resulting high pH. Adding TSP and MAP to urea reduced maximum daily rates of NH₃ loss and delayed the time of maximum NH₃ loss rate by 5 to 10 d. The effect of acidic phosphates on NH₃ loss was related to their effect in reducing pH in the fertilizer microsite and retarding urea hydrolysis. Mixtures of acidic P fertilizers with urea increased soil NH₄ and NO₃ contents. Surface-applied urea fertilizer efficiency could be increased if applied together with TSP or MAP. Fertilizer granules of urea-P mixtures would be beneficial for hay and pasture application, no-till fertilization, or crops where applications are restricted to soil surfaces.

Received for publication April 28, 1992.

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