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This article in SSSAJ

  1. Vol. 57 No. 3, p. 839-845
    Received: Apr 28, 1992

    * Corresponding author(s):
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Urea and Phosphate Interactions in Fertilizer Microsites: Ammonia Volatilization and pH Changes

  1. M. X. Fan and
  2. A. F. Mackenzie 
  1. Department of Renewable Resources, McGill Univ., Macdonald Campus, 21111 Lakeshore Road, Ste Anne De Bellevue, Quebec, Canada H9X 3V9



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.

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