UREA AND PHOSPHATE INTERACTIONS IN FERTILIZER MICROSITES - AMMONIA VOLATILIZATION AND PH CHANGES

Ammonia volatilization from urea fertilizer reduces N fertilizer effic iency 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 hy drolysis, and pH changes with surface-applied urea. Surface soil sampl es from two Typic Humaquepts, a Ste Rosalie clay (very-fine-silty, mix ed, 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 vol atilization increased on both soils as application rates increased fro m 0.5 to 2.0 g N kg -1 soil. Adding TSP and MAP to urea reduced NH3 lo ss from 30 to 90% on both soils compared with urea alone. Ammonia loss decreased as P/urea ratios increased. No significant difference was f ound in NH3 loss between TSP and MAP. Ammonia loss from urea was incre ased with added DAP, because of a resulting high pH. Adding TSP or 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 fertili zer microsite and retarding urea hydrolysis. Mixtures of acidic P fert ilizers with urea increased soil NH4 and NO3 contents. Surface-applied urea fertilizer efficiency could be increased if applied together wit h TSP or MAP. Fertilizer granules of urea-P mixtures would be benefici al for hay and pasture application, no-till fertilization, or crops wh ere applicatons are restricted to soil surfaces.