Nitrogen is the most yield-limiting nutrient for maize crop growth and development as in the most soil kinds and it needs effective management to enhance crop production. Adoption of appropriate management may overcome N loss and enhance its use efficiency in crops which are associated with soil and crop management and include appropriate source and application time. Thus, a field experiment was done to determine mineral N fertilizer source and time of application on agronomic parameters and yield of maize in Nitisols of Kersa and Tiro Afeta Districts South-western Ethiopia during 2017/18. The experiment was laid out in randomized complete block design included eight treatments with three replications. The average soil results showed moderately acidic in reaction, sandy clay texture, low in Total N and available P and medium in K, organic matter and cation exchange capacity at both sites. The collected data was subjected to ANOVA using SAS 9.3 version software. Least significant difference test was used to separate means at 5% level of significant. Grain yield, biomass yield and agronomic efficiency were highly affected by time, rate and N source. The highest grain yield (7067.4 kg ha-1 at Kersa and 8178.1 kg ha-1 at Tiro Afeta) and biomass in average (21.5 t ha-1) were obtained from application of 138 kg ha-1 N from US in splits while the lowest grain yield (3251.4 and 3145.4 kg ha-1) and biomass yield (14.75and 11.39 t ha-1) were recorded from control at both sites, respectively. The highest Agronomic Efficiency of N (41.29 kg kg-1) at Kersa and (52.10 kg kg-1) at Tiro Afeta was obtained from plots treated with 46 kg ha-1 N and 138 kg ha-1 N US respectively. Moreover, application of 138 kg ha-1 N from US in splits provided the maximum net benefit of 61701.93 Ethiopian birr ha-1 with marginal rate of return (415.28%) at Kersa and 72198.05 Ethiopian birr ha-1 with marginal rate of return (1795.28%) at Tiro Afeta. Therefore, application of 138 kg ha-1 N from US (treated with urease inhibitor) in splits is recommended for farmers to maximize maize production and reducing N loss under high soil moisture conditions.

Keywords: Maize, Nitrogen, Urea stable.