Washington navel orange trees grown in clay loam soil were fertilized with
urea-formaldehyde (UF) as a slow release nitrogen fertilizer during two successive
seasons 2002 and 2003 in a private orchard located in Banha, Kalubia governorate,
Egypt. Trees were fertilized with urea at 1000 gm N/tree divided into three equal
doses as control treatment, while urea-formaldehyde was used at three doses 1000,
750 and 500 gm N/tree each of them added either as one application in March or
divided into two equal applications added in March and June. Results indicated that N, P and K contents in the leaves did not affect significantly by
different treatments. In this respect, UF treatments enhanced nitrogen percentage
comparing with urea treatment. So, UF at 1000 gm N/plant raised N content in the
leaves comparing with the other treatments. On the other hand, fertilizing trees with
UF at 500 gm N/tree either as one or two applications raised potassium content than
the other treatments. Concerning the effect on yield and fruit quality, the data reveal that all
treatmerts had a significant effect on number of fruits per tree since; using UF at 500
gm N/tree significantly increased this parameter than the other treatments and gave
the higher values especially when added as one application and resulted in highest
yield weight/tree. On the other hand, no significant differences were detected between
treatments on fruit weight, peel thickness, fruit juice content, total soluble solids,
acidity and ascorbic acid in fruit juice, although the higher UF doses specially 1000
and 750 gm N/plant tended to increase peel thickness. Thus, it seems that fertilizing Washington Navel orange trees with urea-formaldehyde
at 500 gm N/tree as one application added in March is the suitable treatment under
conditions of this study.