Effects of nitrogen fertigation on fruit yield and quality of young 'French' prune trees

In 1991, a 3.6 ha plot was established to determine the effects of urea fer tigation via drip emitters on growth responses and yield of young 'French' prune trees. Urea N (UN32) was applied annually beginning in 1992 and conti nuing through 1996 at 0 (control), 0.11, 0.23, and 0.45 kg actual N per tre e as biweekly applications with one-tenth of the total amount per applicati on from May until September. An additional annual treatment delivered 0.06 kg N per tree if July analysis of leaf N showed 2.3% or lower total N. Frui t yield, number of fruit per kilogram, drying ratio (fresh fruit mass per d ry fruit mass), tree growth (trunk cross-sectional area and mass of pruned shoots), leaf N concentration, and soil nitrate content were assessed as a function of treatment. Dry yields per tree were increased by N treatments c ompared with the control in 1994 and 1996 and when averaged over three year s. Fruit size and drying ratio were not affected by N treatments during any year. Where no N was applied, there was an increased incidence of bacteria l canker, while in plots where 0.11 or more kg N per tree was applied, virt ually no bacterial canker symptoms appeared. Use of leaf analysis to indica te need and timing of N application improved tree performance, but annual r ates of 0.11 to 0.23 kg N per tree should be applied in several increments before tree leaf N reaches or falls below a level near 2.3%. Nitrogen at 0. 45 kg per tree applied yearly did not improve yield significantly over lowe r rates. Furthermore, tree growth during establishment was not accelerated by the 0.45 kg rate and the risk of nitrate leaching was increased. Based o n these results, annual fertigation of 2-5 year old 'French' prune trees wi th 0.11 to 0.23 kg N per tree is a sufficient amount of N to provide for go od fruit yields, tree growth and orchard health, and minimize N leaching in to groundwater.