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

Abstract

SummaryIn 1991, a 3.6 ha plot was established to determine the effects of urea fertigation via drip emitters on growth responses and yield of young `French' prune trees. Urea N (UN32) was applied annually beginning in 1992 and continuing through 1996 at 0 (control), 0.11, 0.23, and 0.45 kg actual N per tree as biweekly applications with one-tenth of the total amount per application 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. Fruit yield, number of fruit per kilogram, drying ratio (fresh fruit mass per dry 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 compared with the control in 1994 and 1996 and when averaged over three years. 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 bacterial canker, while in plots where 0.11 or more kg N per tree was applied, virtually no bacterial canker symptoms appeared. Use of leaf analysis to indicate need and timing of N application improved tree performance, but annual rates 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 lower rates. Furthermore, tree growth during establishment was not accelerated by the 0.45 kg rate and the risk of nitrate leaching was increased. Based on these results, annual fertigation of 2–5 year old `French' prune trees with 0.11 to 0.23 kg N per tree is a sufficient amount of N to provide for good fruit yields, tree growth and orchard health, and minimize N leaching into groundwater.