Improving nitrogen uptake by fertigation in European 'Conference'

The standard nitrogen (N) application in Belgian ‘Conference’ pear orchards is to scatter calcium nitrate granules on the herbicide strip under the trees. However, this N fertilization does not guarantee a sufficient N content in the fruits (minimum 50 mg 100 g^‑1 fresh weight) to allow prolonged storage and good shelf life. In 2018, a fertilization trial started in which the standard N application was compared with N fertigation in an attempt to improve the nitrogen uptake by the tree and to increase the N content in the fruits. Apart from the untreated control (no N fertilization) and the standard N soil application (40 kg N ha^‑1 in spring followed by 20 kg N ha^‑1 in summer), three different fertigation treatments were tested: the standard dose (40 kg N ha^‑1 in spring and 20 kg N ha^‑1 in summer), a reduced N application (-25%; 30 kg N ha^‑1 in spring and 15 kg N ha^‑1 in summer) and a split application throughout the season (6×10 kg N ha^‑1). In each of the three years of study, the mean N content of the fruit in the standard N soil treatment was comparable to that of the different N fertigation treatments. However, in two out of three years there was more variability in the N content of the fruits of this standard treatment compared to that of the N fertigation treatments demonstrating a more homogeneous fruit quality using fertigation. The ¹⁵N isotope was used in the fertilizer to determine the N use efficiency (FNUE) for summer applications in pear fruit. Only a small fraction (<5%) of current summer application reaches the fruit in the year of application, while 5-12% is detected in the fruit of the following year. Differences between standard soil N application versus N fertigation on N levels of the leaves, the fruits and the soil are discussed.