Efficient Use of Nitrogen and Water in High-density Apple Orchards

Abstract

In irrigated apple orchard systems, the magnitude and timing of plant demand for nitrogen (N) and retention of N in the root zone to allow root interception are important factors for efficient management of N fertilizer. Results from five experiments in high-density plantings of apple (Malus domestica) on dwarfing (`Malling 9') rootstocks are reported. All experimental plots received daily drip irrigation and N applied through the irrigation system (fertigation) with different regimes according to experimental design. Labelled fertilizer applications, whole tree excavation and partitioning and removal of N in fruit and senescent leaves were used to assess tree N demand. Nitrogen requirements ranged from 8 to 40 lb/acre (8.8 to 44 kg·ha-1) over the first 6 years after planting and N use efficiency was often low (<30%), likely because supply exceeded demand. Annual growth is supported by N remobilized from storage and taken up by roots. Root uptake of labelled fertilizer was negligible during early spring and the commencement of rapid uptake was associated with the end of remobilization and the start of shoot growth, rendering prebloom fertilizer applications ineffective. Thus timing of N supply to periods of high demand is crucial for improving efficiency. Comparisons were made to determine the effects on N leaching and tree N utilization of irrigation scheduled to meet evaporative demand and irrigation applied at a fixed rate. Water losses beneath the root zone were greater for fixed rate than scheduled irrigation during the coolest months (May, June and September) of irrigation application. Nitrogen leaching followed a similar pattern during times of N fertigation (May and June). Greater N use efficiency was also measured for trees when irrigation was scheduled to meet evaporative demand rather than applied at a fixed rate. The most N efficient management system was for trees receiving a low [50 ppm (mg·L-1)] fertigated N supply, at 0 to 4 or 4 to 8 weeks following bloom with scheduled irrigation.