Effects of blended NPS and urea fertilizer rates on multi-year vegetative growth of apple trees in the Ethiopian highlands

In the nutrient-scarce Ethiopian highlands, proper fertilization is crucial for apple tree growth. A four-year (2019–2022) field trial at the Holeta Agricultural Research Centre, employing a completely randomized complete block design (RCBD), aimed to determine the optimal blended nitrogen, phosphorus, and sulfur (NPS) and urea fertilizers to maximize vegetative growth in young apple trees. Treatment, 250 kg NPS ha⁻¹ + 225 kg urea ha⁻¹, significantly outperformed all other treatments in overall canopy development (P < 0.001). Compared to the Satellite Check, this treatment resulted in a 2.26 m (+ 14.1%) increase in tree height and a 1.16 m3 (+ 96.6%) increase in canopy volume, confirming the important synergistic role of phosphorus (P) and sulfur (S) in promoting vegetative growth. Crucially, the highest trunk cross-sectional area (TCSA) (16.52 cm²) was observed under the nutrient restriction treatment (0 kg NPS + 200 kg urea ha⁻¹). This finding is significant, revealing a physiological trade-off: under P/S fixation ratio constraints, carbon allocation shifts towards persistent lignocellulosic biomass (TCSA), while optimal nutrient distribution promotes canopy expansion (volume). Therefore, in phosphorus-fixing Ethiopian highland soils, the application of 250 kg NPS ha⁻¹ + 225 kg urea ha⁻¹ was determined to be the optimal combination for maximizing the productivity of the canopy.