Does Partial Organic Fertilization Maintain Physiological and Biometric Performance in Apple Trees?

The MOPLUS project, funded by the Portuguese Recovery and Resilience Plan (PRR), aims to enhance soil organic matter, soil structure, and water retention in apple orchards located in the “Maçã de Alcobaça” Protected Geographical Indication area through organic fertilization based on locally available livestock effluents, thereby reducing reliance on synthetic fertilizers under Mediterranean climatic conditions. This study evaluated the physiological and biometric responses of apple trees subjected to four fertilization strategies (M1–M4) in three commercial ‘Gala’ orchards in central Portugal over three growing seasons (2023–2025). Measurements included leaf functional traits, gas exchange, chlorophyll fluorescence, spectral indices, vegetative growth, fruit production per tree and mean fruit weight. Interannual climatic variability and orchard-specific conditions were the dominant drivers of tree response, while fertilization effects were smaller and mainly expressed through interactions with year and orchard. When analyzed within the same orchard, fertilization strategies M2 and particularly M3 maintained physiological performance, vegetative growth, and fruit production per tree at levels comparable to full mineral fertilization. Among treatments, M3 showed the most consistent responses across sites and years, indicating that partial mineral substitution with pig slurry can sustain tree functioning while maintaining or enhancing fruit production per tree. The most restrictive strategy (M4) occasionally showed reduced photosynthetic performance under specific orchard–year combinations, suggesting a threshold effect associated with stronger mineral reduction, but without evidence of generalized physiological stress. Overall, these findings demonstrate that partial substitution of mineral fertilizers by organic amendments—especially pig slurry (M3) and, to a lesser extent, composted cattle manure (M2)—is agronomically viable, allowing apple tree performance and productivity to be maintained while enhancing system resilience under Mediterranean climatic variability. These results also provide practical decision support for site-adapted fertilization management in commercial drip-irrigated apple orchards, supporting reduced mineral fertilizer dependence without compromising productivity.