From Nitrogen Form to Field Performance: A Conceptual Path to Sustainable Nutrient Management in Citrus Production

Background and aims Sustainable citrus production requires nitrogen (N) management strategies that account for soil–plant nutrient interactions. Because N form alters rhizosphere pH and soil solution chemistry, it regulates multi-nutrient availability, particularly calcium (Ca), with consequences for crop performance. This study evaluated how nitrate–ammonium ratios, with or without foliar Ca application, affect soil chemistry, plant nutrition, fruit yield, and nitrogen use efficiency (NUE) in a tropical sweet-orange orchard. Methods A split-plot field experiment was conducted from 2020 to 2023. Subplots received 220 kg N ha⁻¹ yr⁻¹ as calcium nitrate (CN; 100% NO₃⁻), a nitrate-dominant mixture (CN + AN; 75% NO₃⁻:25% NH₄⁺), or ammonium nitrate (AN; 50% NO₃⁻:50% NH₄⁺). Main plots received foliar Ca or no foliar Ca. Soil nutrient availability, leaf nutrient concentrations, fruit yield, and NUE were evaluated. Results Nitrate-dominant fertigation increased soil pH and Ca availability and raised leaf Ca concentration by ~ 15% relative to AN. These changes increased fruit yield by ~ 20% (~ 10 t ha⁻¹) and NUE by ~ 30% across three harvests. Fruit yield correlated positively with leaf Ca concentration and the leaf Ca:N ratio. Foliar Ca application did not enhance leaf Ca concentration or fruit yield, and the additional N supplied via Ca(NO₃)₂ reduced NUE. Conclusion Nitrogen form governed soil–plant nutrient balance and field performance in tropical citrus. Nitrate-based fertigation improved Ca nutrition, fruit yield, and NUE without reliance on foliar Ca inputs. The nitrate-dominant CN + AN mixture performed similarly to exclusive nitrate fertigation, representing a practical and sustainable nutrient management strategy for citrus production.