Circular Fertilization Strategy Using Sulphur with Orange Waste Enhances Soil Health and Broccoli Nutritional and Nutraceutical Quality in Mediterranean Systems

Fertilization strategies are pivotal in sustainable agriculture, affecting both soil health and crop quality. This study investigated the impact of a circular fertilization approach based on agro-industrial residues—specifically, a blend of sulfur bentonite and orange processing waste (RecOrgFert PLUS)—on soil physicochemical and biological properties, as well as the nutritional and nutraceutical quality of broccoli (Brassica oleracea var. italica) grown in Mediterranean conditions (Condofuri, Southern Italy). The effects of RecOrgFert PLUS were compared with those of a synthetic NPK fertilizer, an organic fertilizer (horse manure), and an unfertilized control. Results demonstrated that RecOrgFert PLUS significantly improved soil organic carbon (3.37%), microbial biomass carbon (791 μg C g−1), and key enzymatic activities, indicating enhanced soil biological functioning. Broccoli cultivated under RecOrgFert PLUS also exhibited the highest concentrations of health-promoting compounds, including total phenols (48.87 mg GAE g−1), vitamin C (51.93 mg ASA 100 g−1), and total proteins (82.45 mg BSA g−1). This work provides novel evidence that combining elemental sulphur with orange processing waste not only restores soil fertility but also boosts the nutraceutical and nutritional value of food crops. Unlike previous studies focusing on soil or plant yield alone, this study uniquely integrates soil health indicators with bioactive compound accumulation in broccoli, highlighting the potential of circular bio-based fertilization in functional food production and Mediterranean agroecosystem sustainability.