Impacts of Conventional and Agri-Food Waste-Derived Fertilizers on Durum Wheat Yield, Grain Quality, and Soil Health: A Two-Year Field Study in Greece and Southern Italy

Sustainable fertilization strategies are critical under climate change and the European Green Deal, particularly for Mediterranean cereal systems. Organic fertilizers derived from agro-industrial residues offer promising alternatives to conventional mineral inputs. This study evaluated RecOrgFert, a novel fertilizer composed of sulfur–bentonite and citrus-processing residues, in comparison with NPK (15-15-15) and horse manure across two years in Central Macedonia (Greece) and Apulia (Italy). Using a randomized complete block design, soil chemical and biological properties, plant growth, yield, and grain quality were assessed. RecOrgFert outperformed conventional fertilizers by enhancing soil fertility—raising organic matter 25–27% above control and further increasing it from 2023 to 2024 (up to +75% in Italy, +38% in Greece)—while improving cation exchange capacity, enzymatic activity, and soil water content. Wheat grown with RecOrgFert showed higher protein (up to 15.2%), antioxidant activity (DPPH > 37%, ABTS+ > 26%), and phenolic and flavonoid content, with yields comparable to NPK. The unique sulfur and orange-residue composition distinguish RecOrgFert from standard fertilizers, promoting nutrient cycling, microbial activity, and bioactive compound accumulation. It represents a novel, circular, and climate-smart solution aligned with EU sustainability and circular economy objectives.