Waste-Derived Fertilizers: Conversion Technologies, Circular Bioeconomy Perspectives and Agronomic Value

The urgent need to reduce the environmental footprint of agricultural inputs, along-side the rising cost and limited availability of mineral fertilizers, has encouraged the exploration of organic waste materials as alternative nutrient sources. A wide range of residual streams—from livestock excreta and food industry residues to sewage sludge and combustion by-products—contain valuable nutrients that, if properly recovered and transformed, can support more circular and resilient cropping systems. This re-view examines the potential of converting organic wastes into fertilizers through bio-logical, thermal, and chemical processes, highlighting their main principles, outputs, and practical constraints. It also analyzes the agronomic performance of these waste-derived materials in terms of nutrient release dynamics, impact on soil func-tions, and crop response under different management conditions. Regulatory frame-works and commercialization challenges are considered, with emphasis on current European policies and real-world product examples. The broader contribution of these practices to climate mitigation, resource recovery, and rural development is also dis-cussed. While promising, the implementation of waste-based fertilization strategies requires further efforts in standardization, safety assurance, and farmer engagement. This article provides an integrated overview of the topic and identifies key areas for future research, innovation, and policy development in support of sustainable nutrient management.