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

The urgent need to reduce the environmental footprint of agricultural inputs, alongside the rising cost and limited availability of mineral fertilizers, has encouraged the exploration of organic waste materials as alternative nutrient sources. This review examines how organic and nutrient-rich waste streams—from livestock excreta and food industry residues to sewage sludge and combustion by-products—can be converted into effective fertilizers through biological, thermal, and chemical processes, reducing dependence on mineral fertilizers and supporting more circular and resilient cropping systems. It also analyzes the agronomic performance of these waste-derived materials in terms of nutrient release dynamics, impact on soil functions, and crop response under different management conditions. Examples of commercial waste-derived fertilizers are included, together with their effects on soil quality and crop productivity, and regulatory frameworks 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 discussed. 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.