Linking Soil Health to Cost-Effective Fertilization Strategies: A Three-Year Assessment of Organic, Organo-Mineral, and Mineral Fertilization in Maize

The overuse of mineral fertilization threatens long-term sustainability of through soil degradation, nutrient imbalances, and rising production costs. Sustainable fertilization strategies based on recycled organic materials offer a potential solution to improve soil functioning while maintaining yield and economic viability. In this three-year field study in a Mediterranean maize agroecosystem, we evaluated the effects of organic, organo-mineral, and mineral fertilization on soil chemical al biological properties, maize productivity, and fertilization efficiency. Treatments included mineral NPK, compost, sewage sludge, struvite, and the combinations of organic materials with struvite, at equivalent total nutrient inputs across treatments. To assess soil health, we measured key chemical and biological parameters, including soil organic carbon (SOC), nutrient availability, microbial biomass, and enzyme activity, which were integrated into a unified Soil Health Index (SHI). Organic and organo-mineral treatments progressively enhanced nutrient availability, microbial biomass, and enzyme activities compared with NPK fertilization, leading to higher soil health values over time, particularly under sludge and compost-based treatments. Maize yield did not differ among fertilization strategies, indicating that soil health gains were achieved without productivity tradeoffs. Further, integrating SHI with agronomic and economic metrics revealed that sludge-based fertilization maximized soil health maintained per unit of yield produced and substantially improved fertilizer cost efficiency relative to NPK. These results demonstrate that circular fertilization strategies can improve soil functioning and resource-use efficiency while sustaining maize production, supporting their adoption as a viable pathway toward more resilient and cost-effective agroecosystems.