From Marine and Agricultural Waste to Soil Health: Optimized Co-Composting of Sardine By-Products and Tomato Plant Biomass for Sustainable Agriculture and Environmental Protection

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Agriculture and maritime fishing, two critical international sectors, generate vast quantities of organic by-products and waste, presenting substantial environmental challenges and clashing with principles of sustainability. In the Souss Massa region of Morocco, vegetable cultivation covers approximately 22,300 ha, including 6,860 ha of tomato crops, while the fishing industry (canned, semi-canned, and frozen) produces 40–60% of by-products (e.g., heads, viscera, bones) that are unfit for human consumption. Despite their nutrient richness, these by-products are often underutilized, discarded, or incinerated, creating a compelling need for innovative waste valorization strategies and sustainable local waste management practices. This challenge was addressed through an evaluation of the co-composting of fish by-products with agricultural plant biomass, a promising approach to recycling organic waste into high-value fertilizers and reducing dependency on chemical fertilizers. Conducted in the agriculturally and fishing-intensive Souss Massa region, the study was designed to emphasize circular economy principles through targeted local waste management and soil fertility enhancement. A detailed physicochemical, biological, and statistical analysis was taken for 9 compositions to determine the optimal ratios of substrates necessary for producing high-quality compost. It was found that co-composting fish by-products with tomato plant biomass produces high-quality composts enriched with key nutrients, including nitrogen (1.83–4.65%), phosphorus (62.95–224.12 mg/kg), and potassium (3.15–8.86%). Optimal C/N ratios between 15 and 20 were achieved across treatments (F, E, D, and I), meeting essential agronomic standards. Specifically, Treatment F (25% sardine by-products with 75% tomato stems) demonstrated superior fertilization potential and pathogen suppression compared to other combinations or single-source composts. These composts also maintained stable pH levels, further validating their agronomic suitability. Such nutrient profiles indicate the potential for targeted application in vegetable cropping systems, which require high nutrient availability. The stable pH and pathogen reduction further suggest benefits for maintaining soil microbial balance and plant health, especially in intensive agricultural contexts. This approach exemplifies an effective, sustainable alternative to chemical fertilizers, supporting both sustainable agriculture and circular economy practices. Field application trials and long-term impacts on soil health and crop productivity should be assessed in future studies. The results demonstrated the potential of co-composting to enhance soil quality, reduce waste, and promote an environmentally responsible agricultural model.