Characterization of Mixtures of <i>Rugulopteryx okamurae</i> Compost and Plant Residues to Determine the Most Effective Composition as a Substrate and Source of Nutrients

The increasing invasion of <i>Rugulopteryx okamurae</i> algae along the coast of southeastern Spain has prompted efforts to explore its potential as a resource. Consequently, composting this alga is considered a viable solution for agricultural applications. This study investigates the composting process of mixtures of <i>R. okamurae</i> seaweed with plant residues and characterizes the final compost for its use as a substrate and/or source of nutrients to determine the most effective composition of the mixture. The composting process was conducted using varying proportions of seaweed (100%, 30–35%, and 15%) combined with plant residues (from vegetable plants, fruits, and gardens) and included both washed and unwashed seaweed. The first trials revealed challenges associated with <i>Rugulopteryx okamurae</i>, such as a low C/N ratio and algae washing. Consequently, a second trial was conducted to optimize the mixtures, aiming for a C/N ratio close to 30. Additionally, it was decided not to wash the algae to reduce the electrical conductivity (EC) in the mixtures. The findings indicate that the composting process remains unaffected by high electrical conductivity when algae are unwashed. However, washing the algae before composting did affect the compost quality, as the composts with washed algae with garden waste (S_wP 34.0 dS m⁻¹) had a lower electrical conductivity (EC) than did the unwashed composts with garden waste (SP 51.6 dS m⁻¹ and SFP 64.9 dS m⁻¹). On the other hand, the compost-only horticultural and garden waste (FHP 43.7 dS m⁻¹) had a high EC; therefore, the EC was not increased with low proportions of unwashed algae, as was seen with the compost with 15% unwashed algae (SFHP 47.6 dS m⁻¹). The other quality parameters were not affected by the absence of algae washing at the beginning of the composting process.