Posidonia oceanica-Based Growing Media for Tomato Cultivation: Physicochemical Properties and Plant Responses

Soil degradation and peat extraction limitations have increased the demand for sustainable soilless growing media. This study evaluated the agronomic and physiological viability of growing media derived from revalorized Posidonia oceanica and food waste compost mixed 50:50 (%v/v) with vermiculite, compared to a commercial peat in tomato (Solanum lycopersicum L.) cultivation under greenhouse conditions. Washing reduced the electrical conductivity of Posidonia oceanica debris to 418 μS/cm, enabling its incorporation at a rate of 50% (v/v). However, the Posidonia oceanica-based growing medium showed lower porosity (31.3%) than peat (42.0%) and compost (41.7%), which was associated with reduced shoot and root biomass. Despite lower growth, the tomato plants grown in this medium showed higher foliar concentrations of photosynthetic pigments and higher tissue concentrations of silicon (1141 mg/kg), and macro- and micronutrients. These higher nutrient concentrations were primarily attributed to a concentration effect associated with reduced biomass rather than enhanced nutrient uptake. Although the levels of oxidative stress markers such as H2O2 and MDA were comparable to those in peat, the plants grown in Posidonia oceanica growing media maintained lower root electrolyte leakage (43.0%). Overall, washing effectively reduced Posidonia oceanica salinity and enabled its use at a high incorporation rate, but the tested formulation could not match the agronomic performance of peat or compost.