Poseidonia residues can be used as organic mulch and amendment for lettuce and tomato production.

There is a need for sustainable fertilizers because common mineral fertilizers are increasingly costly and often induce water and air pollution. For instance, seagrass compost could be used as fertilizer in the coastal areas of the Mediterranean, thus also solving the issue of beached resi- dues. Here, we studied organic fertilization by application of seagrass-based compost in a tomato and lettuce crop succes- sion. Composts were made of posidonia and yard wastes at 1/4 w/w fresh weight ratio. Compost was applied at two concentrations: 10 and 20 Mg ha-1 fresh weight. Fresh leaves of posidonia were also tested directly as organic mulch in plots treated with conventional fertilizers. A control treatment in- volved mineral fertilization without mulching soil. Herbicide applications were performed in non-mulched plots immedi- ately before transplanting of tomato and lettuce for controlling the weeds, while mulched treatments did not receive any herbicide application. Results show that 20 Mg ha-1 of posidonia-based compost is equivalent to the inorganic fertil- ization because the nutritional status and yield of tomato and lettuce were statistically similar. No phytotoxicity symptoms were observed in any of the tomato and lettuce plants mulched with posidonia. The same growth rate was observed for toma- to and lettuce growing under conventional management prac- tices: mineral fertilizer and herbicide applications. Overall, our findings show that seagrass-based compost is a promising, sustainable fertilizer for tomato and lettuce.