Evaluation of posidonia seaweed-based compost as a substrate for melon and tomato seedling production

The disposal of seaweed [Posidonia oceanica (L.) Delile] residues represents an environmental and economic problem in the Mediterranean Basin. In this study, two composts (Cp20 and Cp60) were prepared using posidonia residues mixed at different rates [20% or 60% (w/w) on a fresh weight (FW) basis, respectively] with grapevine prunings and vegetable waste.The feasibility of their use as partial substitutes for peat in the growing media used during transplant production of melon and tomato seedlings was assessed. Growing media were prepared by mixing Cp20 or Cp60 with commercial peat at rates of 25%, 50%, or 75% (v/v). Peat at 100% (v/v) was also tested as a control.The main physical and chemical properties of these seven growing media on shoot and root FWs and dry weights (DWs), leaf areas, root morphology, and the mineral composition of shoot tissues were determined. The six growing media obtained using posidonia-based composts had adequate physical and physico-chemical properties. In particular, those based on Cp20 showed higher easily-available water (EAW) values than those containing Cp60 and improved mineral nutrient (K, Fe, Mn and Zn) levels. In general, the growth of melon and tomato seedlings was not affected, or was positively affected, by the presence of either posidonia-based compost in the mixture. Cp20 was a more suitable compost to be added for seedling production, probably due to its lower content of boron (B) and improved physical properties. Tomato seedlings showed a better capacity than melon seedlings to adapt to high B levels. Posidonia-based composts, especially Cp20, may be considered as valuable alternatives (peat substitutes) in growing media used for seedling production.