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b Consiglio per la ricerca e la sperimentazione in agricoltura -Centro per lo studio delle relazioni tra pianta e suolo -Agricultural Research Council, Research Centre of soil-plant system (CRA-RPS),Peat substitution in growth media with renewable organic components such as compost is one of the main challenges for organic nursery production. Two greenhouse experiments were carried out to investigate the performance of tomato seedlings grown on different compost-based growing media. The factors investigated were: (1) dose of compost, (2) level of phosphorus (P) enrichment and, only in the experiment 2, (3) source of P enrichment (rock phosphate enriched compost or rock phosphate amended growing media). Tomato seedling length, diameter, leaf area, leaf dry weight, shoot dry weight and seedling N, P and K concentrations were determined. Growth substrates with 30% compost showed the best seedling growth regardless of the source and level of P enrichment. Furthermore, at higher compost doses (60%) in the growth media, rock phosphate enriched compost (0.59 g P kg 21 compost) produced seedlings that were comparable to the rock phosphate amended substrates (1.18 g P kg 21 substrate). This demonstrates the capacity of the rock phosphate enriched compost in the growing media to supply more P to seedlings than the rock phosphate amended substrate. Moreover, growth media with 60% compost and peat-based growing media with standard rock phosphate application produced similar seedling growth confirming the potential of compost to substitute 60% of the peat in the growth media.

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