Compost-based growing media: influence on growth and nutrient use of bedding plants.

The agronomic performance and the mineral composition and trace element content in Begonia semperflorens "Bellavista F1", Mimulus "Magic x hybridus", Salvia splendens "maestro", and Tagete patula xerecta "Zenith Lemon Yellow", were tested by growing the plants on substrates of white peat and 25-50-75-100% green waste and sewage sludge (80%+20%v/v) compost (CP). A commercial peat medium of black and white peat (2:1v/v) was used as control. At flowering, the agronomic parameters were compared by ANOVA and plant nutritional status was compared by vector analysis. Substrate-species interactions (P<0.001) were evident for all measured parameters. In the 25% CP medium all the species showed an increase or preservation of the studied agronomic parameters. Begonia grown in 25% CP, showed the highest dry weight (DW) and number of flowers. Other treatments were comparable to the control. Mimulus and Salvia showed the highest DW in the 25-50% CP. Mimulus, after a DW increase up to 50% CP, showed the steepest reduction as the CP increased further. Tagete showed no differences in DW up to 50% CP, or in flower number up to 25% CP, compared to the control. The additional increases of CP in the medium showed a DW decrease similar to that of Salvia. Vector analysis showed the use of compost mainly induced a decrease of P concentration in tissues, except for Begonia which remained unchanged. Plant tissues showed a general P reduction due to a dilution effect in the low compost mixtures (25-50%) and a deficiency in the higher CP mixtures. In contrast, an increase of Mg in the aboveground tissues of all species was detectable as compost usage increased, with the exception of Salvia which suffered a Mg deficiency. Vector analysis also highlighted a Ni and partial Fe deficiency in Tagete and Salvia.