Growth of Tomato and Zucchini Seedlings in Orange Waste Compost Media: pH and Implication of Dosage

The high pH and electrical conductivity values of mature compost from solid wastes of citrus processing plants can severely restrict its use as a constituent of growing media in soilless cultivation. In the present work, addition of phosphoric acid was used as a chemically feasible strategy to lower the pH of mature orange waste compost. The growing media for testing were prepared by mixing commercial perlite (Agrilit 3) with increasing dosages (0, 7.5, 15 and 30%, by vol) of orange waste compost after a pH adjustement to 6.5-6.7 with the addition of phosphoric acid. In all potting mixtures the electrical conductivity never exceeded 0.7 dS m-1. Plant growth responses (i.e. fresh and dry biomass, together with root morphological parameters such as root length, surface area, fineness and tissue density) were assessed on tomato and zucchini seedlings grown in potting mixtures under greenhouse conditions for 40 and 20 days, respectively. The results indicated that orange waste compost induced species-specific and dose-dependent responses on plant growth. The growing rates were generally higher in zucchini than in tomato seedlings; moreover, amendments larger than 7.5% determined a reduced growth rate in both plants which became more evident at 30% compost doses. The same responses were induced on root morphological parameters: larger increases in root length, surface area, root dry weight, fineness and tissue density were observed in zucchini than in tomato; moreover, in both plants optimal morphological responses were found at 7.5% compost dose. Overall the results indicated that phosphoric acid may efficiently be used to control high pH values in orange waste compost and make it suitable for wider use as a constituent of perlite-rich media provided the amendment rates are carefully evaluated.