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George Gizas and Dimitrios Savvas

Four successive trials with gypsophila, rose, cucumber, and lettuce grown on different pumice grades were conducted in a heated glasshouse. In the gypsophila experiment, three grades of pumice (0 to 2, 0 to 5, and 0 to 8 mm) were tested, whereas in the rose, cucumber, and lettuce experiments, four pumice grades (0 to 2, 0 to 5, 0 to 8, and 4 to 8 mm) were compared. In each experiment, these pumice grades were combined with two growing systems, specifically placement of the same media volume either in pots or in bags. Furthermore, the physical properties of the tested pumice grades were determined. Overall, gypsophila and cucumber gave the highest yields when grown in pots filled with the two finest pumice grades, whereas the yield obtained from plants grown in bags was poor regardless of particle size range. Coarse pumice restricted gypsophyla and cucumber yield in pot culture to nearly the same degree as in bags. In contrast, lettuce, and to a greater degree roses, exhibited a weaker response to the different pumice grades and growing systems. The two finer pumice grades were characterized by relatively low air-filled porosity, which presumably restricted plant growth and yield as a result of poor root aeration when the media were placed in bags with a height of 8 cm. The coarsest pumice grades were characterized by a steep drop in the water content as the suction increased only a few centimeters above zero, which imposed an appreciable lowering of the overall water content in the pots in comparison with the bags (≈12.5%), as a result of the increased height of the former. Our results indicate that gypsophila and cucumber respond more strongly than roses and lettuce to the variations in the air-to-water ratio in the root zone originating from differences in the physical properties of the growing media.

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George Gizas, Ioannis Tsirogiannis, Maria Bakea, Nikolaos Mantzos and Dimitrios Savvas

The residues of the aquatic plant Posidonia oceanica that are washed ashore, thereby causing environmental problems in coastal areas, can be used as growing media in horticulture. In the present study, the hydraulic characteristics of raw or composted Posidonia residues, coir, and their 1:1 blends (v/v) with pumice were determined, and their agronomic performance was evaluated in a lettuce crop. The mixture of all three substrates with pumice reduced their effective pore space and increased their bulk density. Furthermore, the water and air capacity (determined at a suction of 10 cm) and the easily available water were also reduced by mixing the three tested media with pumice. The relative hydraulic conductivity (Kr) decreased with increasing suction (ψ) in all of the tested media. The highest and the lowest rates of Kr decrease with increasing ψ were observed in the mix of non-composted Posidonia with pumice and in 100% composted Posidonia, respectively. Blending composted or non-composted Posidonia with pumice at a 1:1 ratio raised the rate of Kr decrease with increasing ψ in comparison with 100% composted or 100% non-composted Posidonia, respectively. In contrast, blending coir with pumice reduced the rate of Kr decrease with increasing ψ in comparison with 100% coir. The differences in the mean fresh weight between lettuce plants grown on the six growing media were similar with those in the rate of Kr decrease with increasing ψ. These results indicate that the crucial factor for the yield performance of lettuce grown on the tested growing media was not the air but the water availability. Furthermore, the present results indicate that the actual water availability to plants grown on the tested substrates depends much more on water flux toward roots and concomitantly on their hydraulic conductivity than on the easily available water (i.e., the difference in water content between 10 and 50 cm suction).