An experimental set-up to grow lettuce plants in a closed hydroponic growing system using substrates was made up and tested. Commercial fibber cement tiles were used as growing beds, in which gullies were filled with basaltic gravel. Substrates were laid out over the gravel in a 0.05 m layer depth and covered with a white polyethylene sheet. Lettuce plantlets from polystyrene nursery trays were planted in gaps made by cuttings on the polyethylene sheet. A complete nutrient solution was delivered daily to plants, by means of a drip irrigation system placed just bellow the polyethylene sheet and over the substrate layer. The nutrient solution running off from gullies was conducted back to the catchment tank. Two experiments were carried out, from May 16 to June 24, and from October 1th to October 28, 2003, using the lettuce cultivars Regina and Mimosa, and sand and the organic substrate Plantmax® Folhosas HA as growing media. The pH values of the nutrient solution decreased, but only one pH correction was made during each one of the experimental periods. On the hottest day of the spring experiment, temperature was higher in sand, but did not differ significantly from the organic substrate. Average values reached 5.4ºC above that of the nutrient solution and 0.4ºC above that of the air. Growth and development plant variables showed higher values in plants growing in the organic substrate in autumn, whereas in spring they were recorded on plants grown in sand. The substrates allowed a 92.4% reduction in the time during which the pump was switched on, leading to a simplification in practices of fertigation and control of the nutrient solution.
Lactuca sativa; fertigation; hydroponics; nutrient solution
