The experiment was carried out in a plastic greenhouse of the "Universidade Federal de Santa Maria", Brazil, from August to November 1999. In the 'NFT' (Nutrient Film Technique) five nutrient replacement manners in the nutrient solution were evaluted, based on the electrical conductivity (EC) and Regina and Deisy cultivars growing in production benches made by asbestos cement tiles. In each replication were put fourteen plants and one cultivar in each three channels totalling eighty-four plants in a production bench. We compared the efficiency of nutrient replacement methods on the growth of lettuce as well as the use of the same solution with nutrient replacement during three consecutive growing seasons. The experimental design was of randomized blocks with three replications in a 5x2 factorial scheme. The performance of the cultivars was not influenced by nutrient replacement methods. The absence of nutrient replacement in the solution during one growing season provided higher yield of lettuce. The replacement is not recommended when the objective is the renovation of the solution at the end of the growing season. The nutrient replacement forms, during the first and second growing seasons, did not alter the yield when compared to the complete renewal of the solution at the end of the growing season. To reutilize the same solution, it is advisable to replace nutrients whenever the EC decreases 50% from the initial level.
Lactuca sativa L.; hydroponics; nutrient solution; electrical conductivity
